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.
Analtech offers a unique tapered layer with preadsorbent for capillary-flow preparative separations and precast TLC silica
gel GF rotors with 1000–8000 µm nominal thickness for use with the company's Cyclograph centrifugal instrument (Figure 6)
for PLC applications. It is possible to see the operation of the Cyclograph instrument in a YouTube video.
Figure 6: The Cyclograph centrifugal PLC instrument. (Courtesy of Analtech, Inc.)
HPTLC and TLC International Meetings
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
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
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.