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Characterization of Natural Organic Matter and Humic Substance Isolates by Size Exclusion Chromatography following Reduction with Sodium Borohydride
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  • Hang Li
    Hang Li
    Zachry Department of Civil & Environmental Engineering, Texas A&M University, College Station, Texas 77843, United States
    More by Hang Li
  • Blair Hanson
    Blair Hanson
    Department of Civil, Environmental, and Architectural Engineering  and  Environmental Engineering Program, University of Colorado Boulder, Boulder, Colorado 80303, United States
    More by Blair Hanson
  • Garrett McKay*
    Garrett McKay
    Zachry Department of Civil & Environmental Engineering, Texas A&M University, College Station, Texas 77843, United States
    *Email: [email protected]. Phone: (979) 458-6540.
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    Orcidhttps://orcid.org/0000-0002-6529-0892
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ACS Environmental Au

Cite this: ACS Environ. Au 2024, XXXX, XXX, XXX-XXX
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https://doi.org/10.1021/acsenvironau.4c00075
Published December 20, 2024

© 2024 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Chemical reduction with sodium borohydride has been used for over four decades to probe the presence and function of carbonyl-containing moieties in dissolved organic matter (DOM). One of these structure–property relationships is the attenuation of UV–visible absorbance after borohydride reduction, an effect that has been observed universally across DOM of different origins. We previously demonstrated that DOM with similar bulk physicochemical properties exhibits bifurcating reactivity with borohydride depending on the source (i.e., soil vs. aquatic), as judged by the kinetics of fractional absorbance removal during reduction at a fixed borohydride:DOM mass ratio. This result and data from other studies suggest that a portion of borohydride-reducible chromophores in DOM may be inaccessible to the water solvent, explaining the incomplete absorbance attenuation even at very high borohydride mass excesses. Here, we study the reactivity of five DOM isolates with sodium borohydride via size exclusion chromatography coupled to total organic carbon, absorbance, and fluorescence detectors. Reduction with sodium borohydride resulted in quantifiable yet exceedingly small decreases in DOM molecular weight, suggesting that the reduction of carbonyl groups to alcohols does not markedly impact the DOM secondary structure. Interestingly, higher molecular weight DOM exhibited the most prominent changes in optical properties after reduction, suggesting that larger molecules contain a high proportion of borohydride-reducible moieties. Optical surrogates were inversely correlated to molecular weight across a single isolate, both native and reduced. However, the correlation broke down at lower molecular weights, wherein optical surrogates remained constant with continued decreases in elution volume, suggesting that there is an intrinsic lower limit to the ability of optical surrogates to capture further decreases in molecular weight. Overall, these results provide insights into the DOM structure and help inform future applications of sodium borohydride for studying the DOM source and reactivity.

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Published as part of the ACS Environmental Au special issue “2024 Rising Stars in Environmental Research”.

Introduction

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Naturally occurring dissolved organic matter (DOM) is a mixture of compounds that are present in all water bodies and plays critical roles in natural and engineered systems. (1−3) The role of DOM in these systems is, in part, governed by its chemical and physical structure; however, efforts to resolve DOM structure have been challenged by its heterogeneity and molecular complexity. (1,4) For example, although ultrahigh resolution mass spectrometry reveals that DOM contains many thousands of different molecules, (5−8) resolving precise chemical structure from these data requires additional experiments or analyses. (9−11)
One way to probe DOM’s chemical composition involves selective transformation of key functional groups. (11−13) Monitoring the DOM property of interest before, during, and after such reactions provides a direct link between the chemistry of that transformation and observable properties, provided that appropriate controls are performed to test for the potential impact of reaction conditions. A nonexhaustive list of chemical transformations used to study structure–function relationships in DOM includes ozone, (14−16) reduction with metals (17,18) and electrodes, (19−21) acetylation, (22) and proton exchange. (11,23,24)
One of the most common transformations, spanning over four decades of use, is chemical reduction with sodium borohydride (NaBH4). Borohydride is a common reagent used in organic chemistry for reducing ketones and aldehydes to their corresponding alcohols (Scheme 1). The initial step in the reaction involves hydride transfer from the nucleophilic borohydride (BH4–) to the electrophilic C═O group, with the alcohol formed upon acidic workup. In an early study, Leenheer et al. used NaBH4 to help infer the predominance of aromatic ketone’s contribution to C═O groups in humic substance isolates from the Suwannee River. (25) Tinnacher and Honeyman later used tritiated borohydride as a radiolabeling technique, demonstrating via size exclusion chromatography (SEC) that 3H was uniformly distributed over the molecular size range of Suwannee River fulvic acid. (26) Pioneering work by Del Vecchio, Blough, and colleagues studied the impact of borohydride reduction on absorbance and fluorescence spectra of DOM and humic substances. (27) These studies and others have demonstrated a consistent response of optical properties to borohydride reduction–preferential attenuation of visible absorption, increased and blue-shifted fluorescence emission, and increasing fluorescence quantum yields–for a diverse set of humic substance isolates, (27,28) marine whole waters and isolates, (29−31) and other complex carbonaceous materials (e.g., atmospheric brown carbon). (32) More recently, studies combining mass labeling (using NaBD4) have demonstrated that >30% of formulas detected by negative mode electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry contain a borohydride-reducible moiety. (33) Although DOM can contain both aliphatic and aromatic carbonyls, (34) the latter are likely more important chromophores in sunlit waters. (35,36) Taken together, these prior studies indicate carbonyl moieties play an important role in DOM chemistry.

Scheme 1

Scheme 1. Influence of Borohydride Reduction on Electron Acceptor Moieties, Including Ketones/aldehydes (A) and Quinones (B). Borohydride Reduction of Carbonyl-containing Acceptors is Hypothesized to Disrupt Charge-Transfer Interactions in DOM (C)
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Results from NaBH4 reduction have been broadly used to support a photophysical model in which interacting chromophores play a key role in the emergence of DOM optical and photochemical properties. In this model, electron-poor acceptor moieties (e.g., aromatic aldehydes and ketones, or quinones) interact in their ground or excited electronic state with electron-rich donor moieties (e.g., phenols, alkoxy phenols, and polyphenols). (37−39) Transformation of carbonyl-containing electron acceptors to their corresponding alcohols removes these moieties’ ability to serve as electron acceptors. Loss of such donor–acceptor complexes is consistent with the changes observed after NaBH4 reduction, including increases in spectral slope (less visible absorbance due to charge-transfer) and blue-shifted and enhanced fluorescence emission (increased emission from local excited states due to a decreased rate of excited state proton/electron transfer). It has been argued that the impacts of NaBH4 reduction are inconsistent with reduction of isolated chromophores and fluorophores and that only an electronic interaction model can explain these aggregate observations. (27,38,40)
In prior work seeking to elucidate the role of charge-transfer interactions on the optical properties of DOM, McKay, Korak et al. demonstrated that absorbance and fluorescence spectra of DOM were largely independent of solvent polarity and temperature, which was taken as evidence that donor–acceptor complexes do not play a significant role in DOM optical properties. (41) In response, Del Vecchio and Blough hypothesized that charge-transfer complexes in DOM were thermodynamically stable and isolated from solvent (and therefore not in contact with organic solvent) because they were encapsulated in a hydrophobic DOM core. (40,42) They further argued that the incomplete absorbance attenuation of humic substances during NaBH4 reduction even at very high borohydride doses is evidence for encapsulated charge-transfer complexes. (12,40) A corollary to this hypothesis is that donor–acceptor complexes themselves may play a role in modulating DOM structure. Indeed, past research using SEC, (43,44) NMR, (45) and molecular dynamics simulations (46,47) has suggested that noncovalent interactions between smaller molecules may contribute to DOM three-dimensional structure. Based on this prior literature, we hypothesized that chemical reduction of DOM with NaBH4, which could disrupt charge-transfer contacts, would result in DOM with a lower molecular weight. Isolated charge-transfer complexes may also lead to unique dependence of optical properties of DOM on molecular weight due to varying accessibility of donor–acceptor pairs or differences between fractions in the abundance of borohydride-reducible groups.
To test this hypothesis, we reduced five DOM isolates with NaBH4 and employed SEC to characterize the size distribution of total organic carbon (TOC), absorbance, and fluorescence in native and reduced samples. In addition, apparent fluorescence quantum yields (AQY) of the native and reduced fractions were calculated as a function of elution volume using a recently developed methodology. (48) Spectral changes observed during SEC were compared to those for bulk samples. In addition to testing the impact of borohydride reduction on the molecular weight of DOM, these measurements allow us to examine the size dependence of the borohydride-induced changes in optical properties. Overall, results and analyses from this study provide insight into the distribution of borohydride-reducible groups in DOM, informing future applications of NaBH4 for differentiating sources and reactivity of DOM and other complex mixtures.

Materials and Methods

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Chemicals and DOM Samples

Humic substance and natural organic matter (NOM) isolates were obtained from the International Humic Substances Society, including Elliot Soil humic acid (ESHA 5S102H), Elliot Soil fulvic acid (ESFA 5S102F), Suwannee River humic acid (SRHA 3S101H), Suwannee River fulvic acid (SRFA 3S101F), and Suwannee River natural organic matter (SRNOM, 2R101N). Solutions for reduction experiments and the control group were prepared at 200 mg/L in pH 7 ultrapure water (≥18.2 MΩ-cm, Barnstead Nanopure). NaBH4 stock solution (5 g/L) were made in water preadjusted to pH 12 with NaOH. All humic substance and NOM solutions were filtered with 0.45 μm poly(ether sulfone) syringe filters (Supor) right before the reaction with borohydride. The filters were prerinsed with 20 mL of ultrapure water.

Borohydride Reduction

For the reduction group, 25-fold mass excess of NaBH4 was added to the DOM samples and fully stirred. (12) The control group was prepared at the same DOM concentration but without addition of NaBH4. Both the reduction and control group solutions were adjusted to pH 10 with 2 M NaOH and maintained at this pH for 4 days, in the dark. Although past work has shown minimal changes in spectral shape and intensity of bulk samples during high pH exposure, (28) the control experiment done here ensures that potential changes in spectra across the size gradients produced in SEC are due to reaction with NaBH4 and not hydroxide ions. After 4 days, both groups were adjusted to pH 7 with 2 M HCl to quench residual borohydride.

Analytical Methods

The reduction group and control group samples were diluted to 20 or 40 mg L–1 in 0.01 M phosphate buffer of pH 6.8 and filtered with water-rinsed 0.45 μm poly(ether sulfone) syringe filters. Absorbance and fluorescence spectra were measured on diluted (bulk) samples using an Aqualog (Horiba). The excitation wavelength (λex) ranged from 240 to 800 nm (in 5 nm increments) and the emission wavelength (λem) ranged from 250 to 800 nm (bin setting of 2.33 nm). The sample integration time was 0.2 or 0.5 s depending on the sample absorbance and emission intensity.
Aliquots of the diluted samples were shipped overnight to CU Boulder (Boulder, CO) in ice-packed coolers for SEC analysis as described previously. (48) Briefly, the in-line SEC system employed a Toyopearl HW-50S column with a mobile phase of phosphate buffer with sodium sulfate of pH 6.8, 0.1 M ionic strength, at a flow rate of 1 mL min–1. Samples (1.8 mL, 10 mg L–1) were adjusted to the eluent ionic strength (0.1 M) prior to injection using a concentrated mobile phase solution. Following elution from the SEC column, samples passed through multiple detectors, including an Agilent 1260 Infinity Series G1315D Diode Array Detector (DAD) from 200 to 700 nm with 2 nm increments, 1260 Infinity II Series G7121B Fluorescence Detectors (FLD) at λex = 350 nm and λem = 350–700 nm with 5 nm increments, and a Sievers M9 TOC Analyzer. The TOC analyzer utilizes UV/persulfate oxidation and was calibrated with standard solutions of potassium hydrogen phthalate (KHP). The SEC run time lasted up to 200 min to ensure complete sample elution prior to the next injection.

Data Processing

SEC data included absorbance spectra, fluorescence emission spectra at λex = 350 nm, and TOC measured at each elution volume. Several data treatment procedures were performed on the raw data. Signals measured prior to and after elution of DOM were averaged and used for blank subtraction and to correct for baseline drift in the case of SEC-absorbance data. Fluorescence data were further treated for instrument-specific correction factors, inner filter corrections, and masking of first order Rayleigh scatter. Although emission spectra were collected on the SEC out to 700 nm, they were trimmed at >600 nm due to low signal-to-noise ratios for some samples and large second order Rayleigh scatter. AQY as a function of elution volume were calculated according to Hanson et al. (48) Briefly, the method utilizes salicylic acid (quantum yield of 0.36 at 300 nm excitation), (49) dissolved in the same mobile phase used for sample analysis, as a fluorescence reference (note that salicylic acid was also used for detector alignment). One limitation of AQY calculations is that, because emission data is trimmed, a portion of the fluorescence signal (emission >600 nm) cannot be integrated. This results in a lower integrated fluorescence intensity and consequently a lower AQY. SRHA and ESHA, which have the most red-shifted fluorescence spectra, would have the greatest impact from this limitation.
Because TOC, absorbance, and fluorescence detectors acquired data at different frequencies, chromatographic data were interpolated to achieve a uniform elution volume increment (0.5 mL). A cubic spline interpolation was tested against other methods and found to give the lowest residuals, with smoothing parameters of 1, 0.95, and 0.90 used for spectra, absorbance chromatograms, and fluorescence chromatograms, respectively.
Bulk sample fluorescence spectra were corrected using standard procedures, (50) including blank subtraction, inner filter corrections, Rayleigh and Raman masking, and Raman normalization.

Quality Control Criteria

Quality control criteria were developed for both chromatographic and spectral data. Spectral data between 0 and 20 mL elution volume were used as the baseline. The quality check threshold is determined to be 20 times the 95% cumulative background noise, which is the standard deviation of baseline data. For fluorescence data, inner filter and instrument-specific correction factors were used to calculate non corrected data prior to determining the background noise threshold. Several other methods were attempted for fluorescence quality control, which are described in SI Text S1. Chromatograms and spectra with a maximum signal lower than the threshold were excluded from subsequent data analysis. To further filter noisy data, summed intensities at each elution volume that were less than 10% of the maximum summed intensity were excluded. (51)

Data Analysis

In addition to qualitative analysis of chromatograms, several metrics were calculated to evaluate quantitative differences between the control and reduction group. Inspired by Wünsch et al., (52) we calculated Tucker Congruence Coefficients (TCC) (53,54) and Shape Sensitive Congruence (SSC) to compare chromatograms of the native and reduced samples. TCC describes the difference in peak position between two chromatograms (eq 1)
TCC(s1,s2)=Σs1s2Σs12Σs22
(1)
where s1 and s2 represent chromatograms to be compared (e.g., native and reduced) with identical x-axes. SSC is thought to be a more sensitive quantification of peak differences by including penalty terms α and β that represent the difference in maximum position and area of the two peaks (eq 2)
SSC(s1,s2)=TCC−∑α,β
(2)
where
α(s1,s2)=|Vs1−Vs2|Vmax−Vmin
(3)
β(s1,s2)=|∫s1−∫s2|Vmax−Vmin
(4)
In eqs 3 and 4, |Vs1–Vs2| is the difference in peak position between s1 and s2 in milliliters, Vmax–Vmin represents the range of observed elution volume in milliliters, and |∫s1–∫s2| is the difference in areas of s1 and s2 normalized to their respective maximum peak intensities.
Although TCC > 0.95 is considered a “match” for PARAFAC components, there is no such accepted threshold for SSC, which tends to be lower than TCC, (52) nor is there an accepted threshold for comparing SEC chromatograms. As a first approximation, we derived threshold TCC and SSC values from replicate chromatograms of SRFA, SRHA, and SRNOM (n = 4 per sample, n = 12 total). Complementing this replicate analysis, we also compared chromatograms of native DOM samples that been passed through a column of Sephadex G-10 resin to remove borate salts, which is not expected to modify DOM molecular weight (see Text S2), but has been employed previously in borohydride reduction workflows. (12)
Additional quantitative metrics for comparing spectra included the intensity-weighted shift in wavelength, which was calculated for both absorbance and fluorescence spectra as a function of elution volume (eqs 5 and 6). Ared,i and Anat,i are the absorbance at each absorbance wavelength (λi) of the reduced and native samples, respectively. Fred,i and Fnat,i are the corresponding fluorescence data at λex = 350 nm, and each λem. These parameters describe the increase in steepness of absorbance spectra and decrease in peak emission maxima for the reduced sample compared to the control.
ΔλA=∑λiAred,i∑Ared,i−∑λiAnat,i∑Anat,i
(5)
ΔλF=∑λem,iFred,i∑Fred,i−∑λem,iFnat,i∑Fnat,i
(6)
Finally, optical surrogates were calculated as a function of elution volume. The first, A250/A364, is nearly identical to the widely used E2:E3 ratio (defined as A250/A365). (55,56) A250/A364 was used instead of E2:E3 because the SEC absorbance detector wavelength increment was 2 nm. The second optical surrogate is closely related to the fluorescence index (FI), (57,58) which is defined as the ratio of fluorescence emission at 470 to 520 at 370 nm excitation. In this study, the SEC fluorescence detector was set at 350 nm excitation and the ratio of fluorescence emission at 470 to 520 nm was calculated (F470/F520). While 370 nm has been used in previous studies of bulk DOM fluorescence, SEC results in a significant dilution of samples. The choice of 350 nm relates to the absorbance signal needed for AQY calculations where a lower absorbance threshold of 0.5 mAu was used as a QA/QC measure, below which AQY is not calculated. Because the absorbance spectra of DOM are generally characterized by an exponential decrease with increasing wavelength, 350 nm provides a greater absorbance signal and extends the range of elution volumes for which AQY is calculated.

Results

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Typical Chromatograms for TOC, Absorbance, and Fluorescence Detectors

This study presents chromatograms with elution volume on the x-axis. Elution volume is proportional to retention time (elution volume = retention time × flow rate) and, in SEC, is inversely proportional to molecular size. Although SEC studies often convert column retention time to molecular weight using polymer standards, we chose not to do so (detailed explanation presented in Text S3). The term “molecular weight” will be used in the later sections to describe the distribution of DOM fractions presented by retention time, while recognizing that SEC separates by size.
Figure 1 shows SEC chromatograms for SRFA measured by TOC, absorbance, and fluorescence detectors. Although we use the term TOC, the sample was filtered through 0.45 μM filter prior to injection and thus the signal is attributable to dissolved organic carbon. The absorbance chromatogram shown in Figure 1B is at a wavelength of 350 nm. The fluorescence detector was set to 350 nm excitation and Figure 1C represents the integrated fluorescence emission. Finally, AQY values shown in Figure 1D are calculated utilizing both absorbance and fluorescence data.

Figure 1

Figure 1. Chromatography of SRFA. From top to bottom: TOC, absorbance at 350 nm, integrated fluorescence emission at 350 nm excitation, and fluorescence quantum yield vs elution volume.

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Peak elution volumes for TOC occur just before absorbance and the fluorescence peak elution volume occurs after absorbance, consistent with a prior study of SRFA. (48) Because of the elution volume offset between absorbance and fluorescence, AQY values are highest at elution volumes larger than the peak of both absorbance and fluorescence chromatograms.
Figure 1 also demonstrates the typical impacts of NaBH4 reduction observed in SEC data that will be evaluated in detail below. Native and reduced SRFA exhibit highly similar TOC chromatograms, with reduced SRFA slightly shifted to larger elution volumes (lower molecular weight). The absorbance chromatogram of reduced SRFA is ∼50% lower in intensity at the peak than native SRFA, with integrated fluorescence being larger for the reduced sample. Taken together, these changes result in an increase in AQY at all elution volumes for reduced compared to native SRFA. A notable observation from these data is that optical changes occur across a range of elution volumes (large to small), indicating that SRFA has borohydride-reducible moieties that contribute to optical signals across a range of molecular weights.

Impact of NaBH4 Reduction on TOC Chromatograms

One objective of this study was to test the hypothesis that intermolecular electron donor–acceptor complexes contribute to the three-dimensional structure of DOM (Scheme 1C), as proposed by Blough and Del Vecchio. (40) Because SEC with TOC detection provides an evaluation of the size distribution of all carbon-containing molecules in DOM, we reasoned that comparing TOC chromatograms of native and reduced samples would be a good test of this hypothesis. If NaBH4 reduction disrupts donor–acceptor complexes, TOC chromatograms would be expected to shift toward larger elution volumes (lower molecular weight). This hypothesis was tested using both a qualitative comparison of the chromatograms and a more quantitative approach utilizing TCC and SSC. (52)
Figure 2A–E show that the primary peak of reduced sample’s TOC chromatograms is slightly shifted to larger elution volumes. Although small, the shift to larger elution volume for reduced samples is statistically significant, indicating the possibility that borohydride reduction decreases DOM molecular weight. Importantly, native samples were controlled by maintaining solutions at pH 10 for 4 days (the equivalent pH of borohydride reduced samples) to account for the potential of hydroxide-catalyzed reactions to decrease molecular weight.

Figure 2

Figure 2. TOC chromatograms of native and reduced DOM isolates normalized to peak intensity, including (A) Suwannee River fulvic acid (SRFA), (B) Suwannee River humic acid (SRHA), (C) Suwannee River natural organic matter (SRNOM), (D) Elliot Soil fulvic acid (ESFA), and (E) Elliot Soil humic acid (ESHA). (F) Tucker Congruence Coefficient (TCC) and size sensitive congruence (SSC) for samples under different conditions. SSC is equal to TCC subtracted by penalization terms that consider differences in peak area and peak position. The replicate injections label evaluates the difference between replicate native samples. TCC ± 95% CI and SSC ± 95% CI indicate 95% confidence intervals from four replicates. The native–reduced label evaluates the impact of NaBH4 reduction. An asterisk indicates that the value falls below the lower limit of the corresponding 95% CI.

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Unique to SRHA and SRNOM is the appearance of a peak at ∼27 mL at lower elution volumes that is either absent or less apparent in the native chromatogram. It is important to note that potential complexation of residual borate ions, which were not removed prior to SEC analysis (see Text S2), has been shown to decrease DOM’s electrophoretic mobility. (59) Such complexations would likely cause an increase in molecular size, which may explain the ∼27 mL peak for SRHA and SRNOM.
TCC and SSC values for the native-reduced group (treatment) TOC chromatograms are shown in Figure 2F and are compared to the lower 95% confidence interval of the equivalent values for native replicates (control). The replicates are derived from multiple injections of the same samples, which is considered a threshold for evaluating TCC and SSC of DOM samples. All TCC values for the treatment group are lower than the control TCC (<0.997). Considering SSC, four out of five values for the treatment group are lower than the control, with SRFA (SSC of 0.992) being the exception.

Comparison of Native and Reduced Spectra for Bulk DOM and Size-Separated DOM at the Peak Elution Volume

Before presenting the dependence of optical spectra on elution volume, it is instructive to show spectra for native and reduced SRFA collected for the bulk sample and at the peak elution volume (∼40 mL) during SEC (Figure 3). The spectral shape before and after borohydride reduction are qualitatively similar for bulk and SEC measurements. Absorbance spectra decrease in intensity across the UV–visible range following borohydride reduction, concomitant with an increase and blue-shifted fluorescence emission. Notably, SRFA’s red-edge emission (>550 nm) is largely unaltered after reduction with NaBH4. There is also good quantitative agreement in the magnitude of bulk samples’ optical changes. The fractional absorbance removal, Ared/Anat, reaches a minimum of ∼0.5 for spectra collected at the peak emission volume; a similar minimum (∼0.4) is observed for the bulk sample. The lower Ared/Anat observed for bulk SRFA may also be due to the different signal-to-noise ratios of different instruments. The ratio of reduced to native fluorescence emission, Fred/Fnat, shows similar agreement, with maximum values near 2.5 to 3 at ∼400 nm and a minimum of ∼1 at ∼550 nm.

Figure 3

Figure 3. Absorbance (A and B) and fluorescence spectra (C and D) of Suwannee River fulvic acid at the peak elution volume (A and C) and bulk samples (B and D). The right y-axis (gray lines) shows the ratio of reduced to native spectra. Spectra in (D) are normalized to water Raman scattering, whereas units in (C) are arbitrary. Emission spectra in (C) and (D) are collected at 350 nm excitation.

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Like results for SRFA, good qualitative and quantitative agreement is observed for spectra and their fractional change after NaBH4 reduction for other DOM isolates (Figure S1). Notable exceptions are that Ared/Anat for SRNOM and SRHA tend to be lower for bulk samples compared to Ared/Anat at the peak elution volume.

Impact of NaBH4 Reduction on DOM Absorbance Spectra and Chromatograms

NaBH4 reduction of all DOM isolates results in loss of UV and visible absorbance across a range of elution volumes (35 to 50 mL, Figure 4, Figure S2). Ared/Anat was lowest for elution volumes below ∼42 mL. In contrast, Ared/Anat for SRFA, SRHA, and ESFA remained largely the same at elution volumes above ∼45 mL (Figure 4A,B, and D). These results imply that material at a range of molecular weights (small to large) contains borohydride-reducible moieties, but that these groups play a more significant role in the absorbance of high molecular weight DOM. The inverse relationship between molecular weight and Ared/Anat (higher molecular weight, lower Ared/Anat) agrees with a prior study showing that Ared/Anat for size-fractionated (with a 5 kDa ultrafiltration membrane) SRFA decreased in the order SRFA < 5K, SRFA bulk, SRFA > 5K. (60)

Figure 4

Figure 4. Dependence of fractional absorbance (Ared/Anat) remaining on elution volume. Spectra collected at pH 7. NaBH4 reduction carried out for 4 days at 25-mass fold excess NaBH4. Heatmap: 35 to 50 mL elution volume. Ared/Anat of the bulk sample is indicated by a solid black line.

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A second observation from Figure 4 is that for selected DOM isolates Ared/Anat is less dependent on wavelength for lower elution volumes (higher molecular weight). Ared/Anat for ESFA and ESHA at an elution volume of ∼35 mL (red lines in Figure 4D,E) decreases less with increasing wavelength in comparison to material eluting at larger elution volumes (lower molecular weight). This result is further reflected in the dependence of intensity-averaged absorbance wavelength shifts ΔλA (between native and reduced samples) on elution volume (vide infra). Briefly, ΔλA values for ESHA and ESFA approach zero at lower elution volumes (Figure 5N,O), indicating that absorbance attenuation in high molecular weight material is removed proportionally at different wavelengths.

Figure 5

Figure 5. Absorbance properties of DOM isolates as a function of molecular weight and reduction with NaBH4. Logarithm of area-normalized absorbance spectra (log Anorm) of (A–E) native and (F–J) reduced samples, with the heatmap indicating elution volume. The solid and dashed lines in (A–J) represent the absorbance spectra of the native and reduced bulk samples, respectively. (K–O) Intensity-weighted difference in absorbance wavelength (ΔλA) as a function of elution volume. The dashed lines represent ΔλA for the bulk sample. The chromatogram of absorbance at 350 nm excitation is depicted by the gray line (arbitrary y-axis). The sample labels in (A–E) apply to (F–O).

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To further explore the impact of molecular weight and reduction on absorbance spectral shape, Figure 5 shows logarithmic absorbance spectra (normalized to area) collected as a function of elution volume for native (Figure 5A–E) and reduced (Figure 5F–J) samples. Spectra for the bulk native and reduced samples are shown for comparison using solid and dashed black lines, respectively.
The variation in spectral slope as a function of elution volume is largest for SRFA, SRHA, and ESFA and smallest for SRNOM and ESHA. This is the case for both native and reduced samples. It has been demonstrated for many types of DOM that NaBH4, in general, results in increased spectral slope. (27−32) The change in spectral slope after NaBH4 reduction at each elution volume, judged here by ΔλA, is generally in good agreement with the bulk sample. SRNOM is an exception in that the bulk and SEC sample’s ΔλA differs by ∼5 nm. Most of the variation in spectral slope (for both native and reduced samples) is driven by high molecular weight material (lower elution volume). In contrast, spectra of DOM eluting at larger elution volumes are tightly spaced (blue colors in Figure 5A–J). This observation is consistent with prior studies of highly resolved size fractions of DOM. For example, using SEC coupled to a diode array detector, Wünsch et al. (51) demonstrated that spectral slopes (S300–600) of four Swedish lakes had the most variation between ∼1 to 4 kDa whereas S300–600 values were largely independent of molecular weight below 1000 Da. Similarly, using asymmetric field flow fractionation, Guéguen and Cuss (61) showed that S275–295 and S350–400 were less sensitive to decreasing molecular weight at molecular weights less than 1000 Da relative to spectral slopes measured at >1000 Da.
The above results regarding the impact of borohydride reduction on DOM absorbance are mainly focused on spectra. To explore how NaBH4 impacts the size distribution of absorbing material, Figure 6 plots absorbance chromatograms for native and reduced samples at multiple λex. A comparison of Ared/Anat as a function of elution volume is also shown for comparison, which reveals that higher molecular weight material (lower elution volume) has a greater fractional absorbance removal than lower molecular weight material. This finding is consistent with the lower Ared/Anat across the UV–visible at lower elution volumes (Figure 4). Although subtle, comparison of normalized chromatograms (Figure S3) indicates that reduced samples are slightly shifted to larger elution volumes.

Figure 6

Figure 6. Absorbance chromatograms at multiple wavelengths for all DOM isolates. The gray lines represent the fractional absorbance remaining (Ared/Anat) at the indicated wavelength. Wavelengths indicated apply to all columns in each row, and sample headings in each column apply to all rows.

