Bibliometric Mapping: Eight Decades of Analytical Chemistry, With Special Focus on the Use of Mass SpectrometryClick to copy article linkArticle link copied!
In this Feature we use automatic bibliometric mapping tools to visualize the history of analytical chemistry from the 1920s until the present. In particular, we have focused on the application of mass spectrometry in different fields. The analysis shows major shifts in research focus and use of mass spectrometry. We conclude by discussing the application of bibliometric mapping and visualization tools in analytical chemists’ research.
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Evolution of Topics in Analytical Chemistry 1929–2012
Bibliometrics | The quantitative study of literatures as they are reflected in bibliographies (22) |
Citation network | Network of citation relations between items (e.g., publications, authors or journals) |
CiteSpaceII | Software tool developed by Chen for “detecting and visualizing emerging trends and transient patterns in scientific literature” (5) |
CitNetExplorer | Software tool developed by Van Eck and Waltman “for visualizing and analyzing citation networks of scientific publications” (4) |
Mapping | Positioning of a subset of the publications in a citation network (usually selected based on their citation frequency) in a two-dimensional map in which the vertical dimension indicates time (i.e., the year of publication) and the horizontal dimension indicates the closeness of publications in the citation network. |
Clustering | Partitioning of the publications in a citation network into a number of groups (clusters). Publications assigned to the same group are closely connected to each other in the citation network. |
Co-word map | Map of words (or terms), usually extracted from the titles and abstracts of scientific publications, showing the co-occurrence relations of the words (i.e., the number of publications in which two words occur together). |
HistCite | Software tool developed by Eugene Garfield to “generate chronological maps” of scientific literature based on WoS input (3) |
Sci2 | Software tool developed by a team led by Börner and Boyack that “is a modular toolset specifically designed for the study of science. It supports the temporal, geospatial, topical, and network analysis and visualization of scholarly datasets at the micro (individual), meso (local), and macro (global) levels.” |
VOSviewer | Software tool developed by Van Eck and Waltman “for analyzing bibliometric networks”, (9) in particular networks based on citation and co-occurrence relations |
Mapping | Positioning of the items in a network in a two-dimensional map in such a way that strongly connected items tend to be located close to each other while weakly connected items tend to be located further away from each other. The horizontal and vertical axes have no special meaning. Only the relative distances between items carry meaning in a map. |
Clustering | Partitioning of the items in a network into a number of groups (clusters). Items assigned to the same group are closely connected to each other in the network. |
Web of Science (WoS) | Multidisciplinary bibliographic database produced by Thomson Reuters |
Color | Description | Color | Description | Color | Description | Color | Description |
---|---|---|---|---|---|---|---|
1929–1940 | 1941–1950 | 1951–1960 | 1961–1970 | ||||
Green | Apparatuses | Green | Apparatuses | Cyan | Chromatography | Cyan | Chromatography |
Pink | Gases | Pink | Inorganic chemistry: gases/halogens | Sea green | Electrochemistry | Red | Inorganic chemistry |
Red | Inorganic chemistry | Red | Inorganic chemistry: metals | Red | Inorganic chemistry: metals | Sea green | Electrochemistry |
Cyan | Industrial applications, hydrocarbons and food | Dark blue | Organic and food chemistry | Green | Apparatuses | Yellow | General/editorial and ″informatics″ |
Yellow | General/editorial | Yellow | General/editorial | ||||
Cyan | Industrial applications and hydrocarbons | ||||||
1971–1980 | 1981–1990 | 1991–2000 | 2001–2012 | ||||
Cyan | Chromatography | Yellow | General/editorial | Cyan | Chromatography | Sea green | Detection, electrochemistry and (bio)sensors |
Red | Inorganic chemistry | Sea green | Electrochemistry | Purple | Electrophoresis | Brown | Small molecules and quantitation |
Sea green | Electrochemistry | Red | Inorganic chemistry | Sea green | Inorganic chemistry, electrochemistry and (bio)sensors | Blue | Mass spectrometry |
Yellow | General/editorial | Cyan | Chromatography | Yellow | General/editorial | Mustard | Separations, microfluidics, and theory and simulations |
