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Research Profile

Ultrathin-layer chromatography spotting and detection on the sub-millimeter scale

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Steven C. Powell

Anal. Chem., 2010, 82 (9), pp 3408–3408

DOI: 10.1021/ac100625s

Publication Date (Web): March 22, 2010

Point-and-click chemistry is the dream of many a bench scientist. A recent AC article (DOI 10.1021/ac902945t) offers a glimpse into a future in which ultrathin-layer chromatography (UTLC) might be directed entirely with “instruments” available at an office supply store. Gertrud Morlock and co-workers at the University of Hohenheim (Germany), the University of Alberta, and NRC National Institute for Nanotechnology (both in Canada) report analyte spotting and reading techniques that incorporate common inkjet printers and scanners. Their techniques inexpensively address some of the difficulties inherent in miniaturized planar chromatography, which requires sub-millimeter spot sizes.

High-performance thin-layer chromatography (HPTLC) is an indispensable tool in a variety of fields and is used to separate analytes qualitatively and quantitatively. With UTLC (a recently developed, related technology), the stationary phase is miniaturized even further, from the typical sorbent layer thickness of 100 μm in HPTLC to ≤10 μm in UTLC. The makeup of the sorbent layers for UTLC is an active area of current research with a variety of manifestations, such as deposited SiO₂ spiral nanostructures or commercially available monolithic silica. The result is a small, thin surface that requires less analyte and mobile phase than is required for HPTLC. Unfortunately, “the miniaturized plate formats are very difficult to manage with the current TLC and HPTLC equipment available,” says Morlock. “It’s not convenient at all.”

As a result, UTLC has not been widely adopted by the analytical community, so the researchers sought to create an inexpensive solution. Their approach was to harness the mature technology of inkjet printing, refined over many years as a result of the demands of millions of users worldwide. In a thermal inkjet printer, microengineered chambers are filled with ink by capillary action; a heating element vaporizes the contents of the chamber to eject ink onto the facing paper surface. The array of ink chambers is aligned with heating elements that actuate multiple chambers at a time to create an individual droplet (a single pixel).

Morlock’s team used an inexpensive, commercially available printer with four separate ink cartridges and devised a simple vacuum apparatus in the lab to load cartridges with their test solutions containing dye mixtures. By adjusting the printer driver software and loading UTLC plates into the printer’s interior tray (designed for labeling compact discs), they precisely printed bands and spots.

Two commercial devices (A and B) did no better than an inexpensively adapted inkjet printer (C) in precisely applying volumes of just a few nanoliters to UTLC plates.

For direct comparison, they applied similar dye mixtures with both a commercially available TLC sampler (pressurized spray) and a piezoelectric nanojet. In a variety of analyses, such as the linearity and precision of application, the inket method worked as well or better than the other devices, producing reliably placed volumes in the low-nanoliter range.

“In evaluating this approach, other methods of depositing small spots need to be considered, such as Fenimore’s contact spotter and Liu’s electroosmosis-based nanopipettor, both of which are also capable of depositing a sub-millimeter spot,” says David Nurok of Indiana University−Purdue University Indianapolis. Given the benefits derived from refinements in miniature spot deposition, he adds that “studies like this should be encouraged.”

The plates were developed in small solvent chambers, and the researchers then harnessed another office staple—the flatbed scanner—as an analytical instrument. An inexpensive scanner outperformed two commercial systems commonly applied to TLC-based analyses. Although “the flatbed scanner yielded better signal-to-noise ratios, a fuller comparison would include a modern scanning densitometer, which is likely to be even more sensitive,” points out Nurok. Commercially available UTLC plates, however, are very small and have semi-opaque backing, two qualities that make densitometry difficult with current instruments.

The inkjet apparatus doesn’t require a physical barrier to create above-atmospheric pressures to effect forced flow. This open configuration can be an advantage. The system can “synergistically benefit from print and media technologies through its open, planar format,” says Morlock. “We discovered that printing the mobile phase was possible, [as was] the generation of a minor forced flow. Thus, the potential for a fully online, automated system is clearly possible and has immense potential to speed up routine analysis.”

“The instrumentation described in the article will likely allow UTLC to establish itself,” says Susan Olesik of Ohio State University. “New methods of making highly efficient stationary phases now exist, but they require the application of small sample sizes and the detection of small concentrations of analytes. This article illustrates improvements in instrumentation that will solve both of these problems.”