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To quantitatively describe changes in absorbance between native and reduced chromatograms, Figure S4 shows TCC and SSC values for native-reduced chromatograms compared to TCC and SSC for the column-no column control group. In the standard method for NaBH4 reduction, G-10 resin is used to remove borate. (12) However, Sephadex is also a size exclusion resin (G-10 has an exclusion limit of 700 Da), and prior studies have not rigorously tested whether borate cleanup changes the molecular weight distribution of DOM. Only SRNOM and SRHA were processed with and without the G-10 column (see Text S3), but we consider the values for these samples to be a good threshold for evaluating TCC and SSC of other DOM samples. Based on TCC values, which are derived solely from peak position, one would conclude that there are minimal differences in chromatograms between the native and control groups. However, larger differences are seen when shape is considered. For example, SSC values for absorbance chromatograms for the column-no column group are >0.99 but tend to be lower for the native-control group, especially for SRHA, SRNOM, and ESHA (SSC < 0.98). This result indicates that NaBH4 reduction results in a shift in the size distribution of absorbing DOM at a range of λex, albeit exceedingly small.

Impact of NaBH4 Reduction on DOM Fluorescence Spectra and Chromatograms

NaBH4 reduction results in increasing fluorescence intensity, despite the absorbance decrease, resulting in an increasing AQY. Increasing fluorescence emission and AQY has also been observed for NaBH4 reduction of bulk samples. (27,28) Our results indicate that this increase in AQY occurs at a range of elution volumes (low to high), demonstrating that AQY increases for all molecular weights. This increase in AQY at a range of molecular weights is consistent with a prior study demonstrating that SRFA molecular weight fractions (<5K, bulk, and >5K) had similar enhancements in AQY at different λex (between 2- and 3-fold). (60)
To further explore the impact of molecular weight and NaBH4 reduction on fluorescence spectral shape, Figure 7 shows the intensity-normalized emission spectra collected as a function of elution volume with the SEC fluorescence detector for native (Figure 7A–E) and reduced samples (Figure 7F–J). Spectra for the bulk native and reduced samples are shown for comparison. There are several interesting findings in Figure 7. First, emission spectra of the bulk samples more closely resemble spectra of material eluting at longer elution volumes, which can be explained by the higher AQY of lower molecular weight DOM. Second, there are large differences in peak emission maxima between the minimum and maximum elution volumes for each sample (e.g., > 50 nm for SRHA), with higher molecular weight exhibiting more red edge emission. Notably, the relative intensity of red edge emission is diminished in the reduced samples, suggesting that the emitting species responsible for this feature contain borohydride-reducible moieties. Third, for lower molecular weight material (e.g., at elution volumes greater than ∼45 mL), there is little change in emission maxima with elution volume. In contrast, spectra collected at lower elution volumes (high molecular weight) show a gradual blue-shift as the intensity of the ∼550 nm shoulder peak is diminished. Several prior reports using SEC, asymmetric field flow fractionation, and ultrafiltration have also demonstrated that higher molecular weight DOM tends to have red-shifted emission. (51,61) Finally, all samples besides SRFA exhibited a clear trend of increasing ΔλF with increasing elution volume, with good agreement between ΔλF for bulk and SEC samples. This shows that higher molecular weight material’s emission spectra is more blue-shifted after NaBH4 reduction compared to low molecular weight material.

Figure 7

Figure 7. Fluorescence emission properties of DOM isolates as a function of molecular weight and reduction with NaBH4. Intensity-normalized emission spectra (Fnorm) of (A–E) native and (F–J) reduced samples, with the heatmap indicating elution volume. The solid and dashed lines in (A–J) represent the emission spectra of the native and reduced bulk samples, respectively. (K–O) Intensity-weighted difference in fluorescence emission wavelength (ΔλF) as a function of elution volume. The dashed lines represent ΔλF for the bulk sample. The chromatogram of integrated emission at 350 nm excitation is depicted by the gray line (arbitrary y-axis). The sample labels in (A–E) apply to (F–O).

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In terms of chromatographic shape, the difference in SSC values between the column-no column and native-reduced groups is even more pronounced for fluorescence chromatograms (Figures S5 and S6) than absorbance chromatograms, especially for SRHA and ESHA. SSC values for the column-no column control group are near 0.99, while values for the native-reduced group are less than 0.98 and, in some cases, much lower (e.g., ESHA has SSC < 0.95). In contrast to the largely unidirectional shift in TOC and absorbance chromatograms to lower elution volumes, fluorescence chromatograms of reduced samples are slightly wider than native samples thus exhibiting increased signals (when both chromatograms are normalized to peak intensity) at both lower and higher elution volume. Collectively, these results indicate that the molecular weight of absorbing and emitting material at specific excitation and emission wavelengths is not appreciably changed by NaBH4 reduction.

Size Dependence of Optical Surrogates for Native and Reduced Samples

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SEC coupled to absorbance and fluorescence detectors provides an opportunity to test the underlying relationship between optical surrogates and DOM molecular weight. As highlighted recently, (62) many of these optical surrogates originated in specific contexts that may not be broadly applicable to others. One of the most well-tested contexts is size separation of a single sample, mostly by ultrafiltration, (63,64) with some studies also calculating optical surrogates from absorbance and fluorescence spectra collected during SEC. (51,61,65)
Here, we focus on one absorbance-based surrogate (A250/A364, similar to E2:E3) and one fluorescence-based surrogate (F470/F520 collected at 350 nm excitation, similar to FI). A250/A364 and F470/F520 for native and reduced samples are shown in Figure 8, with values from bulk sample shown for comparison. Both A250/A365 and F470/F520 increase with decreasing molecular weight (larger elution volume), consistent with several continued inquiries employing different size separation techniques (SEC, (51,66) asymmetric field flow fractionation, (61) and ultrafiltration (63,64)), providing support that these optical surrogates are good indicators of molecular weight for a single, size-fractionated sample. Perhaps more interesting is that there appears to be a limit beyond which further decreases in molecular weight (i.e., increasing elution volume) do not result in increased A250/A365 and F470/F520. These results indicate that there is a subset of DOM molecules that violate traditional optical surrogate-molecular weight interpretations. A similar observation (S275–295 and S350–400 did not vary at <800 Da) was reported for Canadian river water samples by Guéguen and Cuss. (61)

Figure 8

Figure 8. Dependence of optical surrogates on molecular weight for native and reduced DOM isolates. (A–E) A250/A364 (similar to E2:E3) as a function of elution volume. The chromatogram of absorbance at 250 nm is depicted by the gray line (arbitrary y-axis). (F–J) F470/F520 at 350 nm excitation (similar to fluorescence index, FI). Blue circles and orange squares represent native and reduced, samples, respectively. The normalized chromatogram of integrated emission at 350 nm excitation is depicted by the gray line (arbitrary y-axis). Optical surrogates for bulk native and reduced samples are indicated by + and *, respectively. The dotted black lines (− - – ) correspond to the ratio of surrogates for the native and reduced samples (right y-axis). The sample labels in (A–E) apply to (F–J).

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Given that prior studies have demonstrated a strong relationship between FI and the emission maximum, (58,67−69) we calculated the intensity-weighted emission wavelength at 350 nm excitation (∑λem,iFred,i∑Fred,i) and plotted these values against F470/F520 (Figure S7). There is a tight relationship in the F470/F520-emission wavelength curve that transcends DOM isolate origin (soil vs aquatic) and NaBH4 reduction status (native vs. reduced), resulting in a continuous exponential decrease in F470/F520 with increasing peak emission wavelength.
The continuous exponential relationship between F470/F520 and intensity-averaged emission maximum motivated us to evaluate whether there was an underlying mathematical explanation. To address this question, we modeled DOM fluorescence spectra using a Gaussian function and then took the ratio of fluorescence intensity at two wavelengths. The result of this derivation shown in eq S2.2 confirms the exponential relationship can be predicted mathematically (see also Text S4).
While the relationship between F470/F520 and intensity-averaged emission maximum is shown in part to be a mathematical result, the relationship between AQY and emission maximum has a fundamental photophysical basis. The fluorescence quantum yield (Φf) can be written as the rate of radiative decay divided by the total rate of singlet excited state decay, Φf = kf/(kf + kic + kisc) where kf, kic, and kisc are rate constants for fluorescence, internal conversion, and intersystem crossing, respectively. Typically >90% of singlet excited state DOM undergoes internal conversion. (39) The energy gap law states that the rate constant for internal conversion is an exponential function of the energy difference between the first excited electronic state (S1) and ground state (S0), kic∝e−ΔES1→S0. (70) Being a mixture of absorbers, we use the intensity-weighted emission maximum as a surrogate for ΔES1→S0; (16) the latter is normally determined by the mirror image rule. Thus, the energy gap law predicts that the AQY will decrease exponentially as a function of emission maximum. Consistent with this prediction, an exponential relationship between AQY and average emission wavelength is observed for most DOM isolates, both native and reduced (Figures S7 and S8). Notable exceptions to this relationship exist. For example, native ESFA shows a region (460–470 nm) in which AQY is insensitive to emission maximum. ESHA also shows behavior different than the Suwannee River isolates. Reduced ESHA exhibits a large range of AQY values over a very small emission wavelength range (500–510), and native ESHA has a large emission wavelength region (510–525 nm) in which AQY is constant. This bifurcation in emission properties of ESHA from other DOM isolates has been noted before in prior studies focused on solvation of DOM isolates in nonpolar organic solvents, (41) borohydride reduction, (28) and fluorescence quenching with cationic nitroxide free radicals. (71)

Discussion

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Here, we discuss the primary results of this study in the context of models for the three-dimensional structure of DOM and its relation to DOM optical properties.
Reduction of five DOM isolates by NaBH4 resulted in statistically significant but exceedingly small decreases in molecular weight observed by TOC and absorbance detectors. NaBH4 reduction converts carbonyls to alcohols, increasing molecular weight by 1 amu. Although minimal, a distinct shift to higher elution volume in TOC chromatograms is observed between 40–50 mL of all DOM isolates, which indicates a selective molecular weight decrease by NaBH4 reduction. The TOC shift is small compared to those typically observed during other chemical (e.g., ozone) (48,72,73) and physical (e.g., ultrafiltration) (64) treatments. Therefore, it is reasonable to conclude that the carbonyl-to-alcohol conversion and any disrupted electron donor–acceptor complexes (Scheme 1C) play a minimal role in DOM secondary structure. Although we cannot completely rule out that intermolecular electron donor–acceptor complexes may be influencing DOM three-dimensional structure, their overall contribution is likely small compared to other intermolecular forces. Consistent with this, a study by Wünsch et al. (51) demonstrated a high degree of similarity in fluorescence spectral shape of bulk and SEC-separated DOM, suggesting a minimal role for intermolecular interactions in the formation of DOM fluorescence. On the other hand, the contribution of intramolecular charge-transfer interactions should also be taken into consideration. Disruption of intramolecular complexes could be expected after borohydride reduction, but the impacts on SEC results would be more difficult to predict. Loss of intramolecular interactions would not decrease molecular weight, but could enable more degrees of freedom that could be manifested in an increased apparent size measured by SEC.
It should be noted that one prior study employing dynamic light scattering (DLS) demonstrated an increase in the average diameter of Aldrich Humic Acid reduced with NaBH4 relative to the native sample. (74) Given prior problems noted with commercial humic acids as a model for humic substances (e.g., interbatch consistency, unreported ash content, and lack of information about source and isolation), (75) we believe the results shown here and those for Aldrich Humic Acid (74) are not necessarily in conflict.
Our results suggest that borohydride-reducible moieties (i.e., aromatic ketones or aldehydes, and quinones) are present throughout DOM’s molecular weight distribution and not isolated in one size class. This conclusion is also supported by Tinnacher and Honeymann (26) who demonstrated a near uniform distribution in radioactivity after labeling SRFA with 3H via tritiated NaBH4. That being said, changes in optical properties of larger sized molecules were more pronounced, such as lower Ared/Anat and more negative shifts in ΔλF. The origin of these more significant changes is currently unclear. One model contends that borohydride-reducible charge-transfer contacts are partially isolated in a hydrophobic core, (40,42) which seems inconsistent with our results. However, it is still possible that solvent-inaccessible borohydride-reducible moieties exist if there are enough solvent-accessible reduction sites. If true, this would suggest that higher molecular weight DOM contains a greater concentration of carbonyl-containing chromophores. Future work could address this question by solid-state 13C NMR analysis of high molecular weight DOM prepared by ultrafiltration.
Finally, our results contribute to the existing literature supporting optical measurements as surrogates for DOM molecular weight. (62) Importantly, the correlations observed between optical surrogates and molecular weight (via elution volume) are for five DOM isolates, native and reduced, developed over a fine size gradient achieved by SEC. Indeed, comparing optical surrogates for native and reduced values demonstrates that other variables besides molecular weight are impacting these surrogates. Although reduced DOM isolates do exhibit a very slight decrease in molecular weight (Figures 2, S3 and S4), the changes in optical surrogates are much more pronounced. In other words, it is likely that modification of the underlying chromophores is responsible for the increase in A250/A364 and F470/F520, not shifts in molecular weight. Consistent with several prior investigations (51,61,76−82) is the observation that absorbance and fluorescence spectral shape converge at lower molecular weights, which is reflected in the constancy of both A250/A364 and F470/F520 at >45 mL of the SEC chromatogram. This result implies that low molecular weight DOM may be insensitive to further spectral shifts, decreasing optical surrogate’s utility for monitoring molecular weight below a certain limit.

Conclusions

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By studying the impact of NaBH4 reduction using size exclusion chromatography coupled to TOC, absorbance, and fluorescence detectors, we demonstrated that
1.

Reduction with NaBH4 results in quantifiable yet exceedingly small decreases in the apparent size of DOM, indicating that intermolecular interactions involving borohydride-accessible carbonyl-containing moieties play a minimal role in modulating the three-dimensional structure of DOM.

2.

Borohydride-reducible groups are present in all size fractions of DOM. However, high molecular weight DOM may have a higher proportion of these groups based on the more pronounced changes in absorbance spectra (Ared/Anat) and fluorescence emission blue shift at lower elution volumes.

3.

The energy gap law is suggested as the photophysical mechanism responsible for the inverse dependence of AQY on emission maximum for DOM of varying size.

4.

Optical surrogates are highly sensitive to NaBH4 reduction despite minimal shifts in DOM molecular weight.

These results provide several potential avenues for future work. First, it would be of interest to perform NaBH4 reduction experiments on size-fractionated DOM (e.g., by ultrafiltration) and then subject samples to SEC analysis. Second, the relative wavelength independence of Ared/Anat for high molecular weight material calls into question the widely stated concept that preferential removal of visible absorption by borohydride reduction is evidence of charge-transfer interactions. Follow up studies could test this hypothesis by measuring Ared/Anat for model organic chromophores known to absorb/emit locally and as donor–acceptor complexes. Third and finally, additional studies are needed to support the mechanism responsible for the small decrease in molecular weight following borohydride reduction. Our hypothesis is that conversion of carbonyl moieties to alcohols results in loss of charge-transfer contacts that decreases DOM molecular weight. This could be tested experimentally using additional size characterization techniques (e.g., asymmetric field flow fractionation, electron microscopy) or computational approaches such as molecular dynamics.

Data Availability

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Data for this article is available in the main manuscript tables and figures or in the Supporting Information. Other data will be made available to upon reasonable request to the authors.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenvironau.4c00075.

  • Additional discussions, tables, and figures; see SI table of contents for specific details (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Hang Li - Zachry Department of Civil & Environmental Engineering, Texas A&M University, College Station, Texas 77843, United States
    • Blair Hanson - Department of Civil, Environmental, and Architectural Engineering  and  Environmental Engineering Program and , University of Colorado Boulder, Boulder, Colorado 80303, United States
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by an NSF CAREER Award (#2237194) to G.M. The authors are grateful for the feedback of three anonymous reviewers which improved the quality of the manuscript.