Green | Apparatuses | Blue | Mass spectrometry | Blue | Mass spectrometry and proteomics | ||
Pink | Gases |
Development and Use of Mass Spectrometry
Color | Description | Color | Description | Color | Description |
---|---|---|---|---|---|
1941–1960 | 1961–1970 | 1971–1980 | |||
Yellow | General and editorial | Mustard | Software | Cyan | Chromatography |
Purple | Hydrocarbons | Green-brown | Sample preparation, separations and derivatization | Brown | Compound quantification and secondary ion MS |
Dark blue | Structural analysis | Purple | Hydrocarbons and organic chemistry | Green | Apparatuses and interfaces (incl. informatics) |
Brown | Quantitation | Green | Apparatuses and interfaces | Sea green | Chemical ionization |
Pink | Gases | Blue | General MS | ||
Gray | Nondiscernible | Red | Inorganic chemistry, metals and isotope ratio MS | ||
Yellow | Editorial | ||||
1981–1990 | 1991–2000 | 2001–2012 | |||
Brown | Compound quantification | Dark blue | MALDI-TOF and proteomics | Blue | MALDI and imaging mass spectrometry |
Cyan | Chromatography | Purple | Chromatography, quantitation and isotope ratio MS | Sea green | Direct analysis (DART etc.), ESI and ICPMS |
Sea green | Chemical ionization | Red | SIMS, surfaces and polymers | Brown | Quantitation (GCMS, LCMS) |
Pink | FAB, FD/mass analyzers and MS/MS | Sea green | Electrospray ionization, quadrupoles, ion traps, FTICR and MS/MS | Green-brown | Sample preparation (labeling, enrichment, purification) |
Green-brown | Secondary ion mass spectrometry, laser desorption and plasma desorption | Dark blue | Proteomics | ||
Gray | Nondiscernible |
Conclusions
Bibliometric Visualization Tools for Your Own Research
Supporting Information
Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
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Biographies
Acknowledgment
We would like to express our gratitude to the American Chemical Society for making data available for this study and technical support. In particular, we would like to thank Catherine Boylan, Emma Moore, David Martinsen, and Jeffrey Krugman. We also thank Rob Marissen (LUMC) and Bjorn Victor (Institute for Tropical Medicine, Antwerp) for technical assistance. Finally, we would like to thank Nees Jan van Eck and Ludo Waltman (both CWTS) and Michael Grayson for fruitful discussions.
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- 30Guenther, A.; Hewitt, C. N.; Erickson, D.; Fall, R.; Geron, C.; Graedel, T.; Harley, P.; Klinger, L.; Lerdau, M.; Mckay, W. A.; Pierce, T.; Scholes, B.; Steinbrecher, R.; Tallamraju, R.; Taylor, J.; Zimmerman, P. J. Geophys. Res.: Atmos. 1995, 100, 8873– 8892Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXmvFKrsb0%253D&md5=d0afe3951cf60f195540d8d0072599afA global model of natural volatile organic compound emissionsGuenther, Alex; Hewitt, C. Nicholas; Erickson, David; Fall, Ray; Geron, Chris; Graedel, Tom; Harley, Peter; Klinger, Lee; Lerdau, Manuel; et al.Journal of Geophysical Research, [Atmospheres] (1995), 100 (D5), 8873-92CODEN: JGRDE3 ISSN:. (American Geophysical Union)Numerical assessments of global air quality and potential changes in atm. chem. constituents require ests. of the surface fluxes of a variety of trace gas species. A global model is developed to est. emissions of volatile org. compds. from natural sources (NVOC). Methane is not considered here and has been described in detail elsewhere. The model has a highly resolved spatial grid (0.5° × 0.5° latitude/longitude) and generates hourly av. emission ests. Chem. species are grouped into four categories: isoprene, monoterpenes, other reactive VOC (ORVOC), and other VOC (OVOC). NVOC emissions from oceans are estd. as a function of geophys. variables from a general circulation model and ocean color satellite data. Emissions from plant foliage are estd. from ecosystem specific biomass and emission factors and algorithms describing light and temp. dependence of NVOC emissions. Foliar d. ests. are based on climatic variables and satellite data. Temporal variations in the model are driven by monthly ests. of biomass and temp. and hourly light ests. The annual global VOC flux is estd. to be 1150 Tg C, composed of 44% isoprene, 11% monoterpenes, 22.5% other reactive VOC, and 22.5% other VOC. Large uncertainties exist for each of these ests. and particularly for compds. other than isoprene and monoterpenes. Tropical woodlands (rain forest, seasonal, drought-deciduous, and savanna) contribute about half of all global natural VOC emissions. Croplands, shrublands and other woodlands contribute 10-20% apiece. Isoprene emissions calcd. for temperate regions are as much as a factor of 5 higher than previous ests.
