Preparative Layer Chromatography

Commercial precoated plates for preparative layer chromatography (PLC) are available from a number of manufacturers (25). These are mostly silica gel with particle size distributions of 5–40 µm, 5–17 µm, or 10–12 µm; layer thickness 0.5–2 mm; organic, gypsum, or no foreign binder; and with or without a fluorescent indicator. A variety of bulk materials are also available from manufacturers for homemade preparation of preparative layers.

Figure 6: The Cyclograph centrifugal PLC instrument. (Courtesy of Analtech, Inc.)

Interested readers should consider attending the next International HPTLC meeting, which will be held in two years. The last meeting (the 21st) was held in July 2011 in Basel, Switzerland. It was attended by 350 participants from 41 countries. More than 200 papers, posters, and tutorials were presented. Information on past and future meetings can be found at http://www.hptlc.com/.

Future Prospects for TLC

The following trends in TLC usage and research are likely to occur in TLC in the next few years:

• Increased use of silica-gel HPTLC versus TLC layers and silica-gel bonded phases versus plain silica gel for improved separations of certain mixtures.
• Increased use of chemometrics in TLC, for example, for mobile-phase optimization through quantitative structure-retention relationship (QSRR) studies and prediction of retention using descriptors such as topological indices, and for the separation of overlapped densitometry peaks when sample constituents cannot be chromatographically resolved and enhancement of selectivity because of noise and background removal (30).
• Increased use of multidimensional planar chromatography (MD-PC) for separation of more complicated multicomponent mixtures by different combinations of 2D development, chromatographic plates with different properties, a variety of mobile-phase compositions, various forced-flow techniques, and multiple development modes (31).
• Continued development of the miniaturized, automated system introduced by Morlock and colleagues (29) with newly prepared UTLC layers having nanoscale and monolithic structures (32).
• Increased development and application of hyphenated techniques, especially TLC–MS and TLC–EDA.
• Increased development of validated methods for industrial quality control (QC) and government regulation of synthetic drug formulations, herbal medicines, and dietary supplements, maintaining these as the primary fields in TLC research.
• Increased use of horizontal chambers to double the number of samples analyzed to drastically save time and money in chromatographic analyses. Morlock and Oellig (33) demonstrated the use of silica-gel TLC–densitometry with an HDC 2 chamber for identification and quantification of 25 water soluble dyes used as food additives; 12 min was required for 40 runs in parallel developed from both ends of a plate toward the center (2 × 20 tracks) using 8 mL of mobile phase up to a migration distance of 50 mm. The total analysis time of all steps of the analysis was 60 min for 40 runs, for an overall time/run of 1.5 min and a solvent consumption of 200 µL. A sample throughput of 1000 samples/8 h day was reached by switching among working stations (application, development, and evaluation) in a 20 min interval, which tripled analysis throughput.
• Expanded application of existing manual–visual semiquantitative TLC screening methods (34) and transfer and validation of these, and additional new methods, to instrumental quantitative HPTLC–densitometry suitable for support of regulatory compliance actions (35) for mislabeled, substandard, and counterfeit pharmaceutical products appearing worldwide, especially in underdeveloped countries.

Authors' note: As this article was being prepared, notice was sent by Hichrom Limited that it would be selling the Whatman HPLC columns (acquired from GE Healthcare). The Whatman/GE Healthcare TLC plates, however, were being discontinued as of mid-April. Merck KGaA (EMD/Millipore in North America) has also discontinued the manufacture of the UTLC monolithic silica gel plates. Unfortunately, this leaves users of any discontinued TLC prepared plates with the task of finding replacement TLC products once their supplies have been depleted.