References

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    Leyva, Dennys; Jaffe, Rudolf; Fernandez-Lima, Francisco
    Analytical Chemistry (Washington, DC, United States) (2020), 92 (17), 11960-11966CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    TIMS-FT-ICR MS is an important alternative to study the isomeric diversity and elemental compn. of complex mixts. While the chem. structure of many compds. in the dissolved org. matter (DOM) remains largely unknown, the high structural diversity has been described at the mol. level using chem. formulas. In this study, we further push the boundaries of TIMS-FT-ICR MS by performing chem. formula-based ion mobility and tandem MS anal. for the structural characterization of DOM. The workflow described is capable to mobility select (R ~ 100) and isolate mol. ion signals (Δm/z = 0.036) in the ICR cell, using single-shot ejections after broadband ejections and MS/MS based on sustained off-resonance irradn. collision-induced dissocn. (SORI-CID). The workflow results are compared to alternative TIMS-q-FT-ICR MS/MS expts. with quadrupole isolation at nominal mass (~ 1 Da). The technol. is demonstrated with isomeric and isobaric mixts. (e.g., 4-methoxy-1-naphthoic acid, 2-methoxy-1-naphthoic acid, decanedioic acid) and applied to the characterization of DOM. The application of this new methodol. to the anal. of a DOM is illustrated by the isolation of the mol. ion [C18H18O10-H]- in the presence of other isobars at nominal mass 393. Five IMS bands were assigned to the heterogeneous ion mobility profile of [C18H18O10-H]-, and candidate structures from the PubChem database were screened based on their ion mobility and the MS/MS matching score. This approach overcomes traditional challenges assocd. with the similarity of fragmentation patterns of DOM samples (e.g., common neutral losses of H2O, CO2, and CH2-H2O) by narrowing down the isomeric candidate structures using the mobility domain.
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  10. 10
    Baluha, D. R.; Blough, N. V.; Del Vecchio, R. Selective Mass Labeling for Linking the Optical Properties of Chromophoric Dissolved Organic Matter to Structure and Composition via Ultrahigh Resolution Electrospray Ionization Mass Spectrometry. Environ. Sci. Technol. 2013, 47 (17), 9891– 9897,  DOI: 10.1021/es402400j
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    10
    Selective Mass Labeling for Linking the Optical Properties of Chromophoric Dissolved Organic Matter to Structure and Composition via Ultrahigh Resolution Electrospray Ionization Mass Spectrometry
    Baluha, Daniel R.; Blough, Neil V.; Del Vecchio, Rossana
    Environmental Science & Technology (2013), 47 (17), 9891-9897CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The mass spectra acquired by ESI FT-ICR MS of untreated, borohydride-reduced, and borodeuteride-reduced samples of Suwannee River fulvic acid (SRFA) and a C18 ext. from the upper Delaware Bay were compared to one another. Treatment of these samples with sodium borodeuteride was shown to produce unique mass labels for species which contain one or two ketone/aldehyde moieties. Approx. 30% of all identified peaks in the two samples were shown to comprise ketone/aldehyde-contg. species. The mol. formulas of the majority of these species had O/C and H/C molar ratios typically attributed to lignin-derived compds. and/or carboxylic rich alicyclic mols. (CRAM). However, the significant loss of UV-vis absorption following redn. supports a lignin-based origin for the optical (and photochem.) properties of these samples. The mass-labeling method described and tested herein shows great promise as a means to further characterize the structure and compn. of complex natural samples, esp. in terms of identifying specific subsets of chem. species that contribute significantly to the optical and photochem. properties of such samples.
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  11. 11
    Zherebker, A.; Shirshin, E.; Rubekina, A.; Kharybin, O.; Kononikhin, A.; Kulikova, N. A.; Zaitsev, K. V.; Roznyatovsky, V. A.; Grishin, Y. K.; Perminova, I. V.; Nikolaev, E. N. Optical Properties of Soil Dissolved Organic Matter Are Related to Acidic Functions of Its Components as Revealed by Fractionation, Selective Deuteromethylation, and Ultrahigh Resolution Mass Spectrometry. Environ. Sci. Technol. 2020, 54 (5), 2667– 2677,  DOI: 10.1021/acs.est.9b05298
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    11
    Optical Properties of Soil Dissolved Organic Matter Are Related to Acidic Functions of Its Components as Revealed by Fractionation, Selective Deuteromethylation, and Ultrahigh Resolution Mass Spectrometry
    Zherebker, Alexander; Shirshin, Evgeny; Rubekina, Anna; Kharybin, Oleg; Kononikhin, Alexey; Kulikova, Natalia A.; Zaitsev, Kirill V.; Roznyatovsky, Vitaliy A.; Grishin, Yuri K.; Perminova, Irina V.; Nikolaev, Evgeny N.
    Environmental Science & Technology (2020), 54 (5), 2667-2677CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The goal of this study was to establish a relationship between the optical properties of soil dissolved org. matter (DOM) and acidic functions carried out by its individual constituents. We obtained 12 fractions of DOM samples using sequential solid phase extn. on nonionic sorbent at steadily lowered pH values: 7, 5, 3, 2, which correspond to low bounds of pKa values of phenols, aliph., and arom. carboxylic acids, and ketoacids. The structural studies were conducted with the use of NMR and selective deuteromethylation of isolated fractions coupled to ultrahigh resoln. mass spectrometry. First, a gradual shift of mol. compns. was obsd. from reduced components to arom. oxidized compds. isolated at pH 7 and 2, resp. Changes in mol. compns. were accompanied by a red shift of fluorescence spectra. Further application of deuteromethylation enabled us to distinguish DOM constituents with different amts. of carboxylic groups. Moreover, identification of structural isomers in a single DOM sample was achieved. Statistical anal. revealed that red shift of fluorescence is facilitated by the increase of a contribution of arom. poly(carboxylic acid)s with high conjugation lengths. Addnl., anal. of the labeled fractionated permafrost thaw DOM directly showed carboxyl-rich alicyclic mols., while the same components from lower-latitude DOM were assigned to lignin-like species.
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  12. 12
    Schendorf, T. M.; Del Vecchio, R.; Koech, K.; Blough, N. V. A standard protocol for NaBH4 reduction of CDOM and HS. Limnol. Oceanogr.:Methods 2016, 14 (6), 414– 423,  DOI: 10.1002/lom3.10100
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    12
    A standard protocol for NaBH4 reduction of CDOM and HS
    Schendorf, Tara Marie; Del Vecchio, Rossana; Koech, Kevin; Blough, Neil V.
    Limnology and Oceanography: Methods (2016), 14 (6), 414-423CODEN: LOMIBY; ISSN:1541-5856. (John Wiley & Sons, Inc.)
    Sodium borohydride (NaBH4) selectively reduces ketones/aldehydes to alcs. and quinones to hydroquinones and has been used to reduce these carbonyl moieties within humic substances (HS) and chromophoric dissolved org. matter (CDOM) to gain insight into their contributions to the optical/photochem. properties of these materials. To further investigate the effects of borohydride redn. on the absorption and emission properties of HS and to develop a std. protocol for this redn., the effects of increasing borohydride concns. were examd. over the course of 96 h. Time courses for the redn. revealed the presence of two kinetically distinguishable pools of carbonyls that were irreversibly reduced. A third pool appears to be reversibly reduced at a low mass excess of NaBH4, but contributes much less to HS absorption. Reduced samples can be adjusted to pH 7 and passed through a size exclusion column (Sephadex G-10) to remove borate salts, thus producing salt-free samples that can be used for further analyses. A 25-fold mass excess of NaBH4 relative to the HS mass produced the largest changes in optical properties without generating excessive amts. of reaction products. Although smaller addnl. changes are obsd. at higher mass excesses, the removal of excess borate becomes substantially more difficult and time consuming. While most changes are complete by 48 h, our results suggest that the reaction should be allowed to proceed for 96 h in the dark at room temp. to ensure that addnl. changes in the optical signals are not obsd.
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  13. 13
    Nebbioso, A.; Piccolo, A. Modification of chemical and conformational properties of natural organic matter by click chemistry as revealed by ESI-Orbitrap mass spectrometry. Anal. Bioanal. Chem. 2015, 407 (28), 8515– 8523,  DOI: 10.1007/s00216-015-9005-7
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  14. 14
    Houska, J.; Salhi, E.; Walpen, N.; von Gunten, U. Oxidant-reactive carbonous moieties in dissolved organic matter: Selective quantification by oxidative titration using chlorine dioxide and ozone. Water Res. 2021, 207, 117790  DOI: 10.1016/j.watres.2021.117790
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    14
    Oxidant-reactive carbonous moieties in dissolved organic matter: Selective quantification by oxidative titration using chlorine dioxide and ozone
    Houska, Joanna; Salhi, Elisabeth; Walpen, Nicolas; von Gunten, Urs
    Water Research (2021), 207 (), 117790CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    The application of oxidants for disinfection or micropollutant abatement during drinking water and wastewater treatment is accompanied by oxidn. of matrix components such as dissolved org. matter (DOM). To improve predictions of the efficiency of oxidn. processes and the formation of oxidn. products, methods to det. concns. of oxidant-reactive phenolic, olefinic or amine-type DOM moieties are crit. Here, a novel selective oxidative titrn. approach is presented, which is based on reaction kinetics of oxidn. reactions towards certain DOM moieties. Phenolic moieties were detd. by oxidative titrn. with ClO2 and O3 for five DOM isolates and two secondary wastewater effluent samples. The detd. concns. of phenolic moieties correlated with the electron-donating capacity (EDC) and the formation of inorg. ClO2-byproducts (HOCl, ClO-2, ClO-3). ClO2-byproduct yields from phenol and DOM isolates and changes due to the application of mol. tagging for phenols revealed a better understanding of oxidant-reactive structures within DOM. Overall, oxidative titrns. with ClO2 and O3 provide a novel and promising tool to quantify oxidant-reactive moieties in complex mixts. such as DOM and can be expanded to other matrixes or oxidants.
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  15. 15
    Wenk, J.; Aeschbacher, M.; Salhi, E.; Canonica, S.; von Gunten, U.; Sander, M. Chemical oxidation of dissolved organic matter by chlorine dioxide, chlorine, and ozone: effects on its optical and antioxidant properties. Environ. Sci. Technol. 2013, 47 (19), 11147– 11156,  DOI: 10.1021/es402516b
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    15
    Chemical Oxidation of Dissolved Organic Matter by Chlorine Dioxide, Chlorine, and Ozone: Effects on Its Optical and Antioxidant Properties
    Wenk, Jannis; Aeschbacher, Michael; Salhi, Elisabeth; Canonica, Silvio; von Gunten, Urs; Sander, Michael
    Environmental Science & Technology (2013), 47 (19), 11147-11156CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    In water treatment dissolved org. matter (DOM) is typically the major sink for chem. oxidants. The resulting changes in DOM, such as its optical properties have been measured to follow the oxidn. processes. However, such measurements contain only limited information on the changes in the oxidn. states of and the reactive moieties in the DOM. We used mediated electrochem. oxidn. to quantify changes in the electron donating capacities (EDCs), and hence the redox states, of 3 different types of DOM during oxidn. with ClO2, Cl (as HOCl/OCl-), and O3. Treatment with ClO2 and HOCl resulted in comparable and prominent decreases in EDCs, while the UV light absorbances of the DOM decreased only slightly. Conversely, ozonization resulted in only small decreases of the EDCs but pronounced absorbance losses of the DOM. These results suggest that ClO2 and HOCl primarily reacted as oxidants by accepting electrons from electron-rich phenolic and hydroquinone moieties in the DOM, while O3 reacted via electrophilic addn. to arom. moieties, followed by ring cleavage. This study highlights the potential of combined EDC-UV measurements to monitor chem. oxidn. of DOM, to assess the nature of the reactive moieties and to study the underlying reaction pathways.
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  16. 16
    Leresche, F.; McKay, G.; Kurtz, T.; von Gunten, U.; Canonica, S.; Rosario-Ortiz, F. L. Effects of ozone on the photochemical and photophysical properties of dissolved organic matter. Environ. Sci. Technol. 2019, 53 (10), 5622– 5632,  DOI: 10.1021/acs.est.8b06410
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    16
    Effects of Ozone on the Photochemical and Photophysical Properties of Dissolved Organic Matter
    Leresche, Frank; McKay, Garrett; Kurtz, Tyler; von Gunten, Urs; Canonica, Silvio; Rosario-Ortiz, Fernando L.
    Environmental Science & Technology (2019), 53 (10), 5622-5632CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    This study focused on the effects of ozonization on the photochem. and photophys. properties of dissolved org. matter (DOM). Upon ozonization, a decrease in DOM absorbance was obsd. in parallel with an increase in singlet O (1O2) and fluorescence quantum yields (Φ1O2 and ΦF). The increase in Φ1O2 was attributed to the formation of quinone-like moieties during ozonization of the phenolic moieties of DOM, while the increase in ΦF can be explained by a significant decrease in the internal conversion rate of the 1st excited singlet state of the DOM (1DOM*). It is a consequence of an increase in the av. energy of the 1st electronic transition (S1 → S0) that was assessed using the wavelength of max. fluorescence emission (λF,max). Also, ozonization did not affect the ratio of the apparent steady-state concns. of excited triplet DOM (3DOM*) and 1O2, indicating that ozonization does not affect the efficiency of 1O2 prodn. from 3DOM*. The consequences of these changes for the phototransformation rates of micropollutants in surface waters were examd. using photochem. model calcns. The decrease in DOM absorbance caused by ozonization leads to an enhancement of direct photolysis rates due to the increased transparency of the H2O. Rates of indirect photooxidn. induced by 1O2 and 3DOM* slightly decrease after ozonization.
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  17. 17
    Macalady, D. L.; Walton-Day, K. New light on a dark subject: On the use of fluorescence data to deduce redox states of natural organic matter (NOM). Aquat. Sci. 2009, 71 (2), 135– 143,  DOI: 10.1007/s00027-009-9174-6
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    17
    New light on a dark subject: On the use of fluorescence data to deduce redox states of natural organic matter (NOM)
    Macalady, Donald L.; Walton-Day, Katherine
    Aquatic Sciences (2009), 71 (2), 135-143CODEN: AQSCEA; ISSN:1015-1621. (Birkhaeuser Verlag)
    This paper reports the use of excitation-emission matrix fluorescence spectroscopy (EEMS), parallel factor statistical anal. (PARAFAC), and oxidn.-redn. expts. to examine the effect of redox conditions on PARAFAC model results for aq. samples rich in natural org. matter. Fifty-four aq. samples from 11 different geog. locations and two plant exts. were analyzed untreated and after chem. treatments or irradn. were used in attempts to change the redox status of the natural org. matter. The EEMS spectra were generated and modeled using a PARAFAC package developed by Cory and McKnight (2005). The PARAFAC model output was examd. for consistency with previously reported relations and with changes expected to occur upon exptl. oxidn. and redn. of aq. samples. Results indicate the implied fraction of total sample fluorescence attributed to quinone-like moieties was consistent (0.64 to 0.78) and greater than that obsd. by Cory and McKnight (2005). The fraction of the quinone-like moieties that was reduced (the reducing index, RI) showed relatively little variation (0.46 to 0.71) despite attempts to alter the redox status of the natural org. matter. The RI changed little after reducing samples using zinc metal, oxidizing at high pH with air, or irradiating with a Xenon lamp. Our results, however, are consistent with the correlations between the fluorescence indexes (FI) of samples and the ratio of PARAFAC fitting parameters suggested by Cory and McKnight (2005), though we used samples with a much narrower range of FI values.
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  18. 18
    Macalady, D. L.; Walton-Day, K. Redox Chemistry and Natural Organic Matter (NOM): Geochemists’ Dream, Analytical Chemists’ Nightmare; American Chemical Society: Washington, DC, 2011; Vol. 1071, pp 85– 111.
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  19. 19
    Maurer, F.; Christl, I.; Kretzschmar, R. Reduction and Reoxidation of Humic Acid: Influence on Spectroscopic Properties and Proton Binding. Environ. Sci. Technol. 2010, 44 (15), 5787– 5792,  DOI: 10.1021/es100594t
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    19
    Reduction and Reoxidation of Humic Acid: Influence on Spectroscopic Properties and Proton Binding
    Maurer, Felix; Christl, Iso; Kretzschmar, Ruben
    Environmental Science & Technology (2010), 44 (15), 5787-5792CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Previous studies on proton and metal binding to humic substances have not considered a potential influence of redn. and oxidn. of functional groups. Therefore, we investigated how proton binding of a purified soil humic acid was affected by redn. Redn. of the humic acid was carried out using an electrochem. cell that allowed us to measure the amts. of electrons and protons involved in redn. reactions. We further applied spectroscopic methods (UV-vis, fluorescence, FT-IR, C-1s NEXAFS) to detect possible chem. changes in the humic acid induced by redn. and reoxidn. The effect of redn. on proton binding was detd. with acid-base titrns. in the pH range 4-10 under controlled redox conditions. During redn., 0.54 mol kg-1 protons and 0.55 mol kg-1 electrons were transferred to humic acid. NICA-Donnan modeling revealed an equiv. increase in proton-reactive sites (0.52 mol kg-1) in the alk. pH-range. Our results indicate that redn. of humic acid increased the amt. of proton-reactive sites by 15% compared to the untreated state. Spectroscopic differences between the untreated and reduced humic acid were minor, apart from a lower UV-vis absorption of the reduced humic acid between 400 and 700 nm.
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  20. 20
    Yang, P.; Jiang, T.; Cong, Z.; Liu, G.; Guo, Y.; Liu, Y.; Shi, J.; Hu, L.; Yin, Y.; Cai, Y.; Jiang, G. Loss and Increase of the Electron Exchange Capacity of Natural Organic Matter during Its Reduction and Reoxidation: The Role of Quinone and Nonquinone Moieties. Environ. Sci. Technol. 2022, 56 (10), 6744– 6753,  DOI: 10.1021/acs.est.1c08927
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    20
    Loss and Increase of the Electron Exchange Capacity of Natural Organic Matter during Its Reduction and Reoxidation: The Role of Quinone and Nonquinone Moieties
    Yang, Peijie; Jiang, Tao; Cong, Zhiyuan; Liu, Guangliang; Guo, Yingying; Liu, Yanwei; Shi, Jianbo; Hu, Ligang; Yin, Yongguang; Cai, Yong; Jiang, Guibin
    Environmental Science & Technology (2022), 56 (10), 6744-6753CODEN: ESTHAG; ISSN:1520-5851. (American Chemical Society)
    Redox-active quinone and nonquinone moieties represent the electron exchange capacity (EEC) of natural org. matter (NOM), playing an important role in the electron transfer link of microbes and transformation of contaminants/metal minerals. However, the corresponding transformation of quinone/phenol and their resp. influence on the EECs during redn. and reoxidn. remain poorly characterized. Besides, it is still controversial whether nonquinones donate or accept electrons. Herein, we demonstrated that reoxidn. of NOM after redn. can form new phenolic/quinone moieties, thus increasing the EEC. The assessment for the EEC, including the electron-donating capacity (EDC) and electron-accepting capacity (EAC), of nonquinones reflects the contribution of sulfur-contg. moieties with considerable EDCs and EACs. In contrast, nitrogen-contg. moieties donate negligible electrons even at Eh = +0.73 V. The contributions of both thiol and amine moieties to the EEC are greatly affected by adjacent functional groups. Meanwhile, aldehydes/ketones did not display an EAC during the electron transfer process of NOM. Furthermore, substantially increased EDC at Eh from +0.61 to +0.73 V could not be fully explained using thiol and phenolic moieties, suggesting the contribution of unknown moieties with high oxidn. potential. The overall findings suggest that the roles of new quinones/phenol (derived from the addn. of oxygen to condensed arom./lignin-like components) during redox dynamic cycling and thiol species should be considered in assessing the electron transfer processes of NOM.
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  21. 21
    McAdams, B. C.; Hudson, J.; Arnold, W. A.; Chin, Y. P. Effects of aquatic dissolved organic matter redox state on adsorption to goethite. Aquat. Sci. 2023, 85 (1), 19  DOI: 10.1007/s00027-022-00912-0
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    21
    Effects of aquatic dissolved organic matter redox state on adsorption to goethite
    McAdams, Brandon C.; Hudson, Jeffrey; Arnold, William A.; Chin, Yu-Ping
    Aquatic Sciences (2023), 85 (1), 19CODEN: AQSCEA; ISSN:1015-1621. (Springer International Publishing AG)
    Abstr.: Direct electrochem. methods were used to alter the redox state of dissolved org. matter (DOM) rich in carbon-hydrogen-oxygen-sulfur (CHOS) and sulfur deficient (CHO) compn. to explore how their reduced and oxidized forms sorbed to iron oxides. The CHOS-rich DOM was isolated from Prairie Pothole wetland surface waters, while Suwannee River natural org. matter (SRNOM) represented a sulfur poor DOM. CHOS-rich DOM showed decreased adsorption to goethite following electrochem. redn. compared to its oxidized or unaltered forms. In contrast, no difference was obsd. between the adsorption of unaltered or reduced SRNOM to goethite. Our results suggest that the org. sulfur moieties in Prairie Pothole DOM are responsible for the decline in adsorption following redn. Changes in DOM mol. wt. after adsorption were obsd. for all samples, however, reduced Prairie Pothole DOM resulted in increased fractionation relative to its unaltered or oxidized forms. This was not obsd. for SRNOM. Furthermore, increases in Fe(II) in soln. were obsd. following interaction of reduced Prairie Pothole DOM with goethite, a phenomenon not seen in oxidized or reduced SRNOM. This observation suggests that reduced CHOS-rich DOM enhanced the reductive dissoln. of goethite, which may play an important role in the cycling and bioavailability of iron in systems where sulfur is important. This work illustrates the potential importance of redox active, high sulfur DOM in aquatic biogeochem. cycles.
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  22. 22
    Trofimova, A.; Hems, R. F.; Liu, T.; Abbatt, J. P. D.; Schnitzler, E. G. Contribution of Charge-Transfer Complexes to Absorptivity of Primary Brown Carbon Aerosol. ACS Earth Space Chem. 2019, 3 (8), 1393– 1401,  DOI: 10.1021/acsearthspacechem.9b00116
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    22
    Contribution of Charge-Transfer Complexes to Absorptivity of Primary Brown Carbon Aerosol
    Trofimova, Alina; Hems, Rachel F.; Liu, Tengyu; Abbatt, Jonathan P. D.; Schnitzler, Elijah G.
    ACS Earth and Space Chemistry (2019), 3 (8), 1393-1401CODEN: AESCCQ; ISSN:2472-3452. (American Chemical Society)
    Light-absorbing org. aerosol, or brown carbon (BrC), has significant but poorly constrained effects on climate. A large fraction of the absorptivity of ambient BrC is unassigned, and org. charge-transfer (CT) complexes have the potential to contribute to this fraction. Here, the contributions of CT complexes to the absorptivity of lab.-generated BrC and ambient aerosol material influenced by biomass burning have been investigated, using a wide range of chem., spectroscopic, and phys. analyses. Chem. functionalization expts. are inconclusive about the role of CT complexes, whereas fluorescence spectra exhibit distinct spectral features indicative of individual chromophores. Detns. of the concn. and temp. dependences of absorbance are more conclusive. In particular, for lab.-generated BrC extd. in either water or methanol, absorbance scaled linearly with orders-of-magnitude changes in concn., indicating that intermol. complexes do not contribute to the absorptivity. Furthermore, whereas the absorbance of BrC exts. in DMSO exhibited a slight temp. dependence, consistent with a 15% contribution from intramol. CT complexes at 15 °C, the complete temp. independence of absorbance of water-sol. exts. from surrogate and ambient BrC indicates a negligible role for CT complexes. Overall, our results find little evidence for CT complexes in the primary BrC studied, suggesting that they do not contribute significantly to the missing absorptivity of ambient BrC.
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  23. 23
    Zhang, C.; Mo, S.; Liu, Z.; Chen, B.; Korshin, G.; Hertkorn, N.; Ni, J.; Yan, M. Interpreting pH-Dependent Differential UV/VIS Absorbance Spectra to Characterize Carboxylic and Phenolic Chromophores in Natural Organic Matter. Water Res. 2023, 244, 120522  DOI: 10.1016/j.watres.2023.120522
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    Allen, A.; Cheng, K.; Mckay, G. Evaluating the pH-dependence of DOM absorbance, fluorescence, and photochemical production of singlet oxygen. Environ. Sci.:Processes Impacts 2023, 25 (12), 1974– 1985,  DOI: 10.1039/D3EM00316G
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    Leenheer, J. A.; Wilson, M. A.; Malcolm, R. L. Presence and potential significance of aromatic-ketone groups in aquatic humic substances. Org. Geochem. 1987, 11 (4), 273– 280,  DOI: 10.1016/0146-6380(87)90038-6
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    Presence and potential significance of aromatic-ketone groups in aquatic humic substances
    Leenheer, J. A.; Wilson, M. A.; Malcolm, R. L.
    Organic Geochemistry (1987), 11 (4), 273-80CODEN: ORGEDE; ISSN:0146-6380.
    Aquatic humic- and fulvic-acid stds. of the International Humic Substances Society were characterized, with emphasis on carbonyl-group nature and content, by 13C NMR spectroscopy, proton NMR spectroscopy, and IR spectroscopy. After comparing spectral results of underivatized humic and fulvic acids with spectral results of chem. modified derivs. that allow improved observation of the carbonyl group, the data clearly indicated that arom. ketone groups comprised the majority of the carbonyl-group content. About 1 ketone group per monocyclic arom. ring was detd. for both humic and fulvic acids. Arom. ketone groups were hypothesized to form by photolytic rearrangements and oxidn. of phenolic ester and hydrocarbon precursors; these groups have potential significance regarding haloform formation in water, reactivity resulting from active H of the Me and methylene adjacent to the ketone groups, and formation of hemiketal and lactol structures. Arom. ketone groups also may be the point of attachment between aliph. and arom. moieties of aquatic humic-substance structure.
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  26. 26
    Tinnacher, R. M.; Honeyman, B. D. A New Method to Radiolabel Natural Organic Matter by Chemical Reduction with Tritiated Sodium Borohydride. Environ. Sci. Technol. 2007, 41 (19), 6776– 6782,  DOI: 10.1021/es070563b
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    26
    A New Method to Radiolabel Natural Organic Matter by Chemical Reduction with Tritiated Sodium Borohydride
    Tinnacher, Ruth M.; Honeyman, Bruce D.
    Environmental Science & Technology (2007), 41 (19), 6776-6782CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    In this paper, we describe a new method for labeling NOM with the radioisotope tritium (3H) using fulvic acid (FA) as the target NOM fraction. During labeling, FA ketone groups are chem. reduced with tritiated sodium borohydride (NaBH4), while the chem. functionality of the carboxyl and phenol groups is preserved. The labeling procedure was optimized in efficiency expts. that detd. the excess concn. of tritiated NaBH4 required for optimum redn. conditions. The chem. characterization of the labeled FA product using FTIR and 1H NMR spectral anal. confirms the proposed reaction mechanism and rules out any significant amts. of impurities or undesirable side reactions. Results from size exclusion chromatog. indicate that the tritium label is distributed uniformly over the whole mol. size range of FA and that it is stable over time and under various pH conditions. Potential differences in FA sorption behavior onto mineral surfaces due to labeling were excluded based on exptl. data. This method produces NOM of high specific activity (e.g., 1.9 mCi mg-1 FA); this permits the tracing of FA at a detection limit of 0.3 μg L-1 FA.
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  27. 27
    Ma, J.; Del Vecchio, R.; Golanoski, K. S.; Boyle, E. S.; Blough, N. V. Optical properties of humic substances and CDOM: Effects of borohydride reduction. Environ. Sci. Technol. 2010, 44 (14), 5395– 5402,  DOI: 10.1021/es100880q
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    27
    Optical Properties of Humic Substances and CDOM: Effects of Borohydride Reduction
    Ma, Jiahai; Del Vecchio, Rossana; Golanoski, Kelli S.; Boyle, Erin S.; Blough, Neil V.
    Environmental Science & Technology (2010), 44 (14), 5395-5402CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Treatment of Suwanee River humic (SRHA) and fulvic (SRFA) acids, a com. lignin (LAC), and a series of solid phase exts. (C18) from the Middle Atlantic Bight (MAB exts.) with NaBH4, a selective reductant of carbonyl-contg. compds. including quinones and arom. ketones, produces a preferential loss of visible absorption (≥50% for SRFA) and substantially enhanced, blue-shifted fluorescence emission (2- to 3-fold increase). Comparison of the results with those obtained from a series of model quinones and hydroquinones demonstrates that these spectral changes cannot be assigned directly to the absorption and emission of visible light by quinones/hydroquinones. Instead, these results are consistent with a charge transfer model in which the visible absorption is due primarily to charge transfer transitions arising among hydroxy- (methoxy-) arom. donors and carbonyl-contg. acceptors. Unlike most of the model hydroquinones, the changes in optical properties of the natural samples following NaBH4 redn. were largely irreversible in the presence of air and following addn. of a Cu2+ catalyst, providing tentative evidence that arom. ketones (or other similar carbonyl-contg. structures) may play a more important role than quinones in the optical properties of these materials.
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  28. 28
    Li, H.; McKay, G. Relationships between the Physicochemical Properties of Dissolved Organic Matter and Its Reaction with Sodium Borohydride. Environ. Sci. Technol. 2021, 55 (15), 10843– 10851,  DOI: 10.1021/acs.est.1c01973
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    28
    Relationships between the Physicochemical Properties of Dissolved Organic Matter and Its Reaction with Sodium Borohydride
    Li, Hang; McKay, Garrett
    Environmental Science & Technology (2021), 55 (15), 10843-10851CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The reaction of dissolved org. matter (DOM) with sodium borohydride has been used to understand the geog. origin of DOM and investigate the photophys. model underlying DOM's optical properties. However, the physicochem. properties of DOM (e.g., mol. size and charge) that influence the kinetics and ultimate reducibility of DOM by borohydride remain poorly characterized. Herein, we studied the kinetics of DOM-borohydride reactions by recording absorbance and fluorescence spectra at a high temporal frequency (every ~ 10 min for 24 h) for a diverse set of DOM isolates of aquatic and soil origin. The reducibility of DOM by sodium borohydride (as judged by relative removal of initial absorbance) varied appreciably among the DOM samples studied, with soil humic substances being less reducible than aquatic humic substances and natural org. matter. While statistically significant correlations were found between the reducibility of DOM and descriptors of mol. size, these descriptors were not able to differentiate the reducibility of soil vs. aquatic DOM isolates that had similar bulk properties. Thus, it appears that the extent of absorbance removal by borohydride is largely driven by the origin of the humic substance isolate (aquatic vs soil) instead of mol. size or charge. Borohydride redn. resulted in increased fluorescence emission across UV and visible excitation wavelengths. However, the enhanced emission at visible excitation decreased over a time period of hours to days, suggesting that redn. of an important subset of DOM chromophores is reversible. This reversibility in fluorescence emission is consistent with the small role of quinones in the absorbance of DOM but suggests a more important role for quinone-contg. charge-transfer contacts in the fluorescence of DOM, particularly at visible excitation wavelengths.
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  29. 29
    Cartisano, C. M.; Del Vecchio, R.; Bianca, M. R.; Blough, N. V. Investigating the sources and structure of chromophoric dissolved organic matter (CDOM) in the North Pacific Ocean (NPO) utilizing optical spectroscopy combined with solid phase extraction and borohydride reduction. Mar. Chem. 2018, 204, 20– 35,  DOI: 10.1016/j.marchem.2018.05.005
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    29
    Investigating the sources and structure of chromophoric dissolved organic matter (CDOM) in the North Pacific Ocean (NPO) utilizing optical spectroscopy combined with solid phase extraction and borohydride reduction
    Cartisano, Carmen M.; Del Vecchio, Rossana; Bianca, Marla R.; Blough, Neil V.
    Marine Chemistry (2018), 204 (), 20-35CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
    Prior optical measurements of waters in the Equatorial Atlantic Ocean (EAO) provided evidence of a major terrestrial "humic-like" component of the CDOM that absorbed in the UV and visible and emitted across the visible, along with a marine component that primarily absorbed and emitted in the UV. Here we extend these measurements to the North Pacific Ocean (NPO) at Station Aloha. Detailed optical measurements of both the natural waters (CDOM) and C18 org. matter exts. of these waters (C18-OM) were acquired before and after sodium borohydride (NaBH4) redn. of samples obtained throughout the water column. Optical properties of the "humic-like" component were relatively uniform with depth below ∼600 m [aCDOM(350) ∼0.08 (m-1), a*CDOM (350) ∼ 0.2 (m-1 mg-1C L), SUVA254 ∼ 0.55 (m-1 mg-1C L),E2:E3 ∼ 10, S300-700 ∼ 0.02 (nm-1), S350-400 ∼ 0.012 (nm-1), SR ∼1.7, F(350/450) ∼ 0.009 (QSE), and .vphi.360 ∼0.026], but were significantly different in surface waters, likely due to photobleaching and biol. activity [aCDOM(350) ∼ 0.026 (m-1), a*CDOM (350) ∼ 0.027 (m-1 mg-1C L), SUVA254 ∼ 0.36 (m-1 mg-1C L), E2:E3 ∼45, S300-700 ∼ 0.03 (nm-1), S350-400 ∼ 0.003 (nm-1), SR ∼6.8, F(350/450) ∼ 0.003 (QSE), and .vphi.350 ∼ 0.024]. Optical properties of the short-wavelength components (UV bands) were more variable with depth. Response to solid phase extn. was also relatively uniform with depth, with preferential extn. of the long-wavelength absorbing/emitting "humic-like" component and virtually no extn. of the the UV absorbing/emitting bands. Response to NaBH4 redn. was also similar down the water column with preferential loss of absorption in the visible region, and enhanced, blue-shifted fluorescence emission. As in the EAO the 'humic-like" component exhibited very similar, although not identical, optical and chem. properties to those obsd. for terrestrially-dominated estuarine and coastal environments, providing evidence that this component originates from a terrestrial source.
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  30. 30
    Andrew, A. A.; Del Vecchio, R.; Subramaniam, A.; Blough, N. V. Chromophoric dissolved organic matter (CDOM) in the Equatorial Atlantic Ocean: Optical properties and their relation to CDOM structure and source. Mar. Chem. 2013, 148 (C), 33– 43,  DOI: 10.1016/j.marchem.2012.11.001
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    30
    Chromophoric dissolved organic matter (CDOM) in the Equatorial Atlantic Ocean: Optical properties and their relation to CDOM structure and source
    Andrew, Andrea A.; Del Vecchio, Rossana; Subramaniam, Ajit; Blough, Neil V.
    Marine Chemistry (2013), 148 (), 33-43CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
    Extensive data exist on the optical properties of CDOM from terrestrial and coastal environments, yet the open oceans have been historically under-sampled. The source and structural basis of marine CDOM optical properties are still debated. Detailed optical measurements were acquired for both untreated and sodium borohydride (NaBH4) reduced natural waters and C18 exts. (C18-OM) across the Equatorial Atlantic Ocean. Except in regions of upwelling or in the vicinity of the Congo River outflow, CDOM absorption coeffs. and visible emission intensity were far smaller for surface waters (aCDOM(355): 0.057-0.162 m-1; λex/λem = 350/450 nm: 0.396-1.431 qse) than for waters below the mixed layer (aCDOM(355) 0.084-0.344 m-1; λex/λem = 350/450 nm: 0.903-3.226 qse), while spectral slopes were higher (surface: 0.019 to 0.025 nm-1; deep: 0.013 to 0.019 nm-1), consistent with photobleaching of CDOM in surface waters. Distinct emission bands were obsd. in the UV, primarily at excitation/emission wavelengths (λex/λem) = 280/320 nm, but also at λex/λem = 300/340, 300/405 and 320/380 nm for some stations and depths. In contrast, visible emission exhibited maxima that continuously red shifted with increasing λex (> 330 nm), a property characteristic of CDOM from estuarine and coastal environments. Further evidence that CDOM in the offshore waters of this region is composed of a major terrestrial component includes: (1) similar spectral dependencies of the emission maxima and fluorescence quantum yields; (2) a large Stokes shift in the emission maxima with short-wavelength excitation (λex = 280 nm); (3) correlation of visible emission intensities with absorption at λex = 280, 320 and 450 nm, with absorption to fluorescence ratios comparable to those found in estuarine and coastal environments; (4) affinity of C18 cartridges for the long wavelength (visible) absorbing and emitting material, but not the UV emitting material; (5) preferential loss of visible absorption and substantially enhanced blue-shifted emission in the visible following borohydride redn. of both the Equatorial Atlantic waters and the C18-OM of these waters. These results support the occurrence in offshore waters of a major terrestrial CDOM component that absorbs in the UV and visible and emits in the visible, as well as marine CDOM components that absorb and emit in the UV. The results further demonstrate that the simultaneous acquisition of complete spectral absorption and emission properties, combined with chem. tests (C-18 extns., borohydride redn.) can provide a far clearer picture of the sources and cycling of CDOM within the oceans.
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  31. 31
    Osburn, C. L.; Kinsey, J. D.; Bianchi, T. S.; Shields, M. R. Formation of planktonic chromophoric dissolved organic matter in the ocean. Mar. Chem. 2019, 209, 1– 13,  DOI: 10.1016/j.marchem.2018.11.010
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    31
    Formation of planktonic chromophoric dissolved organic matter in the ocean
    Osburn, Christopher L.; Kinsey, Joanna D.; Bianchi, Thomas S.; Shields, Michael R.
    Marine Chemistry (2019), 209 (), 1-13CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
    Chromophoric dissolved org. matter (CDOM) is an important fraction of the marine carbon cycle that controls most light absorption and many photochem. and biol. processes in the ocean. Despite its importance, the chem. basis for the formation of oceanic CDOM remains unclear. Currently, CDOM's optical properties are best-described by an electronic interaction (EI) model of charge transfer (CT) complexes which form between electron-rich donors and electron-poor acceptors. While terrigenous compds. such as lignin best fit this model, planktonic sources of CDOM have not yet been tested. Here, we have tested CDOM formed during an incubation expt. using a natural phytoplankton assemblage and throughout active growth, stationary phase and algal biomass decompn. Absorbance of the derived planktonic CDOM generally decreased with increasing wavelength, similar to the ref. Pony Lake (PLFA) and Suwanee River (SRFA) fulvic acid solns. used as models of terrigenous CDOM. Further, after 60 d of microbial degrdn. in the dark, CDOM exhibited fluorescence emission maxima continuously red-shifted into the visible band, consistent with PLFA and SRFA. Redn. of carbonyl-contg. groups, key to CT complex formation, with sodium borohydride (NaBH4) produced coherent results in planktonic CDOM and ref. FAs. Absorption at 350 nm decreased by 50% for planktonic CDOM and by 30% for PLFA and SRFA, with corresponding increases in spectral slope (S) values, indicating preferential loss of absorption well into the visible. Fluorescence likewise responded with enhanced emission at shorter wavelengths. Apparent quantum yields (Φ) were similarly affected. Results from our work support prior observations that phytoplankton and bacteria are important sources of CDOM that color the ocean's "twilight zone". We hypothesize that microbial processing of a variety of source substrates into more complex compds. represented as planktonic CDOM likely represents a semi-refractory pool of DOM in the ocean.
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    Phillips, S. M.; Smith, G. D. Light Absorption by Charge Transfer Complexes in Brown Carbon Aerosols. Environ. Sci. Technol. Lett. 2014, 1 (10), 382– 386,  DOI: 10.1021/ez500263j
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    Light absorption by charge transfer complexes in brown carbon aerosols
    Phillips, Sabrina M.; Smith, Geoffrey D.
    Environmental Science & Technology Letters (2014), 1 (10), 382-386CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
    Recently, it has become apparent that a fraction of the org. species in ambient aerosols absorbs UV-visible light with a potential impact on climate. It is believed that this light-absorbing, sometimes called "brown", carbon originates from biomass and biofuel burning and could be formed through secondary processes in particles or clouds. Here, we identify for the first time charge transfer (CT) complexes as a significant source of light absorption by org. compds. in aerosols. A dense manifold of these complexes, formed from interactions between alc. and carbonyl moieties, accounts for approx. 50% of the absorption (300-600 nm) obsd. for water-extd. ambient particulate matter collected in Athens, GA. Corresponding fluorescence emission spectra with broad, overlapping long-wavelength tails are consistent with efficient energy transfer among a near continuum of such coupled excited states. We postulate that a wide variety of CT complexes are formed as a result of a supramol. assocn. of self-assembling, smaller mols., just as they are in natural humic substances, and that this gives rise to absorption that extends to red wavelengths. These findings imply that light absorption by org. aerosols is governed by a combination of independent as well as interacting chromophores and that both must be included in an accurate representation of aerosol optical properties.
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    Bianca, M. R.; Baluha, D. R.; Gonsior, M.; Schmitt-Kopplin, P.; Del Vecchio, R.; Blough, N. V. Contribution of ketone/aldehyde-containing compounds to the composition and optical properties of Suwannee River fulvic acid revealed by ultrahigh resolution mass spectrometry and deuterium labeling. Anal. Bioanal. Chem. 2020, 412 (6), 1441– 1451,  DOI: 10.1007/s00216-019-02377-x
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    Contribution of ketone/aldehyde-containing compounds to the composition and optical properties of Suwannee River fulvic acid revealed by ultrahigh resolution mass spectrometry and deuterium labeling
    Bianca, Marla R.; Baluha, Daniel R.; Gonsior, Michael; Schmitt-Kopplin, Philippe; Del Vecchio, Rossana; Blough, Neil V.
    Analytical and Bioanalytical Chemistry (2020), 412 (6), 1441-1451CODEN: ABCNBP; ISSN:1618-2642. (Springer)
    Abstr.: A prior method of mass labeling ketone-/aldehyde-contg. species in natural dissolved org. matter (DOM) is further developed and applied. This application involved the treatment of Suwannee River fulvic acid (SRFA) with increasing concns. of sodium borodeuteride (NaBD4), followed by detection of reduced species via neg. mode electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FTICR MS). The extent of redn., as detd. by ESI FTICR MS, resulting from increasing concns. of NaBD4 correlated well with changes in the absorption and emission spectra of the corresponding untreated and borodeuteride-reduced samples, providing evidence that ketone/aldehyde functional groups contribute substantially to the bulk optical properties of SRFA. Furthermore, the differences in the reactivity and abundance of ketone-/aldehyde-contg. species for various regions in Van Krevelen plots were revealed, thus showing how this mass labeling method can be used to provide more detailed structural information about components within complex DOM samples than that provided by the detn. and anal. of mol. formulas alone. Graphical abstr. [graphic not available: see fulltext].
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    Zafiriou, O. C.; Xie, H. X.; Kieber, D. J.; Wang, W.; Song, G. S.; Cohen, N. Cyanohydrin Equilibria Implicate Non-Aromatic Aldehydes in Photochemical Production of Oceanic Carbon Monoxide. Environ. Sci. Technol. 2024, 58 (36), 16066– 16075,  DOI: 10.1021/acs.est.4c04637
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    McNeill, K.; Canonica, S. Triplet state dissolved organic matter in aquatic photochemistry: reaction mechanisms, substrate scope, and photophysical properties. Environ. Sci.:Processes Impacts 2016, 18 (11), 1381– 1399,  DOI: 10.1039/C6EM00408C
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    Reaction mechanisms, substrate scope, and photophysical properties of triplet state dissolved organic matter in aquatic photochemistry
    McNeill, Kristopher; Canonica, Silvio
    Environmental Science: Processes & Impacts (2016), 18 (11), 1381-1399CODEN: ESPICZ; ISSN:2050-7895. (Royal Society of Chemistry)