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- 23Laemmli, U. K. Nature 1970, 227, 680– 68523https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXlsFags7s%253D&md5=fff3e668784b8bb3669f854be60a216bCleavage of structural proteins during the assembly of the head of bacteriophage T4Laemmli, U. K.Nature (London, United Kingdom) (1970), 227 (5259), 680-685CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)Using an improved method of polyacrylamide gel electrophoresis based on the capability of SDS to break down proteins into their individual polypeptide chains, many previously unknown proteins have been found in bacteriophage T4 and some of these have been identified with specific gene products. Four major components of the head are cleaved during the process of assembly, apparently after the precursor proteins have assembled into some large intermediate structure.
- 24Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Matsuo, T. Rapid Commun. Mass Spectrom. 1988, 2, 151– 15324https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK3cXjtVSgug%253D%253D&md5=91ab5c36befd5eacb4910b3fadec653fProtein and polymer analyses up to m/z 100,000 by laser ionization time-of-flight mass spectrometryTanaka, Koichi; Waki, Hiroaki; Ido, Yutaka; Akita, Satoshi; Yoshida, Yoshikazu; Yohida, TamioRapid Communications in Mass Spectrometry (1988), 2 (8), 151-3CODEN: RCMSEF; ISSN:0951-4198.Typical spectra of proteins and polymers were obtained by using a laser ionization time-of-flight mass spectrometer to assess the utility of the spectrometer for high masses.
- 25Arthur, C. L.; Pawliszyn, J. Anal. Chem. 1990, 62, 2145– 214825https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK3cXlsVCmt7k%253D&md5=fbf080ede7c55f6345f2891c18b5f93eSolid phase microextraction with thermal desorption using fused silica optical fibersArthur, Catherine L.; Pawliszyn, JanuszAnalytical Chemistry (1990), 62 (19), 2145-8CODEN: ANCHAM; ISSN:0003-2700.Chem. modified optical fibers allow convenient, in situ extn. of org. contaminants from aq. and water samples and their rapid transfer to a capillary column. The total time of extn. and gas chromatog. anal. is only a few minutes. No solvents are used in the desorption process. The technique involves coating the fiber with an appropriate stationary phase and exposing the coating to an aq. sample. Sample analytes partition into the stationary phase. Exposure times range from 1 to 2 min. The analytes are thermally desorbed on-column in the injector of a gas chromatograph for sepn. and anal. In contrast to traditional, solid phase extns. the proposed technique relies on the formation of an equil. between the sample and the stationary phase, rather than an exhaustive extn. A linear working range can be obtained as expected from partition theory. For example, in the anal. for light chlorinated hydrocarbons a linear range of 0.1-1 ppb was obtained. Selectivity of the extn. can be obtained by either using the appropriate stationary phase or by modifying the compn. of the aq. samples with an org. solvent.