    A review. Excited triplet states of chromophoric dissolved org. matter (3CDOM) play a major role among the reactive intermediates produced upon absorption of sunlight by surface waters. After more than two decades of research on the aquatic photochem. of 3CDOM, the need for improving the knowledge about the photophys. and photochem. properties of these elusive reactive species remains considerable. This crit. review examines the efforts to date to characterize 3CDOM. Information on 3CDOM relies mainly on the use of probe compds. because of the difficulties assocd. with directly observing 3CDOM using transient spectroscopic methods. Singlet mol. oxygen (1O2), which is a product of the reaction between 3CDOM and dissolved oxygen, is probably the simplest indicator that can be used to est. steady-state concns. of 3CDOM. There are two major modes of reaction of 3CDOM with substrates, namely triplet energy transfer or oxidn. (via electron transfer, proton-coupled electron transfer or related mechanisms). Org. mols., including several environmental contaminants, that are susceptible to degrdn. by these two different reaction modes are reviewed. It is proposed that through the use of appropriate sets of probe compds. and model photosensitizers an improved estn. of the distribution of triplet energies and one-electron redn. potentials of 3CDOM can be achieved.
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    Golanoski, K. S.; Fang, S.; Del Vecchio, R.; Blough, N. V. Investigating the Mechanism of Phenol Photooxidation by Humic Substances. Environ. Sci. Technol. 2012, 46 (7), 3912– 3920,  DOI: 10.1021/es300142y
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    Investigating the Mechanism of Phenol Photooxidation by Humic Substances
    Golanoski, Kelli S.; Fang, Shuo; Del Vecchio, Rossana; Blough, Neil V.
    Environmental Science & Technology (2012), 46 (7), 3912-3920CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    To probe the mechanism of the photosensitized loss of phenols by humic substances (HS), the dependence of the initial rate of 2,4,6-trimethylphenol (TMP) loss (RTMP) on dioxygen concn. was examd. both for a variety of untreated as well as borohydride-reduced HS and C18 exts. from the Delaware Bay and Mid-Atlantic Bight. RTMP was inversely proportional to dioxygen concn. at [O2] >50μM, a dependence consistent with reaction with triplet excited states, but not with 1O2 or RO2. Modeling the dependence of RTMP on [O2] provided rate consts. for TMP reaction, O2 quenching, and lifetimes compatible with a triplet intermediate. Borohydride redn. significantly reduced TMP loss, supporting the role of arom. ketone triplets in this process. However, for most samples, the incomplete loss of sensitization following borohydride redn., as well as the inverse dependence of RTMP on [O2] for these samples, suggests that there remains another class of oxidizing triplet sensitizer, perhaps quinones.
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    Del Vecchio, R.; Blough, N. V. On the Origin of the Optical Properties of Humic Substances. Environ. Sci. Technol. 2004, 38 (14), 3885– 3891,  DOI: 10.1021/es049912h
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    On the Origin of the Optical Properties of Humic Substances
    Del Vecchio, Rossana; Blough, Neil V.
    Environmental Science and Technology (2004), 38 (14), 3885-3891CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Absorption and fluorescence spectroscopy and laser photobleaching expts. were employed to probe the origins of the optical properties of humic substances (HSs). Luminescence quantum yields and the wavelengths of max. emission were acquired for Suwannee River humic acid (SRHA) and fulvic acid (SRFA) at an extensive series of excitation wavelengths across the UV and visible. Laser irradn. at a series wavelength across the UV and visible range was further employed to destroy selectively chromophores absorbing at specific wavelengths, using absorption spectroscopy to follow the absorption losses with irradn. time. The results provide unequivocal evidence that the absorption and emission spectra of these materials cannot result solely from a simple linear superposition of the spectra of numerous independent chromophores. Instead, the long wavelength absorption tail of HSs (>350 nm) appears to arise from a continuum of coupled states. We propose that this behavior results from intramol. charge-transfer interactions between hydroxy-arom. donors and quinoid acceptors formed by the partial oxidn. of lignin precursors. We further propose that these donor-acceptor interactions may be a common phenomenon, occurring within all natural hydroxy- or polyhydroxy-arom. polymers that form appropriate acceptors upon partial oxidn. Examples of such species include lignin, polyphenols, tannins, and melanins.
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    Sharpless, C. M.; Blough, N. V. The importance of charge-transfer interactions in determining chromophoric dissolved organic matter (CDOM) optical and photochemical properties. Environ. Sci.:Processes Impacts 2014, 16 (4), 654– 671,  DOI: 10.1039/C3EM00573A
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    The importance of charge-transfer interactions in determining chromophoric dissolved organic matter (CDOM) optical and photochemical properties
    Sharpless, Charles M.; Blough, Neil V.
    Environmental Science: Processes & Impacts (2014), 16 (4), 654-671CODEN: ESPICZ; ISSN:2050-7895. (Royal Society of Chemistry)
    A review. Absorption of sunlight by chromophoric dissolved natural org. matter (CDOM) is environmentally significant because it controls photic zone depth and causes photochem. that affects elemental cycling and contaminant fate. Both the optics (absorbance and fluorescence) and photochem. of CDOM display unusual properties that cannot easily be ascribed to a superposition of individual chromophores. These include (i) broad, unstructured absorbance that decreases monotonically well into the visible and near IR, (ii) fluorescence emission spectra that all fall into a single envelope regardless of the excitation wavelength, and (iii) photobleaching and photochem. quantum yields that decrease monotonically with increasing wavelength. In contrast to a simple superposition model, these phenomena and others can be reasonably well explained by a phys. model in which charge-transfer interactions between electron donating and accepting chromophores within the CDOM control the optical and photophys. properties. This review summarizes current understanding of the processes underlying CDOM photophysics and photochem. as well as their phys. basis.
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    McKay, G. Emerging investigator series: critical review of photophysical models for the optical and photochemical properties of dissolved organic matter. Environ. Sci.:Processes Impacts 2020, 22, 1139– 1165,  DOI: 10.1039/D0EM00056F
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    Emerging investigator series: critical review of photophysical models for the optical and photochemical properties of dissolved organic matter
    McKay, Garrett
    Environmental Science: Processes & Impacts (2020), 22 (5), 1139-1165CODEN: ESPICZ; ISSN:2050-7895. (Royal Society of Chemistry)
    Optical measurements (absorbance and fluorescence) are widely used to track dissolved org. matter (DOM) quantity and quality in natural and engineered systems. Despite many decades of research on the optical properties of DOM, there is a lack of understanding with regards to the underlying photophys. model that is the basis for these optical properties. This review both summarizes advances to date on the photophys. properties of DOM and seeks to critically evaluate the photophys. models for DOM optical properties. Recent studies have refined the quant. understanding of DOM photophys. properties such as excited state lifetimes and energies, rates of different photophys. processes, and quantum yields. Considering fundamental models, more clarity is needed on whether DOM photophys. processes are due to a superposition of non-interacting components (superposition model), or whether a portion of optical signals can be ascribed to electronically interacting moieties, for example in the form of electron donor-acceptor complexes (charge transfer model). Multiple studies over more than two decades have provided evidence for the charge transfer model. Questions have been raised, however, about the broad applicability of the charge transfer model. The charge transfer and superposition model are critically reviewed in light of this current research. Recommendations are given for future studies to help clarify the accuracy of these competing photophys. models.
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    Blough, N. V.; Del Vecchio, R. Comment on The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics. Environ. Sci. Technol. 2018, 52 (9), 5512– 5513,  DOI: 10.1021/acs.est.8b01189
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    Comment on The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics
    Blough, Neil V.; Del Vecchio, Rossana
    Environmental Science & Technology (2018), 52 (9), 5512-5513CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    A polemic. In a recent article by McKay et al. (Environ. Sci. Technol. 2018, 52, 406-414), they argue that charge transfer interactions cannot be important to the optical properties of humic substances and dissolved org. matter because perturbations of solvent temp., viscosity and polarity do not affect the absorbance and fluorescence spectral line shapes. Blough and Del Vecchio challenge why these conclusions may be incorrect.
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    McKay, G.; Korak, J. A.; Erickson, P. R.; Latch, D. E.; McNeill, K.; Rosario-Ortiz, F. L. The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics. Environ. Sci. Technol. 2018, 52 (2), 406– 414,  DOI: 10.1021/acs.est.7b03589
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    The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics
    McKay, Garrett; Korak, Julie A.; Erickson, Paul R.; Latch, Douglas E.; McNeill, Kristopher; Rosario-Ortiz, Fernando L.
    Environmental Science & Technology (2018), 52 (2), 406-414CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The optical properties of dissolved org. matter influence chem. and biol. processes in all aquatic ecosystems. Dissolved org. matter optical properties have been attributed to a charge-transfer model in which donor-acceptor complexes play a primary role. This model was evaluated by measuring the absorbance and fluorescence response of org. matter isolates to changes in solvent temp., viscosity, and polarity, which affect the position and intensity of spectra for known donor-acceptor complexes of org. mols. Absorbance and fluorescence spectral shape were largely unaffected by these changes, indicating that the distribution of absorbing and emitting species was unchanged. Overall, these results call into question the wide applicability of the charge-transfer model for explaining org. matter optical properties and suggest that future research should explore other models for dissolved org. matter photophysics.
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    Del Vecchio, R.; Schendorf, T. M.; Blough, N. V. Contribution of Quinones and Ketones/Aldehydes to the Optical Properties of Humic Substances (HS) and Chromophoric Dissolved Organic Matter (CDOM). Environ. Sci. Technol. 2017, 51 (23), 13624– 13632,  DOI: 10.1021/acs.est.7b04172
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    Contribution of Quinones and Ketones/Aldehydes to the Optical Properties of Humic Substances (HS) and Chromophoric Dissolved Organic Matter (CDOM)
    Del Vecchio, Rossana; Schendorf, Tara Marie; Blough, Neil V.
    Environmental Science & Technology (2017), 51 (23), 13624-13632CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The mol. basis of the optical properties of chromophoric dissolved org. matter (CDOM) and humic substances (HS) remains poorly understood and yet to be studied adequately. This study evaluates the relative contributions of 2 broad classes of carbonyl-contg. compds., ketones/aldehydes vs. quinones, to the absorption and emission properties of a representative suite of HS as well as a lignin sample. Selective redn. of quinones to hydroquinones by addn. of small molar excesses of dithionite to these samples under anoxic conditions produced small or negligible changes in their optical properties; however, when measurable, these changes were largely reversible upon exposure to air, consistent with the reoxidn. of hydroquinones to quinones. With one exception, ests. of quinone content based on dithionite consumption by the HS under anoxic conditions were in good agreement with past electrochem. measurements. In contrast, redn. of ketones/aldehydes to alcs. using excess Na borohydride produced pronounced and largely, but not completely, irreversible changes in the optical properties. The results demonstrate that (arom.) ketones/aldehydes, as opposed to quinones, play a far more prominent role in the optical absorption and emission properties of these HS, consistent with these moieties acting as the primary acceptors in charge-transfer transitions within these samples. As a method, anoxic dithionite titrns. may further allow addnl. insight into the content and impact of quinones/hydroquinones on the optical properties of HS and CDOM.
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    Piccolo, A.; Conte, P.; Cozzolino, A.; Spaccini, R. Molecular Sizes and Association Forces of Humic Substances in Solution; Clapp, C. E.; Hayes, M. H. B.; Senesi, N.; Bloom, P. R.; Jardine, P. M.; W I, Eds.; Soil Science Society of America: Madison, 2001; pp 89– 118.
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    Piccolo, A. The Supramolecular Structure of Humic Substances. Soil Sci. 2001, 166 (11), 810– 832,  DOI: 10.1097/00010694-200111000-00007
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    The supramolecular structure of humic substances
    Piccolo, Alessandro
    Soil Science (2001), 166 (11), 810-832CODEN: SOSCAK; ISSN:0038-075X. (Lippincott Williams & Wilkins)
    A review. The scientific understanding of the mol. size and shape of humic substances (HS) is critically reviewed. The traditional view that HS are polymers in soil is not substantiated by any direct evidence but is assumed only on the basis of lab. expts. with model mols. and unwarranted results produced by incorrectly applying either anal. procedures or math. treatments developed for purified and undisputed biopolymers. A large body of evidence shows an alternative understanding of the conformational nature of HS, which should be regarded as supramol. assocns. of self-assembling heterogeneous and relatively small mols. deriving from the degrdn. and decompn. of dead biol. material. A major aspect of the humic supramol. conformation is that it is stabilized predominantly by weak dispersive forces instead of covalent linkages. Hydrophobic (van der Waals, π-π, CH-π) and hydrogen bonds are responsible for the apparent large mol. size of HS, the former becoming more important with the increase of pH. This innovative understanding of the nature of HS implies a further development of the science and technol. for the control of the chem. and reactivity of natural org. matter in the soil and the environment.
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    Simpson, A. J. Determining the molecular weight, aggregation, structures and interactions of natural organic matter using diffusion ordered spectroscopy. Magn. Reson. Chem. 2002, 40 (13), S72– S82,  DOI: 10.1002/mrc.1106
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    Vialykh, E. A.; McKay, G.; Rosario-Ortiz, F. L. Computational Assessment of the Three-Dimensional Configuration of Dissolved Organic Matter Chromophores and Influence on Absorption Spectra. Environ. Sci. Technol. 2020, 54 (24), 15904– 15913,  DOI: 10.1021/acs.est.0c05860
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    Vialykh, E. A.; Salahub, D. R.; Achari, G.; Cook, R. L.; Langford, C. H. Emergent functional behaviour of humic substances perceived as complex labile aggregates of small organic molecules and oligomers. Environ. Chem. 2019, 16 (7), 505– 516,  DOI: 10.1071/EN19095
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    Emergent functional behaviour of humic substances perceived as complex labile aggregates of small organic molecules and oligomers
    Vialykh, Elena A.; Salahub, Dennis R.; Achari, Gopal; Cook, Robert L.; Langford, Cooper H.
    Environmental Chemistry (2019), 16 (7), 505-516CODEN: ECNHAA; ISSN:1449-8979. (CSIRO Publishing)
    Environmental contextThe correlation of physicochem. characteristics of humic substances with their function is crucial to our understanding of how environmental pollutants interact with humic substances. We have developed an approach that models emergent functions of fulvic and humic acids depending on sample characteristics. The results will be useful for predicting the sequestration of org. contaminants in soil under various conditions. AbstractThe structural organization of humic substances (HS) has been a central question of earth sciences for several decades. The latest exptl. results have led to the recognition of HS as complex mixts. of small mols. and oligomers. We investigate the correlation between the chem. compn. of HS, perceived as labile aggregates, and the emergent functions. Computational modeling was used to help to understand the processes and mechanisms on the mol. scale that occur in different fractions of the HS, fulvic acids (FA) and humic acids (HA), as they interact with metal ions and org. pollutants. The importance of non-covalent interactions in the emergent functions of HS is highlighted. H-bonding, hydrophilic/hydrophobic surface areas and π-stacking interactions play a significant role in aggregation processes as well as in the sorption of environmental pollutants. In a highly hydrophilic system with small mols. (the SRFA-22 model), H-bonding is the main force that drives the aggregation process. However, in a highly arom. and hydrophobic model with larger mol. fragments (SRHA-6), hydrophobic and π-stacking interactions dominate in the aggregation process. The chem. properties of contaminants significantly affect their mechanisms of sorption by HS. The interaction of a polar pollutant, phenol, with HS occurs through H-bonding, whereas non-polar benzene interacts through hydrophobic and π-stacking interactions. The non-polar pollutant results in a much stronger sorption by HS and causes an addnl. structural rearrangement of the aggregates, which make it more stable in the environment.
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    Hanson, B.; Wünsch, U.; Buckley, S.; Fischer, S.; Leresche, F.; Murphy, K.; D’Andrilli, J.; Rosario-Ortiz, F. L. DOM Molecular Weight Fractionation and Fluorescence Quantum Yield Assessment Using a Coupled In-Line SEC Optical Property System. ACS ES&T Water 2022, 2 (12), 2491– 2501,  DOI: 10.1021/acsestwater.2c00318
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    Pozdnyakov, I. P.; Pigliucci, A.; Tkachenko, N.; Plyusnin, V. F.; Vauthey, E.; Lemmetyinen, H. The photophysics of salicylic acid derivatives in aqueous solution. J. Phys. Org. Chem. 2009, 22 (5), 449– 454,  DOI: 10.1002/poc.1489
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    Murphy, K. R.; Butler, K. D.; Spencer, R. G. M.; Stedmon, C. A.; Boehme, J. R.; Aiken, G. R. Measurement of Dissolved Organic Matter Fluorescence in Aquatic Environments: An Interlaboratory Comparison. Environ. Sci. Technol. 2010, 44 (24), 9405– 9412,  DOI: 10.1021/es102362t
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    Measurement of dissolved organic matter fluorescence in aquatic environments: an interlaboratory comparison
    Murphy, Kathleen R.; Butler, Kenna D.; Spencer, Robert G. M.; Stedmon, Colin A.; Boehme, Jennifer R.; Aiken, George R.
    Environmental Science & Technology (2010), 44 (24), 9405-9412CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The fluorescent properties of dissolved org. matter (DOM) are often studied in order to infer DOM characteristics in aquatic environments, including source, quantity, compn., and behavior. While a potentially powerful technique, a single widely implemented std. method for correcting and presenting fluorescence measurements is lacking, leading to difficulties when comparing data collected by different research groups. This paper reports on a large-scale interlab. comparison in which natural samples and well-characterized fluorophores were analyzed in 20 labs. in the US, Europe, and Australia. Shortcomings were evident in several areas, including data quality-assurance, the accuracy of spectral correction factors used to correct EEMs, and the treatment of optically dense samples. Data cor. by participants according to individual lab. procedures were more variable than when cor. under a std. protocol. Wavelength dependency in measurement precision and accuracy were obsd. within and between instruments, even in cor. data. In an effort to reduce future occurrences of similar problems, algorithms for correcting and calibrating EEMs are described, and MATLAB scripts for implementing the study protocol are provided. Combined with the recent expansion of spectral fluorescence stds., this approach will serve to increase the intercomparability of DOM fluorescence studies.
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    Wünsch, U. J.; Stedmon, C. A.; Tranvik, L. J.; Guillemette, F. Unraveling the size-dependent optical properties of dissolved organic matter. Limnol. Oceanogr. 2018, 63 (2), 588– 601
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    Wünsch, U. J.; Bro, R.; Stedmon, C.; Wenig, P.; Murphy, K. Emerging patterns in the global distribution of dissolved organic matter fluorescence. Anal. Methods 2019, 11, 888– 893,  DOI: 10.1039/C8AY02422G
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    De Haan, H.; Werlemark, G.; De Boer, T. Effect of pH on molecular weight and size of fulvic acids in drainage water from peaty grassland in NW Netherlands. Plant Soil 1983, 75 (1), 63– 73,  DOI: 10.1007/BF02178614
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    Peuravuori, J.; Pihlaja, K. Molecular size distribution and spectroscopic properties of aquatic humic substances. Anal. Chim. Acta 1997, 337, 133– 149,  DOI: 10.1016/S0003-2670(96)00412-6
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    Molecular size distribution and spectroscopic properties of aquatic humic substances
    Peuravuori, Juhani; Pihlaja, Kalevi
    Analytica Chimica Acta (1997), 337 (2), 133-150CODEN: ACACAM; ISSN:0003-2670. (Elsevier)
    The no.- and wt.-averaged mol. wts. of different isolated humic fractions and unfractionated org. matter from natural water samples were measured using high-performance size-exclusion chromatog. (HPSEC). The chromatograms obtained, with sodium acetate as the mobile phase on a TSK G3000SW high-speed column, provided illustrative information about the nature of the dissolved org. matter that was not available with phosphate buffer eluents. The calibration of the column system merely with protein stds. generated misleading information about the mol. wt. and size distribution of humic solutes. Instead, the present results support the utilization of polystyrene-sulfonates (PSS) either alone or together with certain other model compds. for the calibration of HPSEC in studying mol. wts. of humic solutes. The data obtained with the HPSEC system indicate that the av. mol. wt. of humic solutes is distinctly lower than is generally assumed. Besides, aquatic humic solutes also contain some very large-sized though minor constituents, leading to a fairly polydisperse mixt. No.-averaged mol. wts. obtained using the HPSEC system were 4-5-fold greater than those from vapor-pressure osmometry. Bulk spectroscopic properties such as molar absorptivity at 280nm and the quotient, E2/E3, correlated strongly with the total aromaticity and averaged mol. wts. of all the humic solutes. This observation suggests that bulk spectroscopic properties can be applied, as a first approxn., for estg. the size of humic solutes and their aromaticity in natural surface waters.
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    Cory, R. M.; Miller, M. P.; McKnight, D. M.; Guerard, J. J.; Miller, P. L. Effect of instrument-specific response on the analysis of fulvic acid fluorescence spectra. Limnol. Oceanogr.:Methods 2010, 8 (2), 67– 78,  DOI: 10.4319/lom.2010.8.67
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    Effect of instrument-specific response on the analysis of fulvic acid fluorescence spectra
    Cory, Rose M.; Miller, Matthew P.; McKnight, Diane M.; Guerard, Jennifer J.; Miller, Penney L.
    Limnology and Oceanography: Methods (2010), 8 (Feb.), 67-78CODEN: LOMIBY; ISSN:1541-5856. (American Society of Limnology and Oceanography)
    Fluorescence spectroscopy has been extensively employed to characterize the source, age, and reactivity of aquatic dissolved org. matter (DOM). However, there is no consensus on the protocols for collecting and correcting DOM fluorescence spectra for the instrument-specific response assocd. with each component on a fluorometer involved in the excitation of DOM and the resulting detection of DOM emission. The central objective of this study was to evaluate the removal of instrument-specific response from DOM fluorescence spectra collected on three different fluorometers using manufacturer-provided emission and excitation correction files. We evaluated criteria and protocols for comparison of removal of instrument response, using quinine sulfate, a well-characterized fluorophore, as well as the International Humic Substance Society's microbially and terrestrially derived ref. end-member fulvic acids: Pony Lake and Suwannee River fulvic acids, resp. Our results demonstrate that sample spectra collected on different fluorometers differed significantly before correction. Although the effectiveness of manufacturer-provided correction factors in removal of instrument response from sample spectra varied by instrument, spectral overlap of the same sample on multiple instruments improved after correction. Our results suggest that conclusions based on anal. of trends within a dataset of DOM fluorescence spectra should be expected to be independent of the fluorometer employed.
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    McKnight, D. M.; Boyer, E. W.; Westerhoff, P. K.; Doran, P. T.; Kulbe, T.; Andersen, D. T. Spectrofluorometric characterization of dissolved organic matter for indication of precursor organic material and aromaticity. Limnol. Oceanogr. 2001, 46 (1), 38– 48,  DOI: 10.4319/lo.2001.46.1.0038
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    Spectrofluorometric characterization of dissolved organic matter for indication of precursor organic material and aromaticity
    McKnight, Diane M.; Boyer, Elizabeth W.; Westerhoff, Paul K.; Doran, Peter T.; Kulbe, Thomas; Andersen, Dale T.
    Limnology and Oceanography (2001), 46 (1), 38-48CODEN: LIOCAH; ISSN:0024-3590. (American Society of Limnology and Oceanography)
    We studied the fluorescence properties of fulvic acids isolated from streams and rivers receiving predominantly terrestrial sources of org. material and from lakes with microbial sources of org. material. Microbially derived fulvic acids have fluorophores with a more sharply defined emission peak occurring at lower wavelengths than fluorophores in terrestrially derived fulvic acids. We show that the ratio of the emission intensity at a wavelength of 450 nm to that at 500 nm, obtained with an excitation of 370 nm, can serve as a simple index to distinguish sources of isolated aquatic fulvic acids. In our study, this index is ∼1.9 for microbially derived fulvic acids and ∼1.4 for terrestrially derived fulvic acids. Fulvic acids isolated from 4 large rivers in the US have a fluorescence index of 1.4-1.5, consistent with predominantly terrestrial sources. For fulvic acid samples isolated from a river, lakes, and groundwaters in a forested watershed, the fluorescence index varied in a manner suggesting different sources for the seepage and streamfed lakes. We identified these distinctive fluorophores in filtered whole water samples from lakes in a desert oasis in Antarctica and in filtered whole water samples collected during snowmelt from a Rocky Mountain stream. The fluorescence index measurement in filtered whole water samples in field studies may augment the interpretation of dissolved org. C sources for understanding C cycling in aquatic ecosystems.
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    Schmitt-Kopplin, P.; Hertkorn, N.; Garrison, A. W.; Freitag, D.; Kettrup, A. Influence of Borate Buffers on the Electrophoretic Behavior of Humic Substances in Capillary Zone Electrophoresis. Anal. Chem. 1998, 70 (18), 3798– 3808,  DOI: 10.1021/ac971223j
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    Influence of Borate Buffers on the Electrophoretic Behavior of Humic Substances in Capillary Zone Electrophoresis
    Schmitt-Kopplin, Ph.; Hertkorn, N.; Garrison, A. W.; Freitag, D.; Kettrup, A.
    Analytical Chemistry (1998), 70 (18), 3798-3808CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    The influence of tetrahydroxyborate ions on the electrophoretic mobility of humic acids was evaluated by capillary electrophoresis (CE). Depending on the molarity of borate ions in the sepn. buffer, the humic acids exhibit electropherograms with sharp peaks consistently extending from a "humic hump". The variations in the migration times of these peaks depend on the concn. of borate ions in the sepn. buffer. The complexation of borate ions and humic acid fractions was also analyzed with 11B and 1H NMR spectroscopy as well as UV spectrophotometry in solns. of the same compn. as the CE sepn. buffers. Supplementary studies with model compds. (flavonoids, phenolic and sugar acids) indicate reaction mechanisms that include the formation of bidentate esters (monocomplexes) as well as spiranes (tetradentate esters or dicomplexes) within the humic substructure. Special attention must be given to the interpretation of CE electropherograms while fingerprinting humic substances with borate buffers since obsd. peaks do not necessarily indicate distinct humic components but may be artifacts caused by the interaction of the borate ions with the humic substances.
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    McKay, G.; Couch, K. D.; Mezyk, S. P.; Rosario-Ortiz, F. L. Investigation of the coupled effects of molecular weight and charge-transfer interactions on the optical and photochemical properties of dissolved organic matter. Environ. Sci. Technol. 2016, 50 (15), 8093– 8102,  DOI: 10.1021/acs.est.6b02109
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    Investigation of the Coupled Effects of Molecular Weight and Charge-Transfer Interactions on the Optical and Photochemical Properties of Dissolved Organic Matter
    McKay, Garrett; Couch, Kylie D.; Mezyk, Stephen P.; Rosario-Ortiz, Fernando L.
    Environmental Science & Technology (2016), 50 (15), 8093-8102CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    We studied the formation of photochem. produced reactive intermediates (RI) from dissolved org. matter (DOM). Specifically, we focused on the effects of variable mol. wt. and chem. redn. on the optical properties of DOM (absorbance and fluorescence) and the formation of singlet oxygen (1O2), DOM triplet excited states (3DOM*), and the hydroxyl radical (•OH). The data are largely evaluated in terms of a charge-transfer (CT) model, but deficiencies in the model to explain the data are pointed out when evident. A total of two sets of samples were studied that were subjected to different treatments; the first set included secondary-treated wastewaters and a wastewater-impacted stream, and the second was a DOM isolate. Treatments included size fractionation and chem. redn. using sodium borohydride. Taken as a whole, the results demonstrate that decreasing mol. wt. and borohydride redn. work in opposition regarding quantum efficiencies for 1O2 and 3DOM* prodn. but in concert for fluorescence and •OH prodn. The optical and photochem. data provide evidence for a limited role of CT interactions occurring in lower-mol.-wt. DOM mols. In addn., the data suggest that the obsd. optical and photochem. properties of DOM are a result of multiple populations of chromophores and that their relative contribution is changed by mol.-wt. fractionation and borohydride redn.
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    Guéguen, C.; Cuss, C. W. Characterization of aquatic dissolved organic matter by asymmetrical flow field-flow fractionation coupled to UV–Visible diode array and excitation emission matrix fluorescence. J. Chromatogr. A 2011, 1218 (27), 4188– 4198,  DOI: 10.1016/j.chroma.2010.12.038
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    Korak, J. A.; McKay, G. Critical review of fluorescence and absorbance measurements as surrogates for the molecular weight and aromaticity of dissolved organic matter. Environ. Sci. Process Impacts 2024, 26 (10), 1663– 1702,  DOI: 10.1039/D4EM00183D
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    Maizel, A. C.; Remucal, C. K. Molecular Composition and Photochemical Reactivity of Size-Fractionated Dissolved Organic Matter. Environ. Sci. Technol. 2017, 51 (4), 2113– 2123,  DOI: 10.1021/acs.est.6b05140
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    Molecular Composition and Photochemical Reactivity of Size-Fractionated Dissolved Organic Matter
    Maizel, Andrew C.; Remucal, Christina K.
    Environmental Science & Technology (2017), 51 (4), 2113-2123CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The photochem. prodn. of reactive species, such as triplet dissolved org. matter (3DOM) and singlet oxygen (1O2), contributes to the degrdn. of aquatic contaminants and is related to an array of DOM structural characteristics, notably mol. wt. In order to relate DOM mol. wt., optical properties, and reactive species prodn., Suwannee River (SRFA) and Pony Lake fulvic acid (PLFA) isolates are fractionated by sequential ultrafiltration, and the resultant fractions are evaluated in terms of mol. compn. and photochem. reactivity. UV-visible measurements of aromaticity increase with mol. wt. in both fulvic acids, while PLFA mol. wt. fractions are shown to be structurally similar by Fourier-transform ion cyclotron resonance mass spectrometry. In addn., Bray-Curtis dissimilarity anal. of formulas identified in the isolates and their size fractions reveal that SRFA and PLFA have distinct mol. compns. Quantum yields of 3DOM, measured by electron and energy transfer probes, and 1O2 decreased with mol. wt. Decreasing [3DOM]ss with mol. wt. is shown to derive from elevated quenching in high mol. wt. fractions, rather than increased 3DOM formation. This work has implications for the photochem. of waters undergoing natural or engineered treatment processes that alter DOM mol. wt., such as photooxidn. and biol. degrdn.
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    Environmental Science & Technology (2020), 54 (3), 1657-1667CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Fluorescence excitation emission matrixes (EEM) and parallel factor (PARAFAC) anal. have been widely used in the characterization of dissolved org. matter (DOM) in the aquatic continuum. However, large sample sets are typically needed for establishing a meaningful EEM-PARAFAC model. Applications of the EEM-PARAFAC technique to individual samples require new approaches. Here, flow field-flow fractionation (FlFFF) combined with offline EEM measurements and PARAFAC anal. was used to elucidate the dynamic changes in DOM compn./optical properties with mol. wt. within individual samples. FlFFF-derived size spectra of ultrafiltration-isolated colloidal DOM show that peak-C related fluorophores (Ex/Em= 350/450 nm) are present mostly in the 1-3 kDa size range, while peak-T assocd. fluorophores (Ex/Em = 275/340 nm) have a bimodal distribution with peaks in both the 1-3 kDa and the >100 kDa size fractions. The integrated EEM spectra from FlFFF size-fractionated subsamples closely resembled the bulk EEM spectra, attesting to the convincing comparability between bulk and FlFFF size-fractionated EEMs. The PARAFAC-derived DOM components are distinctive among individual samples with the predominant components being humic-like in river water, but protein-like in a highly eutrophic lagoon sample. This compelling new approach combining FlFFF and EEM-PARAFAC can be used to decipher the dynamic changes in size spectra and compn. of individual DOM samples from sources to sinks or across the redox/hydrol./trophic interfaces.
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    Large-scale preparative high-performance size-exclusion chromatog. (HPSEC) was performed to sep. different mol. size fractions in milligram quantities from strongly colored dissolved org. matter (DOM) of a freshwater using a very mild conjugate acid-base pair (10 mM acetic acid-sodium acetate soln. at pH 7.0 with an ionic strength of 6 × 10-5) as the mobile phase. The homogeneity-uniformity of different mol. size fractions in relation to their combined original mixt. was verified by an anal. HPSEC system. In addn. to mol. size distribution and basic spectroscopic characteristics, Fourier transform IR spectroscopy was applied to specify structural features for different size fractions. The results demonstrate clearly that only a very small amt. of conjugate org. acid-base pair is required to generate a powerful resoln. for a DOM mixt., and very strong treatments with org. acids are not necessarily needed to reach a better SEC resoln. Most essential is the combined outcome of different HPSEC expts. and detd. structural functionalities which indicate that almost all original DOM solutes are aggregated mixts. consisting of different assocns. possessing various mol. size ranges, which can be sepd. from their integrated whole as nearly homogeneous and uniform species. In summary, the present study strongly speaks for the need to direct the research of natural dissolved and colloidal org. carbon more strongly toward a nanoscale study of supramol. assemblies. More precise knowledge about the primary, secondary, and tertiary structure of dissolved DOM constituents has its essential function, e.g., for environmental protection and utilization of surface waters.
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    Critical analysis of commonly used fluorescence metrics to characterize dissolved organic matter
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    The use of fluorescence spectroscopy for the anal. and characterization of dissolved org. matter (DOM) has gained widespread interest over the past decade, in part because of its ease of use and ability to provide bulk DOM chem. characteristics. However, the lack of std. approaches for anal. and data evaluation has complicated its use. This study utilized comparative statistics to systematically evaluate commonly used fluorescence metrics for DOM characterization to provide insight into the implications for data anal. and interpretation such as peak picking methods, carbon-normalized metrics and the fluorescence index (FI). The uncertainty assocd. with peak picking methods was evaluated, including the reporting of peak intensity and peak position. The linear relationship between fluorescence intensity and dissolved org. carbon (DOC) concn. was found to deviate from linearity at environmentally relevant concns. and simultaneously across all peak regions. Comparative anal. suggests that the loss of linearity is compn. specific and likely due to non-ideal intermol. interactions of the DOM rather than the inner filter effects. For some DOM sources, Peak A deviated from linearity at optical densities a factor of 2 higher than that of Peak C. For carbon-normalized fluorescence intensities, the error assocd. with DOC measurements significantly decreases the ability to distinguish compositional differences. An in-depth anal. of FI detd. that the metric is mostly driven by peak emission wavelength and less by emission spectra slope. This study also demonstrates that fluorescence intensity follows property balance principles, but the fluorescence index does not.
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    The increased frequency and severity of wildfires in forested watersheds has the potential to significantly impact the quantity and quality of water extractable org. matter (WEOM) exported from these ecosystems. This study examd. the optical properties of WEOM from lab. heated soil in order to understand physicochem. changes occurring in the org. matter as a result of heating, as well as test the usefulness of optical parameters for assessing the presence of pyrogenic org. matter. WEOM absorbance and fluorescence spectral shape and intensity varied systematically as a function of soil heating temp. Notably, absorbance and fluorescence intensity, specific UV absorbance, apparent fluorescence quantum yield, specific fluorescence emission intensity, and max. fluorescence emission wavelength exhibited consistent changes with heating temp. and indicated that WEOM in heated soil leachates was lower in mol. wt. and more arom. than in unheated samples. The lower mol. wt. in heated soil WEOM was corroborated with size-exclusion chromatog. measurements. This work increases the understanding of the mol. changes occurring in WEOM as a result of wildfire and indicates that optical measurements (i.e., absorbance and fluorescence) could be used for watershed monitoring of post-fire pyrogenic org. matter.
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    Fluorescence spectroscopy is one of the most frequently used techniques for studying dissolved org. matter (DOM) in natural and engineered systems. However, the spatial distribution and fluorophores, including local and interacting states, within DOM's larger structure remains poorly understood. In this study, we used two nitroxide fluorescence quenchers to evaluate the chem. and spatial heterogeneity of DOM fluorophores. Several results from quenching expts. with cationic 4-amino-TEMPO (tempamine), including downward-curving Stern-Volmer plots and spectral dependent quenching, show that multiple emitting species contribute to the obsd. emission even at a single excitation wavelength. Furthermore, for DOM isolates of diverse geog. origins (soil vs aquatic) and isolation procedures (reverse osmosis vs humic substances), the max. extent of quenching occurs on the red edge of the emission spectra. For soil humic substance isolates, the spectral dependent quenching was significant enough to affect a blue shift in the av. emission wavelength. The same soil humic substance isolates whose emission spectra were blue shifted by tempamine quenching were also blue shifted by decreasing soln. pH and decreasing solvent polarity, which suggests a role for anionic fluorophores (e.g., hydroxybenzoic acids) in long wavelength fluorescence. Finally, curvature in Stern-Volmer plots indicate that between 10 and 50% of emitting species detected by steady-state fluorescence are inaccessible to quenching by tempamine, suggesting that this fraction of fluorophores may be inaccessible to water solvent. Results from this study provide an assessment of the spatial distribution of fluorophores within DOM and help to reconcile prior studies on the role of solvent polarity and pH on DOM fluorescence.
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    The redox behavior of humic substances (HS) has drawn significant attention. It has been recently reported that aquatic HS after NaBH4 redn. exhibit a loss in UV absorption but enhanced and blue-shifted fluorescence emission. This unique property was proven to also apply to terrestrial HS in the current study. To further understand the underlying relationship between the mol. structural changes and the optical changes, multiple techniques were employed to obtain solid spectroscopic evidence for Aldrich humic acid. Attenuated total reflectance IR spectroscopy, Raman spectroscopy and solid-state 13C NMR spectroscopy were used together to demonstrate that the alc.-like moieties increased while the carbonyl moieties decreased in HS mols. during redn., which is responsible for optical properties. In addn., the obtained results could explain the change in mol. size as evidenced by dynamic light scattering. The integrated results presented in this study unambiguously provide solid spectroscopic evidence for the charge transfer model, which is a relatively new concept framework for understanding HS.
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    Seven samples of com. humic acids, purchased from 5 different suppliers, were studied, and their characteristics were compared with humic and fulvic acids isolated from streams, soils, peat, leonardite, and a dopplerite sample. Cross-polarization and magic-angle spinning 13C NMR spectroscopy clearly showed pronounced differences between the com. materials and all other samples. Elemental and IR spectroscopic data do not show such clear-cut differences but can be used as supportive evidence, with the 13C NMR data, to substantiate the above distinctions. As a result of these differences and due to the general lack of information relating to the source, method of isolation, or other pretreatment of the com. materials, these com. products are not considered to be appropriate for use as analogs of true soil and water humic substances, in expts. designed to evaluate the nature and reactivity of humic substances in natural waters and soils.
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    Relationships between the mol. wt. (MW) and fluorescence properties of dissolved org. matter (DOM) are important considerations for studies seeking to connect these properties to water treatment processes. Relationships between the size and fluorescence properties of nine allochthonous DOM sources (i.e. leaf leachates, grass, and headwaters) were measured using asym. flow field-flow fractionation (AF4) with online absorbance and fluorescence detectors. Correlations between optical properties and MW were readily apparent using parallel factor anal. (PARAFAC) coupled to self-organizing maps (SOM): protein/polyphenol-like fluorescence (peaks B and T) was highest at lower mol. wts. (<0.5 kDa), fulvic/humic-like fluorescence (peaks A, C, and M) was highest at mid-wts. (0.5-1 kDa), and humic-like fluorescence (Peaks A + C) was highest at larger mol. wts. (>1 kDa). Proportions of peaks B, T, and A + C were significantly correlated with MW (p < 0.001). The first principal component (PC1, 42% of variation in fluorescence properties) was a significant predictor of sample MW (R2 = 0.63, p < 0.05), while scores on PC2 (27% of total variance) traced a source-based gradient from deciduous leachates/headwaters through to coniferous leachates/headwaters. PC3 (13% of var.) was also correlated with MW (p < 0.005). A secondary peak in peak T fluorescence was assocd. with larger size fractions in aged sources, and scores on PC1 also traced a path from the leachates of fresher leaves, through more humified leaves, to headwaters. Findings are consistent with the hypothesis that the structure of aged DOM arises through supramol. assembly.
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  • Abstract

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    Scheme 1

    Scheme 1. Influence of Borohydride Reduction on Electron Acceptor Moieties, Including Ketones/aldehydes (A) and Quinones (B). Borohydride Reduction of Carbonyl-containing Acceptors is Hypothesized to Disrupt Charge-Transfer Interactions in DOM (C)
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    Figure 1

    Figure 1. Chromatography of SRFA. From top to bottom: TOC, absorbance at 350 nm, integrated fluorescence emission at 350 nm excitation, and fluorescence quantum yield vs elution volume.

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    Figure 2

    Figure 2. TOC chromatograms of native and reduced DOM isolates normalized to peak intensity, including (A) Suwannee River fulvic acid (SRFA), (B) Suwannee River humic acid (SRHA), (C) Suwannee River natural organic matter (SRNOM), (D) Elliot Soil fulvic acid (ESFA), and (E) Elliot Soil humic acid (ESHA). (F) Tucker Congruence Coefficient (TCC) and size sensitive congruence (SSC) for samples under different conditions. SSC is equal to TCC subtracted by penalization terms that consider differences in peak area and peak position. The replicate injections label evaluates the difference between replicate native samples. TCC ± 95% CI and SSC ± 95% CI indicate 95% confidence intervals from four replicates. The native–reduced label evaluates the impact of NaBH4 reduction. An asterisk indicates that the value falls below the lower limit of the corresponding 95% CI.

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    Figure 3

    Figure 3. Absorbance (A and B) and fluorescence spectra (C and D) of Suwannee River fulvic acid at the peak elution volume (A and C) and bulk samples (B and D). The right y-axis (gray lines) shows the ratio of reduced to native spectra. Spectra in (D) are normalized to water Raman scattering, whereas units in (C) are arbitrary. Emission spectra in (C) and (D) are collected at 350 nm excitation.

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    Figure 4

    Figure 4. Dependence of fractional absorbance (Ared/Anat) remaining on elution volume. Spectra collected at pH 7. NaBH4 reduction carried out for 4 days at 25-mass fold excess NaBH4. Heatmap: 35 to 50 mL elution volume. Ared/Anat of the bulk sample is indicated by a solid black line.

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    Figure 5

    Figure 5. Absorbance properties of DOM isolates as a function of molecular weight and reduction with NaBH4. Logarithm of area-normalized absorbance spectra (log Anorm) of (A–E) native and (F–J) reduced samples, with the heatmap indicating elution volume. The solid and dashed lines in (A–J) represent the absorbance spectra of the native and reduced bulk samples, respectively. (K–O) Intensity-weighted difference in absorbance wavelength (ΔλA) as a function of elution volume. The dashed lines represent ΔλA for the bulk sample. The chromatogram of absorbance at 350 nm excitation is depicted by the gray line (arbitrary y-axis). The sample labels in (A–E) apply to (F–O).

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    Figure 6

    Figure 6. Absorbance chromatograms at multiple wavelengths for all DOM isolates. The gray lines represent the fractional absorbance remaining (Ared/Anat) at the indicated wavelength. Wavelengths indicated apply to all columns in each row, and sample headings in each column apply to all rows.

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    Figure 7

    Figure 7. Fluorescence emission properties of DOM isolates as a function of molecular weight and reduction with NaBH4. Intensity-normalized emission spectra (Fnorm) of (A–E) native and (F–J) reduced samples, with the heatmap indicating elution volume. The solid and dashed lines in (A–J) represent the emission spectra of the native and reduced bulk samples, respectively. (K–O) Intensity-weighted difference in fluorescence emission wavelength (ΔλF) as a function of elution volume. The dashed lines represent ΔλF for the bulk sample. The chromatogram of integrated emission at 350 nm excitation is depicted by the gray line (arbitrary y-axis). The sample labels in (A–E) apply to (F–O).

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    Figure 8

    Figure 8. Dependence of optical surrogates on molecular weight for native and reduced DOM isolates. (A–E) A250/A364 (similar to E2:E3) as a function of elution volume. The chromatogram of absorbance at 250 nm is depicted by the gray line (arbitrary y-axis). (F–J) F470/F520 at 350 nm excitation (similar to fluorescence index, FI). Blue circles and orange squares represent native and reduced, samples, respectively. The normalized chromatogram of integrated emission at 350 nm excitation is depicted by the gray line (arbitrary y-axis). Optical surrogates for bulk native and reduced samples are indicated by + and *, respectively. The dotted black lines (− - – ) correspond to the ratio of surrogates for the native and reduced samples (right y-axis). The sample labels in (A–E) apply to (F–J).