- 26Matuszewski, B. K.; Constanzer, M. L.; Chavez-Eng, C. M. Anal. Chem. 2003, 75, 3019– 303026https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXltV2jsL8%253D&md5=614e9fac9418be1ca29a013e6331f7ddStrategies for the assessment of matrix effect in quantitative bioanalytical methods based on HPLC-MS/MSMatuszewski, B. K.; Constanzer, M. L.; Chavez-Eng, C. M.Analytical Chemistry (2003), 75 (13), 3019-3030CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)In recent years, high-performance liq. chromatog. (HPLC) with tandem mass spectrometric (MS/MS) detection has been demonstrated to be a powerful technique for the quant. detn. of drugs and metabolites in biol. fluids. However, the common and early perception that utilization of HPLC-MS/MS practically guarantees selectivity is being challenged by a no. of reported examples of lack of selectivity due to ion suppression or enhancement caused by the sample matrix and interferences from metabolites. In light of these serious method liabilities, questions about how to develop and validate reliable HPLC-MS/MS methods, esp. for supporting long-term human pharmacokinetic studies, are being raised. The central issue is what expts., in addn. to the validation data usually provided for the conventional bioanal. methods, need to be conducted to confirm HPLC-MS/MS assay selectivity and reliability. The current regulatory requirements include the need for the assessment and elimination of the matrix effect in the bioanal. methods, but the exptl. procedures necessary to assess the matrix effect are not detailed. Practical, exptl. approaches for studying, identifying, and eliminating the effect of matrix on the results of quant. analyses by HPLC-MS/MS are described in this paper. Using as an example a set of validation expts. performed for one of our investigational new drug candidates, the concepts of the quant. assessment of the "abs." vs. "relative" matrix effect are introduced. In addn., expts. for the detn. of, the "true" recovery of analytes using HPLC-MS/MS are described eliminating the uncertainty about the effect of matrix on the detn. of this commonly measured method parameter. Detn. of the matrix effect allows the assessment of the reliability and selectivity of an existing HPLC-MS/MS method. If the results of these studies are not satisfactory, the parameters detd. may provide a guide to what changes in the method need to be made to improve assay selectivity. In addn., a direct comparison of the extent of the matrix effect using two different interfaces (a heated nebulizer, HN, and ion spray, ISP) under otherwise the same sample prepn. and chromatog. conditions was made. It was demonstrated that, for the investigational drug under study, the matrix effect was clearly obsd. when ISP interface was utilized but it was absent when the HN interface was employed.
- 27Maréchal, C. N.; Télouk, P.; Albarède, F. Chem. Geol. 1999, 156, 251– 27327https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1MXhslOjurs%253D&md5=a97de3e83766a7c2a3217fcad9189395Precise analysis of copper and zinc isotopic compositions by plasma-source mass spectrometryMarechal, Chloe Nadia; Telouk, Philippe; Albarede, FrancisChemical Geology (1999), 156 (1-4), 251-273CODEN: CHGEAD; ISSN:0009-2541. (Elsevier Science B.V.)The stable isotope geochem. of Cu and Zn is poorly known because of the lack of a suitable anal. technique. We present a procedure for the anal. of Cu and Zn isotope compns. by plasma-source mass spectrometry (Plasma 54) together with a method to purify Cu and Zn from natural samples of silicates, ores, and biol. material. A plasma-source mass spectrometer equipped with a magnetic filter and multiple collection can make up for the instability of the ICP source and provide precise Cu and Zn isotope compns. Instrumental mass fractionation is cor. with respect to the isotopic compn. of a std. of a different element added to the sample (Zn for a Cu sample, Cu for a Zn sample) previously purified by anion-exchange chem. We have adopted the NIST Cu std. (SRM 976) and a Johnson-Mattey Zn soln. as refs. This external normalization leads to an internal precision of 20 ppm and an external reproducibility of 40 ppm or 0.04 per mil (95% confidence level). The isotopic compns. can be obtained on as little as 200 ng of element. Isobaric interferences are small enough to be neglected. Isotopic fractionation is obsd. for Cu on the anion-exchange resin, which requires a full yield to be achieved upon purifn. The exponential law of mass fractionation is shown to provide a more consistent correction than the linear and the power law. It is shown that in the mass spectrometer Cu and Zn isotopes do not fractionate to the same extent. The ratio of instrumental mass biases remains const. over one measurement session. This ratio is detd. from the anal. of mixed std. solns., then is used to correct the isotopic compn. measured for Cu and Zn samples. Some preliminary results show the existence of isotopic variations of up to several per mil amongst natural samples of silicates, ores, sediments, and biol. material, which paves the way for the use of Cu and Zn isotopes as geochem. and biochem. tracers.