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    2. 2
      Thurman, E. M. Organic Geochemistry of Natural Waters; Kluwer, 1985.
      There is no corresponding record for this reference.
    3. 3
      Lim, S.; Shi, J. L.; von Gunten, U.; McCurry, D. L. Ozonation of organic compounds in water and wastewater: A critical review. Water Res. 2022, 213, 118053  DOI: 10.1016/j.watres.2022.118053
      3
      Ozonation of organic compounds in water and wastewater: A critical review
      Lim, Sungeun; Shi, Jiaming Lily; von Gunten, Urs; McCurry, Daniel L.
      Water Research (2022), 213 (), 118053CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      A review. Ozonation has been applied in water treatment for more than a century, first for disinfection, later for oxidn. of inorg. and org. pollutants. In recent years, ozone has been increasingly applied for enhanced municipal wastewater treatment for ecosystem protection and for potable water reuse. These applications triggered significant research efforts on the abatement efficiency of org. contaminants and the ensuing formation of transformation products. This endeavor was accompanied by developments in anal. and computational chem., which allowed to improve the mechanistic understanding of ozone reactions. This crit. review assesses the challenges of ozonation of impaired water qualities such as wastewaters and provides an up-to-date compilation of the recent kinetic and mechanistic findings of ozone reactions with dissolved org. matter, various functional groups (olefins, arom. compds., heterocyclic compds., aliph. nitrogen-contg. compds., sulfur-contg. compds., hydrocarbons, carbanions, β-diketones) and antibiotic resistance genes.
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    4. 4
      Leenheer, J. A.; Croue, J. P. Characterizing aquatic dissolved organic matter. Environ. Sci. Technol. 2003, 37 (1), 18A– 26A,  DOI: 10.1021/es032333c
      4
      Characterizing aquatic dissolved organic matter
      Leenheer, Jerry A.; Croue, Jean-Philippe
      Environmental Science and Technology (2003), 37 (1), 18A-26ACODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      A review. An overview is given on studies for the characterization of the generally unknown mol. structures of dissolved org. matter (DOM) in freshwater aquatic environments as a key to an improved treatment of drinking water. First a definition of DOM is given and then studies on whole water characterization are described. These cover the anal. of total and dissolved org. carbon (TOC and DOC) as well as of biodegradable org. matter (BOM) in the form of biodegradable dissolved org. carbon (BDOC) or as assimilable org. carbon (AOC) and the DOM profiling using resin sorbents. The use of the spectrophotometric anal., the size characterization of natural org. matter (NOM) by size exclusion chromatog. (SEC) and sequential ultrafiltration, and the application of pyrolysis gas chromatog. mass spectrometry (GC-MS) for the study of DOM is also shown. Further a tiered approach developed for preparative DOM fractionation, isolation, and characterization of the DOM fractions is described. With this method dissolved orgs. can be classified based on their polarity (hydrophobic/hydrophilic), acid/neutral/base properties, compd.-class characteristics, and specific compds. characteristics. The DOM fractions are operationally defined based on a sepn. by selective sorption/desorption on selected resin columns. The detn. of compd. classes and specific compds. can be further performed on the isolated DOM fractions.
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    5. 5
      Stenson, A. C.; Marshall, A. G.; Cooper, W. T. Exact Masses and Chemical Formulas of Individual Suwannee River Fulvic Acids from Ultrahigh Resolution Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectra. Anal. Chem. 2003, 75 (6), 1275– 1284,  DOI: 10.1021/ac026106p
      5
      Exact Masses and Chemical Formulas of Individual Suwannee River Fulvic Acids from Ultrahigh Resolution Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectra
      Stenson, Alexandra C.; Marshall, Alan G.; Cooper, William T.
      Analytical Chemistry (2003), 75 (6), 1275-1284CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Mol. formulas have been assigned for 4626 individual Suwannee River fulvic acids based on accurate mass measurements from ions generated by electrospray ionization and obsd. by ultrahigh-resoln. Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). Formula assignments were possible because of the mass accuracy of FTICR MS at high field (9.4 T) and the regular mass spacing patterns found in fulvic acid mixts. Sorting the 4626 individually obsd. ions according to Kendrick mass defect and nominal mass series (z* score) revealed that all could be assigned to 1 of 266 distinct homologous series that differ in oxygen content and double bond equivalence. Tandem mass spectrometry based on IR multiphoton dissocn. identified labile fragments of fulvic acid mols., whose chem. formulas led to plausible structures consistent with degraded lignin as a source of Suwannee River fulvic acids.
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    6. 6
      Bahureksa, W.; Tfaily, M. M.; Boiteau, R. M.; Young, R. B.; Logan, M. N.; McKenna, A. M.; Borch, T. Soil Organic Matter Characterization by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR MS): A Critical Review of Sample Preparation, Analysis, and Data Interpretation. Environ. Sci. Technol. 2021, 55 (14), 9637– 9656,  DOI: 10.1021/acs.est.1c01135
      6
      Soil organic matter characterization by Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS): A critical review of sample preparation, analysis, and data interpretation
      Bahureksa, William; Tfaily, Malak M.; Boiteau, Rene M.; Young, Robert B.; Logan, Merritt N.; McKenna, Amy M.; Borch, Thomas
      Environmental Science & Technology (2021), 55 (14), 9637-9656CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      A review. The biogeochem. cycling of soil org. matter (SOM) plays a central role in regulating soil health, water quality, carbon storage, and greenhouse gas emissions. Thus, many studies have been conducted to reveal how anthropogenic and climate variables affect carbon sequestration and nutrient cycling. Among the anal. techniques used to better understand the speciation and transformation of SOM, Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) is the only technique that has sufficient mass resolving power to sep. and accurately assign elemental compns. to individual SOM mols. The global increase in the application of FTICR MS to address SOM complexity has highlighted the many challenges and opportunities assocd. with SOM sample prepn., FTICR MS anal., and mass spectral interpretation. Here, we provide a crit. review of recent strategies for SOM characterization by FTICR MS with emphasis on SOM sample collection, prepn., anal., and data interpretation. Data processing and visualization methods are presented with suggested workflows that detail the considerations needed for the application of mol. information derived from FTICR MS. Finally, we highlight current research gaps, biases, and future directions needed to improve our understanding of org. matter chem. and cycling within terrestrial ecosystems.
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    7. 7
      Young, R. B.; Pica, N. E.; Sharifan, H.; Chen, H.; Roth, H. K.; Blakney, G. T.; Borch, T.; Higgins, C. P.; Kornuc, J. J.; McKenna, A. M.; Blotevogel, J. PFAS Analysis with Ultrahigh Resolution 21T FT-ICR MS: Suspect and Nontargeted Screening with Unrivaled Mass Resolving Power and Accuracy. Environ. Sci. Technol. 2022, 56 (4), 2455– 2465,  DOI: 10.1021/acs.est.1c08143
      7
      PFAS Analysis with Ultrahigh Resolution 21T FT-ICR MS: Suspect and Nontargeted Screening with Unrivaled Mass Resolving Power and Accuracy
      Young, Robert B.; Pica, Nasim E.; Sharifan, Hamidreza; Chen, Huan; Roth, Holly K.; Blakney, Greg T.; Borch, Thomas; Higgins, Christopher P.; Kornuc, John J.; McKenna, Amy M.; Blotevogel, Jens
      Environmental Science & Technology (2022), 56 (4), 2455-2465CODEN: ESTHAG; ISSN:1520-5851. (American Chemical Society)
      Per- and polyfluoroalkyl substances (PFASs) are a large family of thousands of chems., many of which have been identified using nontargeted time-of-flight and Orbitrap mass spectrometry methods. Comprehensive characterization of complex PFAS mixts. is crit. to assess their environmental transport, transformation, exposure, and uptake. Because 21 T (T) Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) offers the highest available mass resolving power and sub-ppm mass errors across a wide mol. wt. range, we developed a nontargeted 21 T FT-ICR MS method to screen for PFASs in an aq. film-forming foam (AFFF) using suspect screening, a targeted formula database (C, H, Cl, F, N, O, P, S; ≤865 Da), isotopologues, and Kendrick-analogous mass difference networks (KAMDNs). False-pos. PFAS identifications in a natural org. matter (NOM) sample, which served as the neg. control, suggested that a min. length of 3 should be imposed when annotating CF2-homologous series with pos. mass defects. We putatively identified 163 known PFASs during suspect screening, as well as 134 novel PFASs during nontargeted screening, including a suspected polyethoxylated perfluoroalkane sulfonamide series. This study shows that 21 T FT-ICR MS anal. can provide unique insights into complex PFAS compn. and expand our understanding of PFAS chemistries in impacted matrixes.
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    8. 8
      Roth, H. K.; Borch, T.; Young, R. B.; Bahureksa, W.; Blakney, G. T.; Nelson, A. R.; Wilkins, M. J.; McKenna, A. M. Enhanced Speciation of Pyrogenic Organic Matter from Wildfires Enabled by 21 T FT-ICR Mass Spectrometry. Anal. Chem. 2022, 94 (6), 2973– 2980,  DOI: 10.1021/acs.analchem.1c05018
      8
      Enhanced Speciation of Pyrogenic Organic Matter from Wildfires Enabled by 21 T FT-ICR Mass Spectrometry
      Roth, Holly K.; Borch, Thomas; Young, Robert B.; Bahureksa, William; Blakney, Greg T.; Nelson, Amelia R.; Wilkins, Michael J.; McKenna, Amy M.
      Analytical Chemistry (Washington, DC, United States) (2022), 94 (6), 2973-2980CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Wildfires affect soils through the formation of pyrogenic org. matter (pyOM) (e.g., char and soot). While many studies examine the connection between pyOM persistence and carbon (C) compn., nitrogen (N) transformation in wildfire-impacted systems remains poorly understood. Thermal reactions in wildfires transform biomass into a highly complex, polyfunctional, and polydisperse org. mixt. that challenges most mass analyzers. High-field Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is the only mass analyzer that achieves resolving powers sufficient to sep. species that differ in mass by the mass of an electron across a wide mol. wt. range (m/z 150-1500). We report enhanced speciation of org. N by pos.-ion electrospray ionization (ESI) that leverages ultrahigh resolving power (m/Δm50% = 1 800 000 at m/z 400) and mass accuracy (<10-100 ppb) achieved by FT-ICR MS at 21 T. Isobaric overlaps, roughly the mass of an electron (Me- = 548μDa), are resolved across a wide mol. wt. range and are more prevalent in pos. ESI than neg. ESI. The custom-built 21 T FT-ICR MS instrument identifies previously unresolved mass differences in CcHhNnOoSs formulas and assigns more than 30 000 peaks in a pyOM sample. This is the first mol. catalog of pyOM by pos.-ion ESI 21 T FT-ICR MS and presents a method to provide new insight into terrestrial cycling of org. carbon and nitrogen in wildfire impacted ecosystems.
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    9. 9
      Leyva, D.; Jaffe, R.; Fernandez-Lima, F. Structural Characterization of Dissolved Organic Matter at the Chemical Formula Level Using TIMS-FT-ICR MS/MS. Anal. Chem. 2020, 92 (17), 11960– 11966,  DOI: 10.1021/acs.analchem.0c02347
      9
      Structural characterization of dissolved organic matter at the chemical formula level using TIMS-FT-ICR MS/MS
      Leyva, Dennys; Jaffe, Rudolf; Fernandez-Lima, Francisco
      Analytical Chemistry (Washington, DC, United States) (2020), 92 (17), 11960-11966CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      TIMS-FT-ICR MS is an important alternative to study the isomeric diversity and elemental compn. of complex mixts. While the chem. structure of many compds. in the dissolved org. matter (DOM) remains largely unknown, the high structural diversity has been described at the mol. level using chem. formulas. In this study, we further push the boundaries of TIMS-FT-ICR MS by performing chem. formula-based ion mobility and tandem MS anal. for the structural characterization of DOM. The workflow described is capable to mobility select (R ~ 100) and isolate mol. ion signals (Δm/z = 0.036) in the ICR cell, using single-shot ejections after broadband ejections and MS/MS based on sustained off-resonance irradn. collision-induced dissocn. (SORI-CID). The workflow results are compared to alternative TIMS-q-FT-ICR MS/MS expts. with quadrupole isolation at nominal mass (~ 1 Da). The technol. is demonstrated with isomeric and isobaric mixts. (e.g., 4-methoxy-1-naphthoic acid, 2-methoxy-1-naphthoic acid, decanedioic acid) and applied to the characterization of DOM. The application of this new methodol. to the anal. of a DOM is illustrated by the isolation of the mol. ion [C18H18O10-H]- in the presence of other isobars at nominal mass 393. Five IMS bands were assigned to the heterogeneous ion mobility profile of [C18H18O10-H]-, and candidate structures from the PubChem database were screened based on their ion mobility and the MS/MS matching score. This approach overcomes traditional challenges assocd. with the similarity of fragmentation patterns of DOM samples (e.g., common neutral losses of H2O, CO2, and CH2-H2O) by narrowing down the isomeric candidate structures using the mobility domain.
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    10. 10
      Baluha, D. R.; Blough, N. V.; Del Vecchio, R. Selective Mass Labeling for Linking the Optical Properties of Chromophoric Dissolved Organic Matter to Structure and Composition via Ultrahigh Resolution Electrospray Ionization Mass Spectrometry. Environ. Sci. Technol. 2013, 47 (17), 9891– 9897,  DOI: 10.1021/es402400j
      10
      Selective Mass Labeling for Linking the Optical Properties of Chromophoric Dissolved Organic Matter to Structure and Composition via Ultrahigh Resolution Electrospray Ionization Mass Spectrometry
      Baluha, Daniel R.; Blough, Neil V.; Del Vecchio, Rossana
      Environmental Science & Technology (2013), 47 (17), 9891-9897CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The mass spectra acquired by ESI FT-ICR MS of untreated, borohydride-reduced, and borodeuteride-reduced samples of Suwannee River fulvic acid (SRFA) and a C18 ext. from the upper Delaware Bay were compared to one another. Treatment of these samples with sodium borodeuteride was shown to produce unique mass labels for species which contain one or two ketone/aldehyde moieties. Approx. 30% of all identified peaks in the two samples were shown to comprise ketone/aldehyde-contg. species. The mol. formulas of the majority of these species had O/C and H/C molar ratios typically attributed to lignin-derived compds. and/or carboxylic rich alicyclic mols. (CRAM). However, the significant loss of UV-vis absorption following redn. supports a lignin-based origin for the optical (and photochem.) properties of these samples. The mass-labeling method described and tested herein shows great promise as a means to further characterize the structure and compn. of complex natural samples, esp. in terms of identifying specific subsets of chem. species that contribute significantly to the optical and photochem. properties of such samples.
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    11. 11
      Zherebker, A.; Shirshin, E.; Rubekina, A.; Kharybin, O.; Kononikhin, A.; Kulikova, N. A.; Zaitsev, K. V.; Roznyatovsky, V. A.; Grishin, Y. K.; Perminova, I. V.; Nikolaev, E. N. Optical Properties of Soil Dissolved Organic Matter Are Related to Acidic Functions of Its Components as Revealed by Fractionation, Selective Deuteromethylation, and Ultrahigh Resolution Mass Spectrometry. Environ. Sci. Technol. 2020, 54 (5), 2667– 2677,  DOI: 10.1021/acs.est.9b05298
      11
      Optical Properties of Soil Dissolved Organic Matter Are Related to Acidic Functions of Its Components as Revealed by Fractionation, Selective Deuteromethylation, and Ultrahigh Resolution Mass Spectrometry
      Zherebker, Alexander; Shirshin, Evgeny; Rubekina, Anna; Kharybin, Oleg; Kononikhin, Alexey; Kulikova, Natalia A.; Zaitsev, Kirill V.; Roznyatovsky, Vitaliy A.; Grishin, Yuri K.; Perminova, Irina V.; Nikolaev, Evgeny N.
      Environmental Science & Technology (2020), 54 (5), 2667-2677CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The goal of this study was to establish a relationship between the optical properties of soil dissolved org. matter (DOM) and acidic functions carried out by its individual constituents. We obtained 12 fractions of DOM samples using sequential solid phase extn. on nonionic sorbent at steadily lowered pH values: 7, 5, 3, 2, which correspond to low bounds of pKa values of phenols, aliph., and arom. carboxylic acids, and ketoacids. The structural studies were conducted with the use of NMR and selective deuteromethylation of isolated fractions coupled to ultrahigh resoln. mass spectrometry. First, a gradual shift of mol. compns. was obsd. from reduced components to arom. oxidized compds. isolated at pH 7 and 2, resp. Changes in mol. compns. were accompanied by a red shift of fluorescence spectra. Further application of deuteromethylation enabled us to distinguish DOM constituents with different amts. of carboxylic groups. Moreover, identification of structural isomers in a single DOM sample was achieved. Statistical anal. revealed that red shift of fluorescence is facilitated by the increase of a contribution of arom. poly(carboxylic acid)s with high conjugation lengths. Addnl., anal. of the labeled fractionated permafrost thaw DOM directly showed carboxyl-rich alicyclic mols., while the same components from lower-latitude DOM were assigned to lignin-like species.
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    12. 12
      Schendorf, T. M.; Del Vecchio, R.; Koech, K.; Blough, N. V. A standard protocol for NaBH4 reduction of CDOM and HS. Limnol. Oceanogr.:Methods 2016, 14 (6), 414– 423,  DOI: 10.1002/lom3.10100
      12
      A standard protocol for NaBH4 reduction of CDOM and HS
      Schendorf, Tara Marie; Del Vecchio, Rossana; Koech, Kevin; Blough, Neil V.
      Limnology and Oceanography: Methods (2016), 14 (6), 414-423CODEN: LOMIBY; ISSN:1541-5856. (John Wiley & Sons, Inc.)
      Sodium borohydride (NaBH4) selectively reduces ketones/aldehydes to alcs. and quinones to hydroquinones and has been used to reduce these carbonyl moieties within humic substances (HS) and chromophoric dissolved org. matter (CDOM) to gain insight into their contributions to the optical/photochem. properties of these materials. To further investigate the effects of borohydride redn. on the absorption and emission properties of HS and to develop a std. protocol for this redn., the effects of increasing borohydride concns. were examd. over the course of 96 h. Time courses for the redn. revealed the presence of two kinetically distinguishable pools of carbonyls that were irreversibly reduced. A third pool appears to be reversibly reduced at a low mass excess of NaBH4, but contributes much less to HS absorption. Reduced samples can be adjusted to pH 7 and passed through a size exclusion column (Sephadex G-10) to remove borate salts, thus producing salt-free samples that can be used for further analyses. A 25-fold mass excess of NaBH4 relative to the HS mass produced the largest changes in optical properties without generating excessive amts. of reaction products. Although smaller addnl. changes are obsd. at higher mass excesses, the removal of excess borate becomes substantially more difficult and time consuming. While most changes are complete by 48 h, our results suggest that the reaction should be allowed to proceed for 96 h in the dark at room temp. to ensure that addnl. changes in the optical signals are not obsd.
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    13. 13
      Nebbioso, A.; Piccolo, A. Modification of chemical and conformational properties of natural organic matter by click chemistry as revealed by ESI-Orbitrap mass spectrometry. Anal. Bioanal. Chem. 2015, 407 (28), 8515– 8523,  DOI: 10.1007/s00216-015-9005-7
      There is no corresponding record for this reference.
    14. 14
      Houska, J.; Salhi, E.; Walpen, N.; von Gunten, U. Oxidant-reactive carbonous moieties in dissolved organic matter: Selective quantification by oxidative titration using chlorine dioxide and ozone. Water Res. 2021, 207, 117790  DOI: 10.1016/j.watres.2021.117790
      14
      Oxidant-reactive carbonous moieties in dissolved organic matter: Selective quantification by oxidative titration using chlorine dioxide and ozone
      Houska, Joanna; Salhi, Elisabeth; Walpen, Nicolas; von Gunten, Urs
      Water Research (2021), 207 (), 117790CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      The application of oxidants for disinfection or micropollutant abatement during drinking water and wastewater treatment is accompanied by oxidn. of matrix components such as dissolved org. matter (DOM). To improve predictions of the efficiency of oxidn. processes and the formation of oxidn. products, methods to det. concns. of oxidant-reactive phenolic, olefinic or amine-type DOM moieties are crit. Here, a novel selective oxidative titrn. approach is presented, which is based on reaction kinetics of oxidn. reactions towards certain DOM moieties. Phenolic moieties were detd. by oxidative titrn. with ClO2 and O3 for five DOM isolates and two secondary wastewater effluent samples. The detd. concns. of phenolic moieties correlated with the electron-donating capacity (EDC) and the formation of inorg. ClO2-byproducts (HOCl, ClO-2, ClO-3). ClO2-byproduct yields from phenol and DOM isolates and changes due to the application of mol. tagging for phenols revealed a better understanding of oxidant-reactive structures within DOM. Overall, oxidative titrns. with ClO2 and O3 provide a novel and promising tool to quantify oxidant-reactive moieties in complex mixts. such as DOM and can be expanded to other matrixes or oxidants.
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    15. 15
      Wenk, J.; Aeschbacher, M.; Salhi, E.; Canonica, S.; von Gunten, U.; Sander, M. Chemical oxidation of dissolved organic matter by chlorine dioxide, chlorine, and ozone: effects on its optical and antioxidant properties. Environ. Sci. Technol. 2013, 47 (19), 11147– 11156,  DOI: 10.1021/es402516b
      15
      Chemical Oxidation of Dissolved Organic Matter by Chlorine Dioxide, Chlorine, and Ozone: Effects on Its Optical and Antioxidant Properties
      Wenk, Jannis; Aeschbacher, Michael; Salhi, Elisabeth; Canonica, Silvio; von Gunten, Urs; Sander, Michael
      Environmental Science & Technology (2013), 47 (19), 11147-11156CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      In water treatment dissolved org. matter (DOM) is typically the major sink for chem. oxidants. The resulting changes in DOM, such as its optical properties have been measured to follow the oxidn. processes. However, such measurements contain only limited information on the changes in the oxidn. states of and the reactive moieties in the DOM. We used mediated electrochem. oxidn. to quantify changes in the electron donating capacities (EDCs), and hence the redox states, of 3 different types of DOM during oxidn. with ClO2, Cl (as HOCl/OCl-), and O3. Treatment with ClO2 and HOCl resulted in comparable and prominent decreases in EDCs, while the UV light absorbances of the DOM decreased only slightly. Conversely, ozonization resulted in only small decreases of the EDCs but pronounced absorbance losses of the DOM. These results suggest that ClO2 and HOCl primarily reacted as oxidants by accepting electrons from electron-rich phenolic and hydroquinone moieties in the DOM, while O3 reacted via electrophilic addn. to arom. moieties, followed by ring cleavage. This study highlights the potential of combined EDC-UV measurements to monitor chem. oxidn. of DOM, to assess the nature of the reactive moieties and to study the underlying reaction pathways.
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    16. 16
      Leresche, F.; McKay, G.; Kurtz, T.; von Gunten, U.; Canonica, S.; Rosario-Ortiz, F. L. Effects of ozone on the photochemical and photophysical properties of dissolved organic matter. Environ. Sci. Technol. 2019, 53 (10), 5622– 5632,  DOI: 10.1021/acs.est.8b06410
      16
      Effects of Ozone on the Photochemical and Photophysical Properties of Dissolved Organic Matter
      Leresche, Frank; McKay, Garrett; Kurtz, Tyler; von Gunten, Urs; Canonica, Silvio; Rosario-Ortiz, Fernando L.
      Environmental Science & Technology (2019), 53 (10), 5622-5632CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      This study focused on the effects of ozonization on the photochem. and photophys. properties of dissolved org. matter (DOM). Upon ozonization, a decrease in DOM absorbance was obsd. in parallel with an increase in singlet O (1O2) and fluorescence quantum yields (Φ1O2 and ΦF). The increase in Φ1O2 was attributed to the formation of quinone-like moieties during ozonization of the phenolic moieties of DOM, while the increase in ΦF can be explained by a significant decrease in the internal conversion rate of the 1st excited singlet state of the DOM (1DOM*). It is a consequence of an increase in the av. energy of the 1st electronic transition (S1 → S0) that was assessed using the wavelength of max. fluorescence emission (λF,max). Also, ozonization did not affect the ratio of the apparent steady-state concns. of excited triplet DOM (3DOM*) and 1O2, indicating that ozonization does not affect the efficiency of 1O2 prodn. from 3DOM*. The consequences of these changes for the phototransformation rates of micropollutants in surface waters were examd. using photochem. model calcns. The decrease in DOM absorbance caused by ozonization leads to an enhancement of direct photolysis rates due to the increased transparency of the H2O. Rates of indirect photooxidn. induced by 1O2 and 3DOM* slightly decrease after ozonization.
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    17. 17
      Macalady, D. L.; Walton-Day, K. New light on a dark subject: On the use of fluorescence data to deduce redox states of natural organic matter (NOM). Aquat. Sci. 2009, 71 (2), 135– 143,  DOI: 10.1007/s00027-009-9174-6
      17
      New light on a dark subject: On the use of fluorescence data to deduce redox states of natural organic matter (NOM)
      Macalady, Donald L.; Walton-Day, Katherine
      Aquatic Sciences (2009), 71 (2), 135-143CODEN: AQSCEA; ISSN:1015-1621. (Birkhaeuser Verlag)
      This paper reports the use of excitation-emission matrix fluorescence spectroscopy (EEMS), parallel factor statistical anal. (PARAFAC), and oxidn.-redn. expts. to examine the effect of redox conditions on PARAFAC model results for aq. samples rich in natural org. matter. Fifty-four aq. samples from 11 different geog. locations and two plant exts. were analyzed untreated and after chem. treatments or irradn. were used in attempts to change the redox status of the natural org. matter. The EEMS spectra were generated and modeled using a PARAFAC package developed by Cory and McKnight (2005). The PARAFAC model output was examd. for consistency with previously reported relations and with changes expected to occur upon exptl. oxidn. and redn. of aq. samples. Results indicate the implied fraction of total sample fluorescence attributed to quinone-like moieties was consistent (0.64 to 0.78) and greater than that obsd. by Cory and McKnight (2005). The fraction of the quinone-like moieties that was reduced (the reducing index, RI) showed relatively little variation (0.46 to 0.71) despite attempts to alter the redox status of the natural org. matter. The RI changed little after reducing samples using zinc metal, oxidizing at high pH with air, or irradiating with a Xenon lamp. Our results, however, are consistent with the correlations between the fluorescence indexes (FI) of samples and the ratio of PARAFAC fitting parameters suggested by Cory and McKnight (2005), though we used samples with a much narrower range of FI values.
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    18. 18
      Macalady, D. L.; Walton-Day, K. Redox Chemistry and Natural Organic Matter (NOM): Geochemists’ Dream, Analytical Chemists’ Nightmare; American Chemical Society: Washington, DC, 2011; Vol. 1071, pp 85– 111.
      There is no corresponding record for this reference.
    19. 19
      Maurer, F.; Christl, I.; Kretzschmar, R. Reduction and Reoxidation of Humic Acid: Influence on Spectroscopic Properties and Proton Binding. Environ. Sci. Technol. 2010, 44 (15), 5787– 5792,  DOI: 10.1021/es100594t
      19
      Reduction and Reoxidation of Humic Acid: Influence on Spectroscopic Properties and Proton Binding
      Maurer, Felix; Christl, Iso; Kretzschmar, Ruben
      Environmental Science & Technology (2010), 44 (15), 5787-5792CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Previous studies on proton and metal binding to humic substances have not considered a potential influence of redn. and oxidn. of functional groups. Therefore, we investigated how proton binding of a purified soil humic acid was affected by redn. Redn. of the humic acid was carried out using an electrochem. cell that allowed us to measure the amts. of electrons and protons involved in redn. reactions. We further applied spectroscopic methods (UV-vis, fluorescence, FT-IR, C-1s NEXAFS) to detect possible chem. changes in the humic acid induced by redn. and reoxidn. The effect of redn. on proton binding was detd. with acid-base titrns. in the pH range 4-10 under controlled redox conditions. During redn., 0.54 mol kg-1 protons and 0.55 mol kg-1 electrons were transferred to humic acid. NICA-Donnan modeling revealed an equiv. increase in proton-reactive sites (0.52 mol kg-1) in the alk. pH-range. Our results indicate that redn. of humic acid increased the amt. of proton-reactive sites by 15% compared to the untreated state. Spectroscopic differences between the untreated and reduced humic acid were minor, apart from a lower UV-vis absorption of the reduced humic acid between 400 and 700 nm.
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    20. 20
      Yang, P.; Jiang, T.; Cong, Z.; Liu, G.; Guo, Y.; Liu, Y.; Shi, J.; Hu, L.; Yin, Y.; Cai, Y.; Jiang, G. Loss and Increase of the Electron Exchange Capacity of Natural Organic Matter during Its Reduction and Reoxidation: The Role of Quinone and Nonquinone Moieties. Environ. Sci. Technol. 2022, 56 (10), 6744– 6753,  DOI: 10.1021/acs.est.1c08927
      20
      Loss and Increase of the Electron Exchange Capacity of Natural Organic Matter during Its Reduction and Reoxidation: The Role of Quinone and Nonquinone Moieties
      Yang, Peijie; Jiang, Tao; Cong, Zhiyuan; Liu, Guangliang; Guo, Yingying; Liu, Yanwei; Shi, Jianbo; Hu, Ligang; Yin, Yongguang; Cai, Yong; Jiang, Guibin
      Environmental Science & Technology (2022), 56 (10), 6744-6753CODEN: ESTHAG; ISSN:1520-5851. (American Chemical Society)
      Redox-active quinone and nonquinone moieties represent the electron exchange capacity (EEC) of natural org. matter (NOM), playing an important role in the electron transfer link of microbes and transformation of contaminants/metal minerals. However, the corresponding transformation of quinone/phenol and their resp. influence on the EECs during redn. and reoxidn. remain poorly characterized. Besides, it is still controversial whether nonquinones donate or accept electrons. Herein, we demonstrated that reoxidn. of NOM after redn. can form new phenolic/quinone moieties, thus increasing the EEC. The assessment for the EEC, including the electron-donating capacity (EDC) and electron-accepting capacity (EAC), of nonquinones reflects the contribution of sulfur-contg. moieties with considerable EDCs and EACs. In contrast, nitrogen-contg. moieties donate negligible electrons even at Eh = +0.73 V. The contributions of both thiol and amine moieties to the EEC are greatly affected by adjacent functional groups. Meanwhile, aldehydes/ketones did not display an EAC during the electron transfer process of NOM. Furthermore, substantially increased EDC at Eh from +0.61 to +0.73 V could not be fully explained using thiol and phenolic moieties, suggesting the contribution of unknown moieties with high oxidn. potential. The overall findings suggest that the roles of new quinones/phenol (derived from the addn. of oxygen to condensed arom./lignin-like components) during redox dynamic cycling and thiol species should be considered in assessing the electron transfer processes of NOM.
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    21. 21
      McAdams, B. C.; Hudson, J.; Arnold, W. A.; Chin, Y. P. Effects of aquatic dissolved organic matter redox state on adsorption to goethite. Aquat. Sci. 2023, 85 (1), 19  DOI: 10.1007/s00027-022-00912-0
      21
      Effects of aquatic dissolved organic matter redox state on adsorption to goethite
      McAdams, Brandon C.; Hudson, Jeffrey; Arnold, William A.; Chin, Yu-Ping
      Aquatic Sciences (2023), 85 (1), 19CODEN: AQSCEA; ISSN:1015-1621. (Springer International Publishing AG)
      Abstr.: Direct electrochem. methods were used to alter the redox state of dissolved org. matter (DOM) rich in carbon-hydrogen-oxygen-sulfur (CHOS) and sulfur deficient (CHO) compn. to explore how their reduced and oxidized forms sorbed to iron oxides. The CHOS-rich DOM was isolated from Prairie Pothole wetland surface waters, while Suwannee River natural org. matter (SRNOM) represented a sulfur poor DOM. CHOS-rich DOM showed decreased adsorption to goethite following electrochem. redn. compared to its oxidized or unaltered forms. In contrast, no difference was obsd. between the adsorption of unaltered or reduced SRNOM to goethite. Our results suggest that the org. sulfur moieties in Prairie Pothole DOM are responsible for the decline in adsorption following redn. Changes in DOM mol. wt. after adsorption were obsd. for all samples, however, reduced Prairie Pothole DOM resulted in increased fractionation relative to its unaltered or oxidized forms. This was not obsd. for SRNOM. Furthermore, increases in Fe(II) in soln. were obsd. following interaction of reduced Prairie Pothole DOM with goethite, a phenomenon not seen in oxidized or reduced SRNOM. This observation suggests that reduced CHOS-rich DOM enhanced the reductive dissoln. of goethite, which may play an important role in the cycling and bioavailability of iron in systems where sulfur is important. This work illustrates the potential importance of redox active, high sulfur DOM in aquatic biogeochem. cycles.
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    22. 22
      Trofimova, A.; Hems, R. F.; Liu, T.; Abbatt, J. P. D.; Schnitzler, E. G. Contribution of Charge-Transfer Complexes to Absorptivity of Primary Brown Carbon Aerosol. ACS Earth Space Chem. 2019, 3 (8), 1393– 1401,  DOI: 10.1021/acsearthspacechem.9b00116
      22
      Contribution of Charge-Transfer Complexes to Absorptivity of Primary Brown Carbon Aerosol
      Trofimova, Alina; Hems, Rachel F.; Liu, Tengyu; Abbatt, Jonathan P. D.; Schnitzler, Elijah G.
      ACS Earth and Space Chemistry (2019), 3 (8), 1393-1401CODEN: AESCCQ; ISSN:2472-3452. (American Chemical Society)
      Light-absorbing org. aerosol, or brown carbon (BrC), has significant but poorly constrained effects on climate. A large fraction of the absorptivity of ambient BrC is unassigned, and org. charge-transfer (CT) complexes have the potential to contribute to this fraction. Here, the contributions of CT complexes to the absorptivity of lab.-generated BrC and ambient aerosol material influenced by biomass burning have been investigated, using a wide range of chem., spectroscopic, and phys. analyses. Chem. functionalization expts. are inconclusive about the role of CT complexes, whereas fluorescence spectra exhibit distinct spectral features indicative of individual chromophores. Detns. of the concn. and temp. dependences of absorbance are more conclusive. In particular, for lab.-generated BrC extd. in either water or methanol, absorbance scaled linearly with orders-of-magnitude changes in concn., indicating that intermol. complexes do not contribute to the absorptivity. Furthermore, whereas the absorbance of BrC exts. in DMSO exhibited a slight temp. dependence, consistent with a 15% contribution from intramol. CT complexes at 15 °C, the complete temp. independence of absorbance of water-sol. exts. from surrogate and ambient BrC indicates a negligible role for CT complexes. Overall, our results find little evidence for CT complexes in the primary BrC studied, suggesting that they do not contribute significantly to the missing absorptivity of ambient BrC.
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    23. 23
      Zhang, C.; Mo, S.; Liu, Z.; Chen, B.; Korshin, G.; Hertkorn, N.; Ni, J.; Yan, M. Interpreting pH-Dependent Differential UV/VIS Absorbance Spectra to Characterize Carboxylic and Phenolic Chromophores in Natural Organic Matter. Water Res. 2023, 244, 120522  DOI: 10.1016/j.watres.2023.120522
      There is no corresponding record for this reference.
    24. 24
      Allen, A.; Cheng, K.; Mckay, G. Evaluating the pH-dependence of DOM absorbance, fluorescence, and photochemical production of singlet oxygen. Environ. Sci.:Processes Impacts 2023, 25 (12), 1974– 1985,  DOI: 10.1039/D3EM00316G
      There is no corresponding record for this reference.
    25. 25
      Leenheer, J. A.; Wilson, M. A.; Malcolm, R. L. Presence and potential significance of aromatic-ketone groups in aquatic humic substances. Org. Geochem. 1987, 11 (4), 273– 280,  DOI: 10.1016/0146-6380(87)90038-6
      25
      Presence and potential significance of aromatic-ketone groups in aquatic humic substances
      Leenheer, J. A.; Wilson, M. A.; Malcolm, R. L.
      Organic Geochemistry (1987), 11 (4), 273-80CODEN: ORGEDE; ISSN:0146-6380.
      Aquatic humic- and fulvic-acid stds. of the International Humic Substances Society were characterized, with emphasis on carbonyl-group nature and content, by 13C NMR spectroscopy, proton NMR spectroscopy, and IR spectroscopy. After comparing spectral results of underivatized humic and fulvic acids with spectral results of chem. modified derivs. that allow improved observation of the carbonyl group, the data clearly indicated that arom. ketone groups comprised the majority of the carbonyl-group content. About 1 ketone group per monocyclic arom. ring was detd. for both humic and fulvic acids. Arom. ketone groups were hypothesized to form by photolytic rearrangements and oxidn. of phenolic ester and hydrocarbon precursors; these groups have potential significance regarding haloform formation in water, reactivity resulting from active H of the Me and methylene adjacent to the ketone groups, and formation of hemiketal and lactol structures. Arom. ketone groups also may be the point of attachment between aliph. and arom. moieties of aquatic humic-substance structure.
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    26. 26
      Tinnacher, R. M.; Honeyman, B. D. A New Method to Radiolabel Natural Organic Matter by Chemical Reduction with Tritiated Sodium Borohydride. Environ. Sci. Technol. 2007, 41 (19), 6776– 6782,  DOI: 10.1021/es070563b
      26
      A New Method to Radiolabel Natural Organic Matter by Chemical Reduction with Tritiated Sodium Borohydride
      Tinnacher, Ruth M.; Honeyman, Bruce D.
      Environmental Science & Technology (2007), 41 (19), 6776-6782CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      In this paper, we describe a new method for labeling NOM with the radioisotope tritium (3H) using fulvic acid (FA) as the target NOM fraction. During labeling, FA ketone groups are chem. reduced with tritiated sodium borohydride (NaBH4), while the chem. functionality of the carboxyl and phenol groups is preserved. The labeling procedure was optimized in efficiency expts. that detd. the excess concn. of tritiated NaBH4 required for optimum redn. conditions. The chem. characterization of the labeled FA product using FTIR and 1H NMR spectral anal. confirms the proposed reaction mechanism and rules out any significant amts. of impurities or undesirable side reactions. Results from size exclusion chromatog. indicate that the tritium label is distributed uniformly over the whole mol. size range of FA and that it is stable over time and under various pH conditions. Potential differences in FA sorption behavior onto mineral surfaces due to labeling were excluded based on exptl. data. This method produces NOM of high specific activity (e.g., 1.9 mCi mg-1 FA); this permits the tracing of FA at a detection limit of 0.3 μg L-1 FA.
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    27. 27
      Ma, J.; Del Vecchio, R.; Golanoski, K. S.; Boyle, E. S.; Blough, N. V. Optical properties of humic substances and CDOM: Effects of borohydride reduction. Environ. Sci. Technol. 2010, 44 (14), 5395– 5402,  DOI: 10.1021/es100880q
      27
      Optical Properties of Humic Substances and CDOM: Effects of Borohydride Reduction
      Ma, Jiahai; Del Vecchio, Rossana; Golanoski, Kelli S.; Boyle, Erin S.; Blough, Neil V.
      Environmental Science & Technology (2010), 44 (14), 5395-5402CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Treatment of Suwanee River humic (SRHA) and fulvic (SRFA) acids, a com. lignin (LAC), and a series of solid phase exts. (C18) from the Middle Atlantic Bight (MAB exts.) with NaBH4, a selective reductant of carbonyl-contg. compds. including quinones and arom. ketones, produces a preferential loss of visible absorption (≥50% for SRFA) and substantially enhanced, blue-shifted fluorescence emission (2- to 3-fold increase). Comparison of the results with those obtained from a series of model quinones and hydroquinones demonstrates that these spectral changes cannot be assigned directly to the absorption and emission of visible light by quinones/hydroquinones. Instead, these results are consistent with a charge transfer model in which the visible absorption is due primarily to charge transfer transitions arising among hydroxy- (methoxy-) arom. donors and carbonyl-contg. acceptors. Unlike most of the model hydroquinones, the changes in optical properties of the natural samples following NaBH4 redn. were largely irreversible in the presence of air and following addn. of a Cu2+ catalyst, providing tentative evidence that arom. ketones (or other similar carbonyl-contg. structures) may play a more important role than quinones in the optical properties of these materials.
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    28. 28
      Li, H.; McKay, G. Relationships between the Physicochemical Properties of Dissolved Organic Matter and Its Reaction with Sodium Borohydride. Environ. Sci. Technol. 2021, 55 (15), 10843– 10851,  DOI: 10.1021/acs.est.1c01973
      28
      Relationships between the Physicochemical Properties of Dissolved Organic Matter and Its Reaction with Sodium Borohydride
      Li, Hang; McKay, Garrett
      Environmental Science & Technology (2021), 55 (15), 10843-10851CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The reaction of dissolved org. matter (DOM) with sodium borohydride has been used to understand the geog. origin of DOM and investigate the photophys. model underlying DOM's optical properties. However, the physicochem. properties of DOM (e.g., mol. size and charge) that influence the kinetics and ultimate reducibility of DOM by borohydride remain poorly characterized. Herein, we studied the kinetics of DOM-borohydride reactions by recording absorbance and fluorescence spectra at a high temporal frequency (every ~ 10 min for 24 h) for a diverse set of DOM isolates of aquatic and soil origin. The reducibility of DOM by sodium borohydride (as judged by relative removal of initial absorbance) varied appreciably among the DOM samples studied, with soil humic substances being less reducible than aquatic humic substances and natural org. matter. While statistically significant correlations were found between the reducibility of DOM and descriptors of mol. size, these descriptors were not able to differentiate the reducibility of soil vs. aquatic DOM isolates that had similar bulk properties. Thus, it appears that the extent of absorbance removal by borohydride is largely driven by the origin of the humic substance isolate (aquatic vs soil) instead of mol. size or charge. Borohydride redn. resulted in increased fluorescence emission across UV and visible excitation wavelengths. However, the enhanced emission at visible excitation decreased over a time period of hours to days, suggesting that redn. of an important subset of DOM chromophores is reversible. This reversibility in fluorescence emission is consistent with the small role of quinones in the absorbance of DOM but suggests a more important role for quinone-contg. charge-transfer contacts in the fluorescence of DOM, particularly at visible excitation wavelengths.
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    29. 29
      Cartisano, C. M.; Del Vecchio, R.; Bianca, M. R.; Blough, N. V. Investigating the sources and structure of chromophoric dissolved organic matter (CDOM) in the North Pacific Ocean (NPO) utilizing optical spectroscopy combined with solid phase extraction and borohydride reduction. Mar. Chem. 2018, 204, 20– 35,  DOI: 10.1016/j.marchem.2018.05.005
      29
      Investigating the sources and structure of chromophoric dissolved organic matter (CDOM) in the North Pacific Ocean (NPO) utilizing optical spectroscopy combined with solid phase extraction and borohydride reduction
      Cartisano, Carmen M.; Del Vecchio, Rossana; Bianca, Marla R.; Blough, Neil V.
      Marine Chemistry (2018), 204 (), 20-35CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
      Prior optical measurements of waters in the Equatorial Atlantic Ocean (EAO) provided evidence of a major terrestrial "humic-like" component of the CDOM that absorbed in the UV and visible and emitted across the visible, along with a marine component that primarily absorbed and emitted in the UV. Here we extend these measurements to the North Pacific Ocean (NPO) at Station Aloha. Detailed optical measurements of both the natural waters (CDOM) and C18 org. matter exts. of these waters (C18-OM) were acquired before and after sodium borohydride (NaBH4) redn. of samples obtained throughout the water column. Optical properties of the "humic-like" component were relatively uniform with depth below ∼600 m [aCDOM(350) ∼0.08 (m-1), a*CDOM (350) ∼ 0.2 (m-1 mg-1C L), SUVA254 ∼ 0.55 (m-1 mg-1C L),E2:E3 ∼ 10, S300-700 ∼ 0.02 (nm-1), S350-400 ∼ 0.012 (nm-1), SR ∼1.7, F(350/450) ∼ 0.009 (QSE), and .vphi.360 ∼0.026], but were significantly different in surface waters, likely due to photobleaching and biol. activity [aCDOM(350) ∼ 0.026 (m-1), a*CDOM (350) ∼ 0.027 (m-1 mg-1C L), SUVA254 ∼ 0.36 (m-1 mg-1C L), E2:E3 ∼45, S300-700 ∼ 0.03 (nm-1), S350-400 ∼ 0.003 (nm-1), SR ∼6.8, F(350/450) ∼ 0.003 (QSE), and .vphi.350 ∼ 0.024]. Optical properties of the short-wavelength components (UV bands) were more variable with depth. Response to solid phase extn. was also relatively uniform with depth, with preferential extn. of the long-wavelength absorbing/emitting "humic-like" component and virtually no extn. of the the UV absorbing/emitting bands. Response to NaBH4 redn. was also similar down the water column with preferential loss of absorption in the visible region, and enhanced, blue-shifted fluorescence emission. As in the EAO the 'humic-like" component exhibited very similar, although not identical, optical and chem. properties to those obsd. for terrestrially-dominated estuarine and coastal environments, providing evidence that this component originates from a terrestrial source.
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    30. 30
      Andrew, A. A.; Del Vecchio, R.; Subramaniam, A.; Blough, N. V. Chromophoric dissolved organic matter (CDOM) in the Equatorial Atlantic Ocean: Optical properties and their relation to CDOM structure and source. Mar. Chem. 2013, 148 (C), 33– 43,  DOI: 10.1016/j.marchem.2012.11.001
      30
      Chromophoric dissolved organic matter (CDOM) in the Equatorial Atlantic Ocean: Optical properties and their relation to CDOM structure and source
      Andrew, Andrea A.; Del Vecchio, Rossana; Subramaniam, Ajit; Blough, Neil V.
      Marine Chemistry (2013), 148 (), 33-43CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
      Extensive data exist on the optical properties of CDOM from terrestrial and coastal environments, yet the open oceans have been historically under-sampled. The source and structural basis of marine CDOM optical properties are still debated. Detailed optical measurements were acquired for both untreated and sodium borohydride (NaBH4) reduced natural waters and C18 exts. (C18-OM) across the Equatorial Atlantic Ocean. Except in regions of upwelling or in the vicinity of the Congo River outflow, CDOM absorption coeffs. and visible emission intensity were far smaller for surface waters (aCDOM(355): 0.057-0.162 m-1; λex/λem = 350/450 nm: 0.396-1.431 qse) than for waters below the mixed layer (aCDOM(355) 0.084-0.344 m-1; λex/λem = 350/450 nm: 0.903-3.226 qse), while spectral slopes were higher (surface: 0.019 to 0.025 nm-1; deep: 0.013 to 0.019 nm-1), consistent with photobleaching of CDOM in surface waters. Distinct emission bands were obsd. in the UV, primarily at excitation/emission wavelengths (λex/λem) = 280/320 nm, but also at λex/λem = 300/340, 300/405 and 320/380 nm for some stations and depths. In contrast, visible emission exhibited maxima that continuously red shifted with increasing λex (> 330 nm), a property characteristic of CDOM from estuarine and coastal environments. Further evidence that CDOM in the offshore waters of this region is composed of a major terrestrial component includes: (1) similar spectral dependencies of the emission maxima and fluorescence quantum yields; (2) a large Stokes shift in the emission maxima with short-wavelength excitation (λex = 280 nm); (3) correlation of visible emission intensities with absorption at λex = 280, 320 and 450 nm, with absorption to fluorescence ratios comparable to those found in estuarine and coastal environments; (4) affinity of C18 cartridges for the long wavelength (visible) absorbing and emitting material, but not the UV emitting material; (5) preferential loss of visible absorption and substantially enhanced blue-shifted emission in the visible following borohydride redn. of both the Equatorial Atlantic waters and the C18-OM of these waters. These results support the occurrence in offshore waters of a major terrestrial CDOM component that absorbs in the UV and visible and emits in the visible, as well as marine CDOM components that absorb and emit in the UV. The results further demonstrate that the simultaneous acquisition of complete spectral absorption and emission properties, combined with chem. tests (C-18 extns., borohydride redn.) can provide a far clearer picture of the sources and cycling of CDOM within the oceans.
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    31. 31
      Osburn, C. L.; Kinsey, J. D.; Bianchi, T. S.; Shields, M. R. Formation of planktonic chromophoric dissolved organic matter in the ocean. Mar. Chem. 2019, 209, 1– 13,  DOI: 10.1016/j.marchem.2018.11.010
      31
      Formation of planktonic chromophoric dissolved organic matter in the ocean
      Osburn, Christopher L.; Kinsey, Joanna D.; Bianchi, Thomas S.; Shields, Michael R.
      Marine Chemistry (2019), 209 (), 1-13CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
      Chromophoric dissolved org. matter (CDOM) is an important fraction of the marine carbon cycle that controls most light absorption and many photochem. and biol. processes in the ocean. Despite its importance, the chem. basis for the formation of oceanic CDOM remains unclear. Currently, CDOM's optical properties are best-described by an electronic interaction (EI) model of charge transfer (CT) complexes which form between electron-rich donors and electron-poor acceptors. While terrigenous compds. such as lignin best fit this model, planktonic sources of CDOM have not yet been tested. Here, we have tested CDOM formed during an incubation expt. using a natural phytoplankton assemblage and throughout active growth, stationary phase and algal biomass decompn. Absorbance of the derived planktonic CDOM generally decreased with increasing wavelength, similar to the ref. Pony Lake (PLFA) and Suwanee River (SRFA) fulvic acid solns. used as models of terrigenous CDOM. Further, after 60 d of microbial degrdn. in the dark, CDOM exhibited fluorescence emission maxima continuously red-shifted into the visible band, consistent with PLFA and SRFA. Redn. of carbonyl-contg. groups, key to CT complex formation, with sodium borohydride (NaBH4) produced coherent results in planktonic CDOM and ref. FAs. Absorption at 350 nm decreased by 50% for planktonic CDOM and by 30% for PLFA and SRFA, with corresponding increases in spectral slope (S) values, indicating preferential loss of absorption well into the visible. Fluorescence likewise responded with enhanced emission at shorter wavelengths. Apparent quantum yields (Φ) were similarly affected. Results from our work support prior observations that phytoplankton and bacteria are important sources of CDOM that color the ocean's "twilight zone". We hypothesize that microbial processing of a variety of source substrates into more complex compds. represented as planktonic CDOM likely represents a semi-refractory pool of DOM in the ocean.
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    32. 32
      Phillips, S. M.; Smith, G. D. Light Absorption by Charge Transfer Complexes in Brown Carbon Aerosols. Environ. Sci. Technol. Lett. 2014, 1 (10), 382– 386,  DOI: 10.1021/ez500263j
      32
      Light absorption by charge transfer complexes in brown carbon aerosols
      Phillips, Sabrina M.; Smith, Geoffrey D.
      Environmental Science & Technology Letters (2014), 1 (10), 382-386CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
      Recently, it has become apparent that a fraction of the org. species in ambient aerosols absorbs UV-visible light with a potential impact on climate. It is believed that this light-absorbing, sometimes called "brown", carbon originates from biomass and biofuel burning and could be formed through secondary processes in particles or clouds. Here, we identify for the first time charge transfer (CT) complexes as a significant source of light absorption by org. compds. in aerosols. A dense manifold of these complexes, formed from interactions between alc. and carbonyl moieties, accounts for approx. 50% of the absorption (300-600 nm) obsd. for water-extd. ambient particulate matter collected in Athens, GA. Corresponding fluorescence emission spectra with broad, overlapping long-wavelength tails are consistent with efficient energy transfer among a near continuum of such coupled excited states. We postulate that a wide variety of CT complexes are formed as a result of a supramol. assocn. of self-assembling, smaller mols., just as they are in natural humic substances, and that this gives rise to absorption that extends to red wavelengths. These findings imply that light absorption by org. aerosols is governed by a combination of independent as well as interacting chromophores and that both must be included in an accurate representation of aerosol optical properties.
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    33. 33
      Bianca, M. R.; Baluha, D. R.; Gonsior, M.; Schmitt-Kopplin, P.; Del Vecchio, R.; Blough, N. V. Contribution of ketone/aldehyde-containing compounds to the composition and optical properties of Suwannee River fulvic acid revealed by ultrahigh resolution mass spectrometry and deuterium labeling. Anal. Bioanal. Chem. 2020, 412 (6), 1441– 1451,  DOI: 10.1007/s00216-019-02377-x
      33
      Contribution of ketone/aldehyde-containing compounds to the composition and optical properties of Suwannee River fulvic acid revealed by ultrahigh resolution mass spectrometry and deuterium labeling
      Bianca, Marla R.; Baluha, Daniel R.; Gonsior, Michael; Schmitt-Kopplin, Philippe; Del Vecchio, Rossana; Blough, Neil V.
      Analytical and Bioanalytical Chemistry (2020), 412 (6), 1441-1451CODEN: ABCNBP; ISSN:1618-2642. (Springer)
      Abstr.: A prior method of mass labeling ketone-/aldehyde-contg. species in natural dissolved org. matter (DOM) is further developed and applied. This application involved the treatment of Suwannee River fulvic acid (SRFA) with increasing concns. of sodium borodeuteride (NaBD4), followed by detection of reduced species via neg. mode electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FTICR MS). The extent of redn., as detd. by ESI FTICR MS, resulting from increasing concns. of NaBD4 correlated well with changes in the absorption and emission spectra of the corresponding untreated and borodeuteride-reduced samples, providing evidence that ketone/aldehyde functional groups contribute substantially to the bulk optical properties of SRFA. Furthermore, the differences in the reactivity and abundance of ketone-/aldehyde-contg. species for various regions in Van Krevelen plots were revealed, thus showing how this mass labeling method can be used to provide more detailed structural information about components within complex DOM samples than that provided by the detn. and anal. of mol. formulas alone. Graphical abstr. [graphic not available: see fulltext].
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    34. 34
      Zafiriou, O. C.; Xie, H. X.; Kieber, D. J.; Wang, W.; Song, G. S.; Cohen, N. Cyanohydrin Equilibria Implicate Non-Aromatic Aldehydes in Photochemical Production of Oceanic Carbon Monoxide. Environ. Sci. Technol. 2024, 58 (36), 16066– 16075,  DOI: 10.1021/acs.est.4c04637
      There is no corresponding record for this reference.
    35. 35
      McNeill, K.; Canonica, S. Triplet state dissolved organic matter in aquatic photochemistry: reaction mechanisms, substrate scope, and photophysical properties. Environ. Sci.:Processes Impacts 2016, 18 (11), 1381– 1399,  DOI: 10.1039/C6EM00408C
      35
      Reaction mechanisms, substrate scope, and photophysical properties of triplet state dissolved organic matter in aquatic photochemistry
      McNeill, Kristopher; Canonica, Silvio
      Environmental Science: Processes & Impacts (2016), 18 (11), 1381-1399CODEN: ESPICZ; ISSN:2050-7895. (Royal Society of Chemistry)
      A review. Excited triplet states of chromophoric dissolved org. matter (3CDOM) play a major role among the reactive intermediates produced upon absorption of sunlight by surface waters. After more than two decades of research on the aquatic photochem. of 3CDOM, the need for improving the knowledge about the photophys. and photochem. properties of these elusive reactive species remains considerable. This crit. review examines the efforts to date to characterize 3CDOM. Information on 3CDOM relies mainly on the use of probe compds. because of the difficulties assocd. with directly observing 3CDOM using transient spectroscopic methods. Singlet mol. oxygen (1O2), which is a product of the reaction between 3CDOM and dissolved oxygen, is probably the simplest indicator that can be used to est. steady-state concns. of 3CDOM. There are two major modes of reaction of 3CDOM with substrates, namely triplet energy transfer or oxidn. (via electron transfer, proton-coupled electron transfer or related mechanisms). Org. mols., including several environmental contaminants, that are susceptible to degrdn. by these two different reaction modes are reviewed. It is proposed that through the use of appropriate sets of probe compds. and model photosensitizers an improved estn. of the distribution of triplet energies and one-electron redn. potentials of 3CDOM can be achieved.
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    36. 36
      Golanoski, K. S.; Fang, S.; Del Vecchio, R.; Blough, N. V. Investigating the Mechanism of Phenol Photooxidation by Humic Substances. Environ. Sci. Technol. 2012, 46 (7), 3912– 3920,  DOI: 10.1021/es300142y
      36
      Investigating the Mechanism of Phenol Photooxidation by Humic Substances
      Golanoski, Kelli S.; Fang, Shuo; Del Vecchio, Rossana; Blough, Neil V.
      Environmental Science & Technology (2012), 46 (7), 3912-3920CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      To probe the mechanism of the photosensitized loss of phenols by humic substances (HS), the dependence of the initial rate of 2,4,6-trimethylphenol (TMP) loss (RTMP) on dioxygen concn. was examd. both for a variety of untreated as well as borohydride-reduced HS and C18 exts. from the Delaware Bay and Mid-Atlantic Bight. RTMP was inversely proportional to dioxygen concn. at [O2] >50μM, a dependence consistent with reaction with triplet excited states, but not with 1O2 or RO2. Modeling the dependence of RTMP on [O2] provided rate consts. for TMP reaction, O2 quenching, and lifetimes compatible with a triplet intermediate. Borohydride redn. significantly reduced TMP loss, supporting the role of arom. ketone triplets in this process. However, for most samples, the incomplete loss of sensitization following borohydride redn., as well as the inverse dependence of RTMP on [O2] for these samples, suggests that there remains another class of oxidizing triplet sensitizer, perhaps quinones.
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    37. 37
      Del Vecchio, R.; Blough, N. V. On the Origin of the Optical Properties of Humic Substances. Environ. Sci. Technol. 2004, 38 (14), 3885– 3891,  DOI: 10.1021/es049912h
      37
      On the Origin of the Optical Properties of Humic Substances
      Del Vecchio, Rossana; Blough, Neil V.
      Environmental Science and Technology (2004), 38 (14), 3885-3891CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Absorption and fluorescence spectroscopy and laser photobleaching expts. were employed to probe the origins of the optical properties of humic substances (HSs). Luminescence quantum yields and the wavelengths of max. emission were acquired for Suwannee River humic acid (SRHA) and fulvic acid (SRFA) at an extensive series of excitation wavelengths across the UV and visible. Laser irradn. at a series wavelength across the UV and visible range was further employed to destroy selectively chromophores absorbing at specific wavelengths, using absorption spectroscopy to follow the absorption losses with irradn. time. The results provide unequivocal evidence that the absorption and emission spectra of these materials cannot result solely from a simple linear superposition of the spectra of numerous independent chromophores. Instead, the long wavelength absorption tail of HSs (>350 nm) appears to arise from a continuum of coupled states. We propose that this behavior results from intramol. charge-transfer interactions between hydroxy-arom. donors and quinoid acceptors formed by the partial oxidn. of lignin precursors. We further propose that these donor-acceptor interactions may be a common phenomenon, occurring within all natural hydroxy- or polyhydroxy-arom. polymers that form appropriate acceptors upon partial oxidn. Examples of such species include lignin, polyphenols, tannins, and melanins.
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    38. 38
      Sharpless, C. M.; Blough, N. V. The importance of charge-transfer interactions in determining chromophoric dissolved organic matter (CDOM) optical and photochemical properties. Environ. Sci.:Processes Impacts 2014, 16 (4), 654– 671,  DOI: 10.1039/C3EM00573A
      38
      The importance of charge-transfer interactions in determining chromophoric dissolved organic matter (CDOM) optical and photochemical properties
      Sharpless, Charles M.; Blough, Neil V.
      Environmental Science: Processes & Impacts (2014), 16 (4), 654-671CODEN: ESPICZ; ISSN:2050-7895. (Royal Society of Chemistry)
      A review. Absorption of sunlight by chromophoric dissolved natural org. matter (CDOM) is environmentally significant because it controls photic zone depth and causes photochem. that affects elemental cycling and contaminant fate. Both the optics (absorbance and fluorescence) and photochem. of CDOM display unusual properties that cannot easily be ascribed to a superposition of individual chromophores. These include (i) broad, unstructured absorbance that decreases monotonically well into the visible and near IR, (ii) fluorescence emission spectra that all fall into a single envelope regardless of the excitation wavelength, and (iii) photobleaching and photochem. quantum yields that decrease monotonically with increasing wavelength. In contrast to a simple superposition model, these phenomena and others can be reasonably well explained by a phys. model in which charge-transfer interactions between electron donating and accepting chromophores within the CDOM control the optical and photophys. properties. This review summarizes current understanding of the processes underlying CDOM photophysics and photochem. as well as their phys. basis.
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    39. 39
      McKay, G. Emerging investigator series: critical review of photophysical models for the optical and photochemical properties of dissolved organic matter. Environ. Sci.:Processes Impacts 2020, 22, 1139– 1165,  DOI: 10.1039/D0EM00056F
      39
      Emerging investigator series: critical review of photophysical models for the optical and photochemical properties of dissolved organic matter
      McKay, Garrett
      Environmental Science: Processes & Impacts (2020), 22 (5), 1139-1165CODEN: ESPICZ; ISSN:2050-7895. (Royal Society of Chemistry)
      Optical measurements (absorbance and fluorescence) are widely used to track dissolved org. matter (DOM) quantity and quality in natural and engineered systems. Despite many decades of research on the optical properties of DOM, there is a lack of understanding with regards to the underlying photophys. model that is the basis for these optical properties. This review both summarizes advances to date on the photophys. properties of DOM and seeks to critically evaluate the photophys. models for DOM optical properties. Recent studies have refined the quant. understanding of DOM photophys. properties such as excited state lifetimes and energies, rates of different photophys. processes, and quantum yields. Considering fundamental models, more clarity is needed on whether DOM photophys. processes are due to a superposition of non-interacting components (superposition model), or whether a portion of optical signals can be ascribed to electronically interacting moieties, for example in the form of electron donor-acceptor complexes (charge transfer model). Multiple studies over more than two decades have provided evidence for the charge transfer model. Questions have been raised, however, about the broad applicability of the charge transfer model. The charge transfer and superposition model are critically reviewed in light of this current research. Recommendations are given for future studies to help clarify the accuracy of these competing photophys. models.
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    40. 40
      Blough, N. V.; Del Vecchio, R. Comment on The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics. Environ. Sci. Technol. 2018, 52 (9), 5512– 5513,  DOI: 10.1021/acs.est.8b01189
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      Comment on The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics
      Blough, Neil V.; Del Vecchio, Rossana
      Environmental Science & Technology (2018), 52 (9), 5512-5513CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      A polemic. In a recent article by McKay et al. (Environ. Sci. Technol. 2018, 52, 406-414), they argue that charge transfer interactions cannot be important to the optical properties of humic substances and dissolved org. matter because perturbations of solvent temp., viscosity and polarity do not affect the absorbance and fluorescence spectral line shapes. Blough and Del Vecchio challenge why these conclusions may be incorrect.
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    41. 41
      McKay, G.; Korak, J. A.; Erickson, P. R.; Latch, D. E.; McNeill, K.; Rosario-Ortiz, F. L. The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics. Environ. Sci. Technol. 2018, 52 (2), 406– 414,  DOI: 10.1021/acs.est.7b03589
      41
      The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics
      McKay, Garrett; Korak, Julie A.; Erickson, Paul R.; Latch, Douglas E.; McNeill, Kristopher; Rosario-Ortiz, Fernando L.
      Environmental Science & Technology (2018), 52 (2), 406-414CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The optical properties of dissolved org. matter influence chem. and biol. processes in all aquatic ecosystems. Dissolved org. matter optical properties have been attributed to a charge-transfer model in which donor-acceptor complexes play a primary role. This model was evaluated by measuring the absorbance and fluorescence response of org. matter isolates to changes in solvent temp., viscosity, and polarity, which affect the position and intensity of spectra for known donor-acceptor complexes of org. mols. Absorbance and fluorescence spectral shape were largely unaffected by these changes, indicating that the distribution of absorbing and emitting species was unchanged. Overall, these results call into question the wide applicability of the charge-transfer model for explaining org. matter optical properties and suggest that future research should explore other models for dissolved org. matter photophysics.
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    42. 42
      Del Vecchio, R.; Schendorf, T. M.; Blough, N. V. Contribution of Quinones and Ketones/Aldehydes to the Optical Properties of Humic Substances (HS) and Chromophoric Dissolved Organic Matter (CDOM). Environ. Sci. Technol. 2017, 51 (23), 13624– 13632,  DOI: 10.1021/acs.est.7b04172
      42
      Contribution of Quinones and Ketones/Aldehydes to the Optical Properties of Humic Substances (HS) and Chromophoric Dissolved Organic Matter (CDOM)
      Del Vecchio, Rossana; Schendorf, Tara Marie; Blough, Neil V.
      Environmental Science & Technology (2017), 51 (23), 13624-13632CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The mol. basis of the optical properties of chromophoric dissolved org. matter (CDOM) and humic substances (HS) remains poorly understood and yet to be studied adequately. This study evaluates the relative contributions of 2 broad classes of carbonyl-contg. compds., ketones/aldehydes vs. quinones, to the absorption and emission properties of a representative suite of HS as well as a lignin sample. Selective redn. of quinones to hydroquinones by addn. of small molar excesses of dithionite to these samples under anoxic conditions produced small or negligible changes in their optical properties; however, when measurable, these changes were largely reversible upon exposure to air, consistent with the reoxidn. of hydroquinones to quinones. With one exception, ests. of quinone content based on dithionite consumption by the HS under anoxic conditions were in good agreement with past electrochem. measurements. In contrast, redn. of ketones/aldehydes to alcs. using excess Na borohydride produced pronounced and largely, but not completely, irreversible changes in the optical properties. The results demonstrate that (arom.) ketones/aldehydes, as opposed to quinones, play a far more prominent role in the optical absorption and emission properties of these HS, consistent with these moieties acting as the primary acceptors in charge-transfer transitions within these samples. As a method, anoxic dithionite titrns. may further allow addnl. insight into the content and impact of quinones/hydroquinones on the optical properties of HS and CDOM.
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    43. 43
      Piccolo, A.; Conte, P.; Cozzolino, A.; Spaccini, R. Molecular Sizes and Association Forces of Humic Substances in Solution; Clapp, C. E.; Hayes, M. H. B.; Senesi, N.; Bloom, P. R.; Jardine, P. M.; W I, Eds.; Soil Science Society of America: Madison, 2001; pp 89– 118.
      There is no corresponding record for this reference.
    44. 44
      Piccolo, A. The Supramolecular Structure of Humic Substances. Soil Sci. 2001, 166 (11), 810– 832,  DOI: 10.1097/00010694-200111000-00007
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      The supramolecular structure of humic substances
      Piccolo, Alessandro
      Soil Science (2001), 166 (11), 810-832CODEN: SOSCAK; ISSN:0038-075X. (Lippincott Williams & Wilkins)
      A review. The scientific understanding of the mol. size and shape of humic substances (HS) is critically reviewed. The traditional view that HS are polymers in soil is not substantiated by any direct evidence but is assumed only on the basis of lab. expts. with model mols. and unwarranted results produced by incorrectly applying either anal. procedures or math. treatments developed for purified and undisputed biopolymers. A large body of evidence shows an alternative understanding of the conformational nature of HS, which should be regarded as supramol. assocns. of self-assembling heterogeneous and relatively small mols. deriving from the degrdn. and decompn. of dead biol. material. A major aspect of the humic supramol. conformation is that it is stabilized predominantly by weak dispersive forces instead of covalent linkages. Hydrophobic (van der Waals, π-π, CH-π) and hydrogen bonds are responsible for the apparent large mol. size of HS, the former becoming more important with the increase of pH. This innovative understanding of the nature of HS implies a further development of the science and technol. for the control of the chem. and reactivity of natural org. matter in the soil and the environment.
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    45. 45
      Simpson, A. J. Determining the molecular weight, aggregation, structures and interactions of natural organic matter using diffusion ordered spectroscopy. Magn. Reson. Chem. 2002, 40 (13), S72– S82,  DOI: 10.1002/mrc.1106
      There is no corresponding record for this reference.
    46. 46
      Vialykh, E. A.; McKay, G.; Rosario-Ortiz, F. L. Computational Assessment of the Three-Dimensional Configuration of Dissolved Organic Matter Chromophores and Influence on Absorption Spectra. Environ. Sci. Technol. 2020, 54 (24), 15904– 15913,  DOI: 10.1021/acs.est.0c05860
      There is no corresponding record for this reference.
    47. 47
      Vialykh, E. A.; Salahub, D. R.; Achari, G.; Cook, R. L.; Langford, C. H. Emergent functional behaviour of humic substances perceived as complex labile aggregates of small organic molecules and oligomers. Environ. Chem. 2019, 16 (7), 505– 516,  DOI: 10.1071/EN19095
      47
      Emergent functional behaviour of humic substances perceived as complex labile aggregates of small organic molecules and oligomers
      Vialykh, Elena A.; Salahub, Dennis R.; Achari, Gopal; Cook, Robert L.; Langford, Cooper H.
      Environmental Chemistry (2019), 16 (7), 505-516CODEN: ECNHAA; ISSN:1449-8979. (CSIRO Publishing)
      Environmental contextThe correlation of physicochem. characteristics of humic substances with their function is crucial to our understanding of how environmental pollutants interact with humic substances. We have developed an approach that models emergent functions of fulvic and humic acids depending on sample characteristics. The results will be useful for predicting the sequestration of org. contaminants in soil under various conditions. AbstractThe structural organization of humic substances (HS) has been a central question of earth sciences for several decades. The latest exptl. results have led to the recognition of HS as complex mixts. of small mols. and oligomers. We investigate the correlation between the chem. compn. of HS, perceived as labile aggregates, and the emergent functions. Computational modeling was used to help to understand the processes and mechanisms on the mol. scale that occur in different fractions of the HS, fulvic acids (FA) and humic acids (HA), as they interact with metal ions and org. pollutants. The importance of non-covalent interactions in the emergent functions of HS is highlighted. H-bonding, hydrophilic/hydrophobic surface areas and π-stacking interactions play a significant role in aggregation processes as well as in the sorption of environmental pollutants. In a highly hydrophilic system with small mols. (the SRFA-22 model), H-bonding is the main force that drives the aggregation process. However, in a highly arom. and hydrophobic model with larger mol. fragments (SRHA-6), hydrophobic and π-stacking interactions dominate in the aggregation process. The chem. properties of contaminants significantly affect their mechanisms of sorption by HS. The interaction of a polar pollutant, phenol, with HS occurs through H-bonding, whereas non-polar benzene interacts through hydrophobic and π-stacking interactions. The non-polar pollutant results in a much stronger sorption by HS and causes an addnl. structural rearrangement of the aggregates, which make it more stable in the environment.