- 28Bligh, E. G.; Dyer, W. J. Can. J. Biochem. Physiol. 1959, 37, 911– 91728https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaG1MXhtVSgt70%253D&md5=5a4deec3e7679aae43f391d79034c81cA rapid method of total lipide extraction and purificationBligh, E. G.; Dyer, W. J.Canadian Journal of Biochemistry and Physiology (1959), 37 (), 911-17CODEN: CJBPAZ; ISSN:0576-5544.The wet tissue is homogenized with a mixt. of CHCl3 and MeOH to form a miscible system with the H2O in the tissue. Diln. with CHCl3 and H2O seps. the homogenate into 2 layers, the CHCl3 layer contg. all the lipides and the methanolic layer contg. all the non-lipides. A purified lipide ext. is obtained merely by isolating the CHCl3 layer. The method has been applied to fish muscle and may easily be adapted to use with other tissues.
- 29van den Dool, H.; Kratz, P. D. J. Chromatogr. 1963, 11, 463– 47129https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaF2cXmtV2l&md5=ae68b96eb94d5182f4bceac83804aba8A generalization of the retention index system including linear temperature programmed gas-liquid partition chromatographyVan den Dool, H.; Kratz, P. D.Journal of Chromatography (1963), 11 (4), 463-71CODEN: JOCRAM; ISSN:0021-9673.Calcd. retention indexes are tabulated for numerous esters chromatographed by temp. programming with 25% Silicone Rubber SE 30 and 25% Carbowax 20M on Celite. A modified Kovats equation (CA 53, 8765g) is used to det. retention indexes.
- 30Guenther, A.; Hewitt, C. N.; Erickson, D.; Fall, R.; Geron, C.; Graedel, T.; Harley, P.; Klinger, L.; Lerdau, M.; Mckay, W. A.; Pierce, T.; Scholes, B.; Steinbrecher, R.; Tallamraju, R.; Taylor, J.; Zimmerman, P. J. Geophys. Res.: Atmos. 1995, 100, 8873– 889230https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXmvFKrsb0%253D&md5=d0afe3951cf60f195540d8d0072599afA global model of natural volatile organic compound emissionsGuenther, Alex; Hewitt, C. Nicholas; Erickson, David; Fall, Ray; Geron, Chris; Graedel, Tom; Harley, Peter; Klinger, Lee; Lerdau, Manuel; et al.Journal of Geophysical Research, [Atmospheres] (1995), 100 (D5), 8873-92CODEN: JGRDE3 ISSN:. (American Geophysical Union)Numerical assessments of global air quality and potential changes in atm. chem. constituents require ests. of the surface fluxes of a variety of trace gas species. A global model is developed to est. emissions of volatile org. compds. from natural sources (NVOC). Methane is not considered here and has been described in detail elsewhere. The model has a highly resolved spatial grid (0.5° × 0.5° latitude/longitude) and generates hourly av. emission ests. Chem. species are grouped into four categories: isoprene, monoterpenes, other reactive VOC (ORVOC), and other VOC (OVOC). NVOC emissions from oceans are estd. as a function of geophys. variables from a general circulation model and ocean color satellite data. Emissions from plant foliage are estd. from ecosystem specific biomass and emission factors and algorithms describing light and temp. dependence of NVOC emissions. Foliar d. ests. are based on climatic variables and satellite data. Temporal variations in the model are driven by monthly ests. of biomass and temp. and hourly light ests. The annual global VOC flux is estd. to be 1150 Tg C, composed of 44% isoprene, 11% monoterpenes, 22.5% other reactive VOC, and 22.5% other VOC. Large uncertainties exist for each of these ests. and particularly for compds. other than isoprene and monoterpenes. Tropical woodlands (rain forest, seasonal, drought-deciduous, and savanna) contribute about half of all global natural VOC emissions. Croplands, shrublands and other woodlands contribute 10-20% apiece. Isoprene emissions calcd. for temperate regions are as much as a factor of 5 higher than previous ests.