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    48. 48
      Hanson, B.; Wünsch, U.; Buckley, S.; Fischer, S.; Leresche, F.; Murphy, K.; D’Andrilli, J.; Rosario-Ortiz, F. L. DOM Molecular Weight Fractionation and Fluorescence Quantum Yield Assessment Using a Coupled In-Line SEC Optical Property System. ACS ES&T Water 2022, 2 (12), 2491– 2501,  DOI: 10.1021/acsestwater.2c00318
      There is no corresponding record for this reference.
    49. 49
      Pozdnyakov, I. P.; Pigliucci, A.; Tkachenko, N.; Plyusnin, V. F.; Vauthey, E.; Lemmetyinen, H. The photophysics of salicylic acid derivatives in aqueous solution. J. Phys. Org. Chem. 2009, 22 (5), 449– 454,  DOI: 10.1002/poc.1489
      There is no corresponding record for this reference.
    50. 50
      Murphy, K. R.; Butler, K. D.; Spencer, R. G. M.; Stedmon, C. A.; Boehme, J. R.; Aiken, G. R. Measurement of Dissolved Organic Matter Fluorescence in Aquatic Environments: An Interlaboratory Comparison. Environ. Sci. Technol. 2010, 44 (24), 9405– 9412,  DOI: 10.1021/es102362t
      50
      Measurement of dissolved organic matter fluorescence in aquatic environments: an interlaboratory comparison
      Murphy, Kathleen R.; Butler, Kenna D.; Spencer, Robert G. M.; Stedmon, Colin A.; Boehme, Jennifer R.; Aiken, George R.
      Environmental Science & Technology (2010), 44 (24), 9405-9412CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The fluorescent properties of dissolved org. matter (DOM) are often studied in order to infer DOM characteristics in aquatic environments, including source, quantity, compn., and behavior. While a potentially powerful technique, a single widely implemented std. method for correcting and presenting fluorescence measurements is lacking, leading to difficulties when comparing data collected by different research groups. This paper reports on a large-scale interlab. comparison in which natural samples and well-characterized fluorophores were analyzed in 20 labs. in the US, Europe, and Australia. Shortcomings were evident in several areas, including data quality-assurance, the accuracy of spectral correction factors used to correct EEMs, and the treatment of optically dense samples. Data cor. by participants according to individual lab. procedures were more variable than when cor. under a std. protocol. Wavelength dependency in measurement precision and accuracy were obsd. within and between instruments, even in cor. data. In an effort to reduce future occurrences of similar problems, algorithms for correcting and calibrating EEMs are described, and MATLAB scripts for implementing the study protocol are provided. Combined with the recent expansion of spectral fluorescence stds., this approach will serve to increase the intercomparability of DOM fluorescence studies.
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    51. 51
      Wünsch, U. J.; Stedmon, C. A.; Tranvik, L. J.; Guillemette, F. Unraveling the size-dependent optical properties of dissolved organic matter. Limnol. Oceanogr. 2018, 63 (2), 588– 601
      There is no corresponding record for this reference.
    52. 52
      Wünsch, U. J.; Bro, R.; Stedmon, C.; Wenig, P.; Murphy, K. Emerging patterns in the global distribution of dissolved organic matter fluorescence. Anal. Methods 2019, 11, 888– 893,  DOI: 10.1039/C8AY02422G
      There is no corresponding record for this reference.
    53. 53
      Lorenzo-Seva, U.; ten Berge, J. M. F. Tucker’s Congruence Coefficient as a Meaningful Index of Factor Similarity. Methodology 2006, 2 (2), 57– 64,  DOI: 10.1027/1614-2241.2.2.57
      There is no corresponding record for this reference.
    54. 54
      Tucker, L. R. Personnel Research Section Report No. 984. Department of the Army: Washington D.C.; 1951.
      There is no corresponding record for this reference.
    55. 55
      De Haan, H.; Werlemark, G.; De Boer, T. Effect of pH on molecular weight and size of fulvic acids in drainage water from peaty grassland in NW Netherlands. Plant Soil 1983, 75 (1), 63– 73,  DOI: 10.1007/BF02178614
      There is no corresponding record for this reference.
    56. 56
      Peuravuori, J.; Pihlaja, K. Molecular size distribution and spectroscopic properties of aquatic humic substances. Anal. Chim. Acta 1997, 337, 133– 149,  DOI: 10.1016/S0003-2670(96)00412-6
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      Molecular size distribution and spectroscopic properties of aquatic humic substances
      Peuravuori, Juhani; Pihlaja, Kalevi
      Analytica Chimica Acta (1997), 337 (2), 133-150CODEN: ACACAM; ISSN:0003-2670. (Elsevier)
      The no.- and wt.-averaged mol. wts. of different isolated humic fractions and unfractionated org. matter from natural water samples were measured using high-performance size-exclusion chromatog. (HPSEC). The chromatograms obtained, with sodium acetate as the mobile phase on a TSK G3000SW high-speed column, provided illustrative information about the nature of the dissolved org. matter that was not available with phosphate buffer eluents. The calibration of the column system merely with protein stds. generated misleading information about the mol. wt. and size distribution of humic solutes. Instead, the present results support the utilization of polystyrene-sulfonates (PSS) either alone or together with certain other model compds. for the calibration of HPSEC in studying mol. wts. of humic solutes. The data obtained with the HPSEC system indicate that the av. mol. wt. of humic solutes is distinctly lower than is generally assumed. Besides, aquatic humic solutes also contain some very large-sized though minor constituents, leading to a fairly polydisperse mixt. No.-averaged mol. wts. obtained using the HPSEC system were 4-5-fold greater than those from vapor-pressure osmometry. Bulk spectroscopic properties such as molar absorptivity at 280nm and the quotient, E2/E3, correlated strongly with the total aromaticity and averaged mol. wts. of all the humic solutes. This observation suggests that bulk spectroscopic properties can be applied, as a first approxn., for estg. the size of humic solutes and their aromaticity in natural surface waters.
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    57. 57
      Cory, R. M.; Miller, M. P.; McKnight, D. M.; Guerard, J. J.; Miller, P. L. Effect of instrument-specific response on the analysis of fulvic acid fluorescence spectra. Limnol. Oceanogr.:Methods 2010, 8 (2), 67– 78,  DOI: 10.4319/lom.2010.8.67
      57
      Effect of instrument-specific response on the analysis of fulvic acid fluorescence spectra
      Cory, Rose M.; Miller, Matthew P.; McKnight, Diane M.; Guerard, Jennifer J.; Miller, Penney L.
      Limnology and Oceanography: Methods (2010), 8 (Feb.), 67-78CODEN: LOMIBY; ISSN:1541-5856. (American Society of Limnology and Oceanography)
      Fluorescence spectroscopy has been extensively employed to characterize the source, age, and reactivity of aquatic dissolved org. matter (DOM). However, there is no consensus on the protocols for collecting and correcting DOM fluorescence spectra for the instrument-specific response assocd. with each component on a fluorometer involved in the excitation of DOM and the resulting detection of DOM emission. The central objective of this study was to evaluate the removal of instrument-specific response from DOM fluorescence spectra collected on three different fluorometers using manufacturer-provided emission and excitation correction files. We evaluated criteria and protocols for comparison of removal of instrument response, using quinine sulfate, a well-characterized fluorophore, as well as the International Humic Substance Society's microbially and terrestrially derived ref. end-member fulvic acids: Pony Lake and Suwannee River fulvic acids, resp. Our results demonstrate that sample spectra collected on different fluorometers differed significantly before correction. Although the effectiveness of manufacturer-provided correction factors in removal of instrument response from sample spectra varied by instrument, spectral overlap of the same sample on multiple instruments improved after correction. Our results suggest that conclusions based on anal. of trends within a dataset of DOM fluorescence spectra should be expected to be independent of the fluorometer employed.
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    58. 58
      McKnight, D. M.; Boyer, E. W.; Westerhoff, P. K.; Doran, P. T.; Kulbe, T.; Andersen, D. T. Spectrofluorometric characterization of dissolved organic matter for indication of precursor organic material and aromaticity. Limnol. Oceanogr. 2001, 46 (1), 38– 48,  DOI: 10.4319/lo.2001.46.1.0038
      58
      Spectrofluorometric characterization of dissolved organic matter for indication of precursor organic material and aromaticity
      McKnight, Diane M.; Boyer, Elizabeth W.; Westerhoff, Paul K.; Doran, Peter T.; Kulbe, Thomas; Andersen, Dale T.
      Limnology and Oceanography (2001), 46 (1), 38-48CODEN: LIOCAH; ISSN:0024-3590. (American Society of Limnology and Oceanography)
      We studied the fluorescence properties of fulvic acids isolated from streams and rivers receiving predominantly terrestrial sources of org. material and from lakes with microbial sources of org. material. Microbially derived fulvic acids have fluorophores with a more sharply defined emission peak occurring at lower wavelengths than fluorophores in terrestrially derived fulvic acids. We show that the ratio of the emission intensity at a wavelength of 450 nm to that at 500 nm, obtained with an excitation of 370 nm, can serve as a simple index to distinguish sources of isolated aquatic fulvic acids. In our study, this index is ∼1.9 for microbially derived fulvic acids and ∼1.4 for terrestrially derived fulvic acids. Fulvic acids isolated from 4 large rivers in the US have a fluorescence index of 1.4-1.5, consistent with predominantly terrestrial sources. For fulvic acid samples isolated from a river, lakes, and groundwaters in a forested watershed, the fluorescence index varied in a manner suggesting different sources for the seepage and streamfed lakes. We identified these distinctive fluorophores in filtered whole water samples from lakes in a desert oasis in Antarctica and in filtered whole water samples collected during snowmelt from a Rocky Mountain stream. The fluorescence index measurement in filtered whole water samples in field studies may augment the interpretation of dissolved org. C sources for understanding C cycling in aquatic ecosystems.
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    59. 59
      Schmitt-Kopplin, P.; Hertkorn, N.; Garrison, A. W.; Freitag, D.; Kettrup, A. Influence of Borate Buffers on the Electrophoretic Behavior of Humic Substances in Capillary Zone Electrophoresis. Anal. Chem. 1998, 70 (18), 3798– 3808,  DOI: 10.1021/ac971223j
      59
      Influence of Borate Buffers on the Electrophoretic Behavior of Humic Substances in Capillary Zone Electrophoresis
      Schmitt-Kopplin, Ph.; Hertkorn, N.; Garrison, A. W.; Freitag, D.; Kettrup, A.
      Analytical Chemistry (1998), 70 (18), 3798-3808CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      The influence of tetrahydroxyborate ions on the electrophoretic mobility of humic acids was evaluated by capillary electrophoresis (CE). Depending on the molarity of borate ions in the sepn. buffer, the humic acids exhibit electropherograms with sharp peaks consistently extending from a "humic hump". The variations in the migration times of these peaks depend on the concn. of borate ions in the sepn. buffer. The complexation of borate ions and humic acid fractions was also analyzed with 11B and 1H NMR spectroscopy as well as UV spectrophotometry in solns. of the same compn. as the CE sepn. buffers. Supplementary studies with model compds. (flavonoids, phenolic and sugar acids) indicate reaction mechanisms that include the formation of bidentate esters (monocomplexes) as well as spiranes (tetradentate esters or dicomplexes) within the humic substructure. Special attention must be given to the interpretation of CE electropherograms while fingerprinting humic substances with borate buffers since obsd. peaks do not necessarily indicate distinct humic components but may be artifacts caused by the interaction of the borate ions with the humic substances.
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    60. 60
      McKay, G.; Couch, K. D.; Mezyk, S. P.; Rosario-Ortiz, F. L. Investigation of the coupled effects of molecular weight and charge-transfer interactions on the optical and photochemical properties of dissolved organic matter. Environ. Sci. Technol. 2016, 50 (15), 8093– 8102,  DOI: 10.1021/acs.est.6b02109
      60
      Investigation of the Coupled Effects of Molecular Weight and Charge-Transfer Interactions on the Optical and Photochemical Properties of Dissolved Organic Matter
      McKay, Garrett; Couch, Kylie D.; Mezyk, Stephen P.; Rosario-Ortiz, Fernando L.
      Environmental Science & Technology (2016), 50 (15), 8093-8102CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      We studied the formation of photochem. produced reactive intermediates (RI) from dissolved org. matter (DOM). Specifically, we focused on the effects of variable mol. wt. and chem. redn. on the optical properties of DOM (absorbance and fluorescence) and the formation of singlet oxygen (1O2), DOM triplet excited states (3DOM*), and the hydroxyl radical (•OH). The data are largely evaluated in terms of a charge-transfer (CT) model, but deficiencies in the model to explain the data are pointed out when evident. A total of two sets of samples were studied that were subjected to different treatments; the first set included secondary-treated wastewaters and a wastewater-impacted stream, and the second was a DOM isolate. Treatments included size fractionation and chem. redn. using sodium borohydride. Taken as a whole, the results demonstrate that decreasing mol. wt. and borohydride redn. work in opposition regarding quantum efficiencies for 1O2 and 3DOM* prodn. but in concert for fluorescence and •OH prodn. The optical and photochem. data provide evidence for a limited role of CT interactions occurring in lower-mol.-wt. DOM mols. In addn., the data suggest that the obsd. optical and photochem. properties of DOM are a result of multiple populations of chromophores and that their relative contribution is changed by mol.-wt. fractionation and borohydride redn.
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    61. 61
      Guéguen, C.; Cuss, C. W. Characterization of aquatic dissolved organic matter by asymmetrical flow field-flow fractionation coupled to UV–Visible diode array and excitation emission matrix fluorescence. J. Chromatogr. A 2011, 1218 (27), 4188– 4198,  DOI: 10.1016/j.chroma.2010.12.038
      There is no corresponding record for this reference.
    62. 62
      Korak, J. A.; McKay, G. Critical review of fluorescence and absorbance measurements as surrogates for the molecular weight and aromaticity of dissolved organic matter. Environ. Sci. Process Impacts 2024, 26 (10), 1663– 1702,  DOI: 10.1039/D4EM00183D
      There is no corresponding record for this reference.
    63. 63
      Maizel, A. C.; Remucal, C. K. Molecular Composition and Photochemical Reactivity of Size-Fractionated Dissolved Organic Matter. Environ. Sci. Technol. 2017, 51 (4), 2113– 2123,  DOI: 10.1021/acs.est.6b05140
      63
      Molecular Composition and Photochemical Reactivity of Size-Fractionated Dissolved Organic Matter
      Maizel, Andrew C.; Remucal, Christina K.
      Environmental Science & Technology (2017), 51 (4), 2113-2123CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The photochem. prodn. of reactive species, such as triplet dissolved org. matter (3DOM) and singlet oxygen (1O2), contributes to the degrdn. of aquatic contaminants and is related to an array of DOM structural characteristics, notably mol. wt. In order to relate DOM mol. wt., optical properties, and reactive species prodn., Suwannee River (SRFA) and Pony Lake fulvic acid (PLFA) isolates are fractionated by sequential ultrafiltration, and the resultant fractions are evaluated in terms of mol. compn. and photochem. reactivity. UV-visible measurements of aromaticity increase with mol. wt. in both fulvic acids, while PLFA mol. wt. fractions are shown to be structurally similar by Fourier-transform ion cyclotron resonance mass spectrometry. In addn., Bray-Curtis dissimilarity anal. of formulas identified in the isolates and their size fractions reveal that SRFA and PLFA have distinct mol. compns. Quantum yields of 3DOM, measured by electron and energy transfer probes, and 1O2 decreased with mol. wt. Decreasing [3DOM]ss with mol. wt. is shown to derive from elevated quenching in high mol. wt. fractions, rather than increased 3DOM formation. This work has implications for the photochem. of waters undergoing natural or engineered treatment processes that alter DOM mol. wt., such as photooxidn. and biol. degrdn.
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    64. 64
      Mostafa, S.; Korak, J. A.; Shimabuku, K.; Glover, C. M.; Rosario-Ortiz, F. L. Relation between optical properties and Formation of reactive intermediates from different size fractions of organic matter. In Advances in the Physicochemical Characterization of Dissolved Organic Matter: Impact on Natural and Engineered Systems; American Chemical Society: Washington, DC, 2014; Vol. 1160, pp 159– 179.
      There is no corresponding record for this reference.
    65. 65
      Lin, H.; Guo, L. Variations in Colloidal DOM Composition with Molecular Weight within Individual Water Samples as Characterized by Flow Field-Flow Fractionation and EEM-PARAFAC Analysis. Environ. Sci. Technol. 2020, 54 (3), 1657– 1667,  DOI: 10.1021/acs.est.9b07123
      65
      Variations in Colloidal DOM Composition with Molecular Weight within Individual Water Samples as Characterized by Flow Field-Flow Fractionation and EEM-PARAFAC Analysis
      Lin, Hui; Guo, Laodong
      Environmental Science & Technology (2020), 54 (3), 1657-1667CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Fluorescence excitation emission matrixes (EEM) and parallel factor (PARAFAC) anal. have been widely used in the characterization of dissolved org. matter (DOM) in the aquatic continuum. However, large sample sets are typically needed for establishing a meaningful EEM-PARAFAC model. Applications of the EEM-PARAFAC technique to individual samples require new approaches. Here, flow field-flow fractionation (FlFFF) combined with offline EEM measurements and PARAFAC anal. was used to elucidate the dynamic changes in DOM compn./optical properties with mol. wt. within individual samples. FlFFF-derived size spectra of ultrafiltration-isolated colloidal DOM show that peak-C related fluorophores (Ex/Em= 350/450 nm) are present mostly in the 1-3 kDa size range, while peak-T assocd. fluorophores (Ex/Em = 275/340 nm) have a bimodal distribution with peaks in both the 1-3 kDa and the >100 kDa size fractions. The integrated EEM spectra from FlFFF size-fractionated subsamples closely resembled the bulk EEM spectra, attesting to the convincing comparability between bulk and FlFFF size-fractionated EEMs. The PARAFAC-derived DOM components are distinctive among individual samples with the predominant components being humic-like in river water, but protein-like in a highly eutrophic lagoon sample. This compelling new approach combining FlFFF and EEM-PARAFAC can be used to decipher the dynamic changes in size spectra and compn. of individual DOM samples from sources to sinks or across the redox/hydrol./trophic interfaces.
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    66. 66
      Peuravuori, J.; Pihlaja, K. Preliminary Study of Lake Dissolved Organic Matter in Light of Nanoscale Supramolecular Assembly. Environ. Sci. Technol. 2004, 38 (22), 5958– 5967,  DOI: 10.1021/es040041l
      66
      Preliminary study of lake dissolved organic matter in light of nanoscale supramolecular assembly
      Peuravuori, Juhani; Pihlaja, Kalevi
      Environmental Science and Technology (2004), 38 (22), 5958-5967CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Large-scale preparative high-performance size-exclusion chromatog. (HPSEC) was performed to sep. different mol. size fractions in milligram quantities from strongly colored dissolved org. matter (DOM) of a freshwater using a very mild conjugate acid-base pair (10 mM acetic acid-sodium acetate soln. at pH 7.0 with an ionic strength of 6 × 10-5) as the mobile phase. The homogeneity-uniformity of different mol. size fractions in relation to their combined original mixt. was verified by an anal. HPSEC system. In addn. to mol. size distribution and basic spectroscopic characteristics, Fourier transform IR spectroscopy was applied to specify structural features for different size fractions. The results demonstrate clearly that only a very small amt. of conjugate org. acid-base pair is required to generate a powerful resoln. for a DOM mixt., and very strong treatments with org. acids are not necessarily needed to reach a better SEC resoln. Most essential is the combined outcome of different HPSEC expts. and detd. structural functionalities which indicate that almost all original DOM solutes are aggregated mixts. consisting of different assocns. possessing various mol. size ranges, which can be sepd. from their integrated whole as nearly homogeneous and uniform species. In summary, the present study strongly speaks for the need to direct the research of natural dissolved and colloidal org. carbon more strongly toward a nanoscale study of supramol. assemblies. More precise knowledge about the primary, secondary, and tertiary structure of dissolved DOM constituents has its essential function, e.g., for environmental protection and utilization of surface waters.
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    67. 67
      Korak, J. A.; McKay, G. Meta-Analysis of Optical Surrogates for the Characterization of Dissolved Organic Matter. Environ. Sci. Technol. 2024, 58 (17), 7380– 7392,  DOI: 10.1021/acs.est.3c10627
      There is no corresponding record for this reference.
    68. 68
      Korak, J. A.; Dotson, A. D.; Summers, R. S.; Rosario-Ortiz, F. L. Critical analysis of commonly used fluorescence metrics to characterize dissolved organic matter. Water Res. 2014, 49, 327– 338,  DOI: 10.1016/j.watres.2013.11.025
      68
      Critical analysis of commonly used fluorescence metrics to characterize dissolved organic matter
      Korak, Julie A.; Dotson, Aaron D.; Summers, R. Scott; Rosario-Ortiz, Fernando L.
      Water Research (2014), 49 (), 327-338CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      The use of fluorescence spectroscopy for the anal. and characterization of dissolved org. matter (DOM) has gained widespread interest over the past decade, in part because of its ease of use and ability to provide bulk DOM chem. characteristics. However, the lack of std. approaches for anal. and data evaluation has complicated its use. This study utilized comparative statistics to systematically evaluate commonly used fluorescence metrics for DOM characterization to provide insight into the implications for data anal. and interpretation such as peak picking methods, carbon-normalized metrics and the fluorescence index (FI). The uncertainty assocd. with peak picking methods was evaluated, including the reporting of peak intensity and peak position. The linear relationship between fluorescence intensity and dissolved org. carbon (DOC) concn. was found to deviate from linearity at environmentally relevant concns. and simultaneously across all peak regions. Comparative anal. suggests that the loss of linearity is compn. specific and likely due to non-ideal intermol. interactions of the DOM rather than the inner filter effects. For some DOM sources, Peak A deviated from linearity at optical densities a factor of 2 higher than that of Peak C. For carbon-normalized fluorescence intensities, the error assocd. with DOC measurements significantly decreases the ability to distinguish compositional differences. An in-depth anal. of FI detd. that the metric is mostly driven by peak emission wavelength and less by emission spectra slope. This study also demonstrates that fluorescence intensity follows property balance principles, but the fluorescence index does not.
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    69. 69
      McKay, G.; Hohner, A. K.; Rosario-Ortiz, F. L. Use of optical properties for evaluating the presence of pyrogenic organic matter in thermally altered soil leachates. Environ. Sci.:Processes Impacts 2020, 22, 981– 992,  DOI: 10.1039/C9EM00413K
      69
      Use of optical properties for evaluating the presence of pyrogenic organic matter in thermally altered soil leachates
      McKay, Garrett; Hohner, Amanda K.; Rosario-Ortiz, Fernando L.
      Environmental Science: Processes & Impacts (2020), 22 (4), 981-992CODEN: ESPICZ; ISSN:2050-7895. (Royal Society of Chemistry)
      The increased frequency and severity of wildfires in forested watersheds has the potential to significantly impact the quantity and quality of water extractable org. matter (WEOM) exported from these ecosystems. This study examd. the optical properties of WEOM from lab. heated soil in order to understand physicochem. changes occurring in the org. matter as a result of heating, as well as test the usefulness of optical parameters for assessing the presence of pyrogenic org. matter. WEOM absorbance and fluorescence spectral shape and intensity varied systematically as a function of soil heating temp. Notably, absorbance and fluorescence intensity, specific UV absorbance, apparent fluorescence quantum yield, specific fluorescence emission intensity, and max. fluorescence emission wavelength exhibited consistent changes with heating temp. and indicated that WEOM in heated soil leachates was lower in mol. wt. and more arom. than in unheated samples. The lower mol. wt. in heated soil WEOM was corroborated with size-exclusion chromatog. measurements. This work increases the understanding of the mol. changes occurring in WEOM as a result of wildfire and indicates that optical measurements (i.e., absorbance and fluorescence) could be used for watershed monitoring of post-fire pyrogenic org. matter.
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    70. 70
      Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular Photochemistry of Organic Molecules; University Science Books: Sausalito, CA, 2012.
      There is no corresponding record for this reference.
    71. 71
      Li, H.; McKay, G. Fluorescence Quenching of Humic Substances and Natural Organic Matter by Nitroxide Free Radicals. Environ. Sci. Technol. 2023, 57 (1), 719– 729,  DOI: 10.1021/acs.est.2c02220
      71
      Fluorescence Quenching of Humic Substances and Natural Organic Matter by Nitroxide Free Radicals
      Li, Hang; McKay, Garrett
      Environmental Science & Technology (2023), 57 (1), 719-729CODEN: ESTHAG; ISSN:1520-5851. (American Chemical Society)
      Fluorescence spectroscopy is one of the most frequently used techniques for studying dissolved org. matter (DOM) in natural and engineered systems. However, the spatial distribution and fluorophores, including local and interacting states, within DOM's larger structure remains poorly understood. In this study, we used two nitroxide fluorescence quenchers to evaluate the chem. and spatial heterogeneity of DOM fluorophores. Several results from quenching expts. with cationic 4-amino-TEMPO (tempamine), including downward-curving Stern-Volmer plots and spectral dependent quenching, show that multiple emitting species contribute to the obsd. emission even at a single excitation wavelength. Furthermore, for DOM isolates of diverse geog. origins (soil vs aquatic) and isolation procedures (reverse osmosis vs humic substances), the max. extent of quenching occurs on the red edge of the emission spectra. For soil humic substance isolates, the spectral dependent quenching was significant enough to affect a blue shift in the av. emission wavelength. The same soil humic substance isolates whose emission spectra were blue shifted by tempamine quenching were also blue shifted by decreasing soln. pH and decreasing solvent polarity, which suggests a role for anionic fluorophores (e.g., hydroxybenzoic acids) in long wavelength fluorescence. Finally, curvature in Stern-Volmer plots indicate that between 10 and 50% of emitting species detected by steady-state fluorescence are inaccessible to quenching by tempamine, suggesting that this fraction of fluorophores may be inaccessible to water solvent. Results from this study provide an assessment of the spatial distribution of fluorophores within DOM and help to reconcile prior studies on the role of solvent polarity and pH on DOM fluorescence.
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    72. 72
      Wert, E. C.; Gonzales, S.; Dong, M. M.; Rosario-Ortiz, F. L. Evaluation of enhanced coagulation pretreatment to improve ozone oxidation efficiency in wastewater. Water Res. 2011, 45 (16), 5191– 5199,  DOI: 10.1016/j.watres.2011.07.021
      There is no corresponding record for this reference.
    73. 73
      Buckley, S.; Leresche, F.; Hanson, B.; Rosario-Ortiz, F. L. Decoupling Optical Response and Photochemical Formation of Singlet Oxygen in Size Isolated Fractions of Ozonated Dissolved Organic Matter. Environ. Sci. Technol. 2023, 57 (14), 5603– 5610,  DOI: 10.1021/acs.est.2c08155
      There is no corresponding record for this reference.
    74. 74
      Guo, R.; Ma, J. Reduction-induced molecular signature of humic substances: structural evidence for optical changes. RSC Adv. 2014, 4 (49), 25880,  DOI: 10.1039/c4ra03051f
      74
      Reduction-induced molecular signature of humic substances: structural evidence for optical changes
      Guo, Rongrong; Ma, Jiahai
      RSC Advances (2014), 4 (49), 25880-25885CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      The redox behavior of humic substances (HS) has drawn significant attention. It has been recently reported that aquatic HS after NaBH4 redn. exhibit a loss in UV absorption but enhanced and blue-shifted fluorescence emission. This unique property was proven to also apply to terrestrial HS in the current study. To further understand the underlying relationship between the mol. structural changes and the optical changes, multiple techniques were employed to obtain solid spectroscopic evidence for Aldrich humic acid. Attenuated total reflectance IR spectroscopy, Raman spectroscopy and solid-state 13C NMR spectroscopy were used together to demonstrate that the alc.-like moieties increased while the carbonyl moieties decreased in HS mols. during redn., which is responsible for optical properties. In addn., the obtained results could explain the change in mol. size as evidenced by dynamic light scattering. The integrated results presented in this study unambiguously provide solid spectroscopic evidence for the charge transfer model, which is a relatively new concept framework for understanding HS.
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    75. 75
      Malcolm, R. L.; Maccarthy, P. Limitations in the Use of Commercial Humic Acids in Water and Soil Research. Environ. Sci. Technol. 1986, 20 (9), 904– 911,  DOI: 10.1021/es00151a009
      75
      Limitations in the use of commercial humic acids in water and soil research
      Malcolm, Ronald L.; MacCarthy, Patrick
      Environmental Science and Technology (1986), 20 (9), 904-11CODEN: ESTHAG; ISSN:0013-936X.
      Seven samples of com. humic acids, purchased from 5 different suppliers, were studied, and their characteristics were compared with humic and fulvic acids isolated from streams, soils, peat, leonardite, and a dopplerite sample. Cross-polarization and magic-angle spinning 13C NMR spectroscopy clearly showed pronounced differences between the com. materials and all other samples. Elemental and IR spectroscopic data do not show such clear-cut differences but can be used as supportive evidence, with the 13C NMR data, to substantiate the above distinctions. As a result of these differences and due to the general lack of information relating to the source, method of isolation, or other pretreatment of the com. materials, these com. products are not considered to be appropriate for use as analogs of true soil and water humic substances, in expts. designed to evaluate the nature and reactivity of humic substances in natural waters and soils.
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    76. 76
      Xue, J.-P.; Cuss, C. W.; Wang, Y.; Javed, M. B.; Noernberg, T.; Pelletier, R.; Shotyk, W. Size-Resolved Fluorescence Underscores Negligible Interaction of Dissolved Organic Matter During Conservative Mixing in a Large Boreal River. Front. Environ. Chem. 2022, 3, 937721  DOI: 10.3389/fenvc.2022.937721
      There is no corresponding record for this reference.
    77. 77
      Gao, Z.; Guéguen, C. Size distribution of absorbing and fluorescing DOM in Beaufort Sea, Canada Basin. Deep Sea Res., Part I 2017, 121, 30– 37,  DOI: 10.1016/j.dsr.2016.12.014
      There is no corresponding record for this reference.
    78. 78
      Cuss, C. W.; Guéguen, C. Relationships between molecular weight and fluorescence properties for size-fractionated dissolved organic matter from fresh and aged sources. Water Res. 2015, 68, 487– 497,  DOI: 10.1016/j.watres.2014.10.013
      78
      Relationships between molecular weight and fluorescence properties for size-fractionated dissolved organic matter from fresh and aged sources
      Cuss, C. W.; Gueguen, C.
      Water Research (2015), 68 (), 487-497CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      Relationships between the mol. wt. (MW) and fluorescence properties of dissolved org. matter (DOM) are important considerations for studies seeking to connect these properties to water treatment processes. Relationships between the size and fluorescence properties of nine allochthonous DOM sources (i.e. leaf leachates, grass, and headwaters) were measured using asym. flow field-flow fractionation (AF4) with online absorbance and fluorescence detectors. Correlations between optical properties and MW were readily apparent using parallel factor anal. (PARAFAC) coupled to self-organizing maps (SOM): protein/polyphenol-like fluorescence (peaks B and T) was highest at lower mol. wts. (<0.5 kDa), fulvic/humic-like fluorescence (peaks A, C, and M) was highest at mid-wts. (0.5-1 kDa), and humic-like fluorescence (Peaks A + C) was highest at larger mol. wts. (>1 kDa). Proportions of peaks B, T, and A + C were significantly correlated with MW (p < 0.001). The first principal component (PC1, 42% of variation in fluorescence properties) was a significant predictor of sample MW (R2 = 0.63, p < 0.05), while scores on PC2 (27% of total variance) traced a source-based gradient from deciduous leachates/headwaters through to coniferous leachates/headwaters. PC3 (13% of var.) was also correlated with MW (p < 0.005). A secondary peak in peak T fluorescence was assocd. with larger size fractions in aged sources, and scores on PC1 also traced a path from the leachates of fresher leaves, through more humified leaves, to headwaters. Findings are consistent with the hypothesis that the structure of aged DOM arises through supramol. assembly.
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    79. 79
      Floge, S. A.; Wells, M. L. Variation in colloidal chromophoric dissolved organic matter in the Damariscotta Estuary, Maine. Limnol. Oceanogr. 2007, 52 (1), 32– 45,  DOI: 10.4319/lo.2007.52.1.0032
      There is no corresponding record for this reference.
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      Zanardi-Lamardo, E.; Moore, C. A.; Zika, R. G. Seasonal variation in molecular mass and optical properties of chromophoric dissolved organic material in coastal waters of southwest Florida. Mar. Chem. 2004, 89 (1–4), 37– 54,  DOI: 10.1016/j.marchem.2004.02.018
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      Wu, F. C.; Evans, R. D.; Dillon, P. J. Separation and Characterization of NOM by High-Performance Liquid Chromatography and On-Line Three-Dimensional Excitation Emission Matrix Fluorescence Detection. Environ. Sci. Technol. 2003, 37 (16), 3687– 3693,  DOI: 10.1021/es020244e
      81
      Separation and Characterization of NOM by High-Performance Liquid Chromatography and On-Line Three-Dimensional Excitation Emission Matrix Fluorescence Detection
      Wu, F. C.; Evans, R. D.; Dillon, P. J.
      Environmental Science and Technology (2003), 37 (16), 3687-3693CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      By using high-performance size exclusion chromatog. (HPSEC) and reversed-phase high-performance liq. chromatog. with online three-dimensional excitation emission matrix fluorescence detection, we measured fluorescence properties of natural org. matter (NOM) as a function of mol. size (MS) and polarity. The work was carried out with Suwannee River Fulvic Acid, Aldrich Humic Acid, and a naturally occurring river NOM sample. Significant differences in fluorescence max. pattern were found as NOM was sepd. chromatog. based on MS and polarity. There existed a strong relationship between MS, fluorescence pattern, and polarity. Humic-, fulvic-, and protein-like fluorescence fractions had distinct hydrophilic/hydrophobic nature. The results suggest that HPSEC may be better for characterizing major fulvic-like fluorescence and smaller MS fractions but not those having humic- and protein-like fluorescence and larger MS, which may be strongly adsorbed onto the HPSEC column because of their hydrophobic nature. This study has significant implications for further understanding the nature of NOM and its complexation with trace metals.
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      Her, N.; Amy, G.; McKnight, D.; Sohn, J.; Yoon, Y. Characterization of DOM as a function of MW by fluorescence EEM and HPLC-SEC using UVA, DOC, and fluorescence detection. Water Res. 2003, 37 (17), 4295– 4303,  DOI: 10.1016/S0043-1354(03)00317-8
      There is no corresponding record for this reference.

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