- 31Weibel, D.; Wong, S.; Lockyer, N.; Blenkinsopp, P.; Hill, R.; Vickerman, J. C. Anal. Chem. 2003, 75, 1754– 176431https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXhsF2ksL0%253D&md5=c5285a6e622631788b42df8908e25ba0A C60 Primary Ion Beam System for Time of Flight Secondary Ion Mass Spectrometry: Its Development and Secondary Ion Yield CharacteristicsWeibel, Daniel; Wong, Steve; Lockyer, Nicholas; Blenkinsopp, Paul; Hill, Rowland; Vickerman, John C.Analytical Chemistry (2003), 75 (7), 1754-1764CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)A buckminsterfullerene (C60)-based primary ion beam system was developed for routine application in TOF-SIMS anal. of org. materials. The ion beam system is described, and its performance is characterized. Nanoamp beam currents of C60+ are obtainable in continuous current mode. C602+ can be obtained in pulsed mode. At 10 keV, the beam can be focused to <3 μm with 0.1 nA currents. TOF-SIMS studies of mol. solids and a no. of polymer systems in monolayer and thick film forms are reported. Very significant enhancement of secondary ion yields, particularly at higher mass, were obsd. using 10-keV C60+ for all samples other than PTFE, as compared to those obsd. from 10 keV Ga+ primary ions. Three materials (PS2000, Irganox 1010, PET) were studied in detail to study primary ion-induced disappearance (damage) cross sections to det. the increase in secondary ion formation efficiency. The C60 disappearance cross sections obsd. from monolayer film PS2000 and self-supporting PET film are close to those obsd. from Ga+. The resulting C60 efficiencies are 30-100 times those obsd. from gallium. The cross sections obsd. from C60 bombardment of multilayer mol. solids are ∼100 times less, such that essentially zero damage sputtering is possible. The resulting efficiencies are >103 greater than from gallium. Also C60 primary ions do not generate any more low-mass fragments than any other ion beam system does. C60 is a very favorable ion beam system for TOF-SIMS, delivering high yield, close to 10% total yield, favoring high-mass ions, and on thick samples, offering the possibility of anal. well beyond the static limit.
- 32Domon, B.; Costello, C. E. Glycoconjugate J. 1988, 5, 397– 40932https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL1MXitVyks7g%253D&md5=28f6fddf1255322fe7362d3d48256104A systematic nomenclature for carbohydrate fragmentations in FAB-MS/MS spectra of glycoconjugatesDomon, Bruno; Costello, Catherine E.Glycoconjugate Journal (1988), 5 (4), 397-409CODEN: GLJOEW; ISSN:0282-0080.A summary of the ion types obsd. in the fast-atom-bombardment mass spectrometry (FAB-MS) and collision induced decompn. (CID) MS/MS spectra of glycoconjugates (glycosphingolipids, glycopeptides, glycosides, and carbohydrates) is presented. The variety of product ion types that arise by cleavages within the carbohydrate moieties has prompted the introduction of a systemic nomenclature to designate these ions. The proposed nomenclature has been developed primarily for FAB-MS, but can be used as well for other ionization techniques [field desorption (FD), direct chem. ionization (DCI), laser desorption/Fourier transform (LD/FT), etc.], and is applicable to spectra recorded in either the pos. or neg. ion mode during both MS and MS/MS expts. Ai, Bi, and Ci labels are used to designate fragments contg. a terminal (non-reducing end) sugar unit, whereas Xj, Yj, and Zj represent ions still contg. the aglycon (or the reducing sugar unit). Subscripts indicate the position relative to the termini analogous to the system used in peptides, and superscripts indicate cleavages within carbohydrate rings.
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