Pilot Technique Thin-layer Chromatography

Thin-layer chromatography is a simple, fast technique that often provides complete separation of fairly complex mixtures. Any sample type that has been successfully separated by thin-layer chromatography on silica or alumina plates can be separated by means of liquid chromatography. 

Mobile phase optimization using HPLC can easily be carried out using fully automated instruments, but this approach is extremely time-consuming. Conversely, modem thin-layer chromatographic techniques allow several mobile phases to be tested in parallel within a very short period of time. Furthermore, different methods are available to visualize the sample components, and the thin-layer chromatogram immediately provides information about the presence of products in the sample that remain at the point of application. 

Taking some basic mles into account, the mobile phase composition established by means of TLC can be used directly for preparative chromatographic applications. A pragmatic approach using TLC as a pilot technique for normal-phase preparative chromatographic separations is elucidated in this chapter. 

Similar to the retention factor in HPLC the retardation factorRf describes retention behavior during TLC experiments. It represents the ratio of the distance migrated by the sample to the distance traveled by the solvent front. Boundaries are 0 Rf 1. For Rf=0, the product does not migrate from the origin and for Rf= 1, the product is not retained. Rf values are calculated to two decimal places, while some authors prefer to tabulate values as whole numbers, as hRf values (equivalent to 100Rf) (Poole, 2003). 

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Reuke, S. Hauck, H.E. Thin layer chromatography as a pilot technique for HPLC demonstrated for pesticide samples. Fresenius J. Anal. Chem. 1995, 351, 739 744. Guiochon, G. Katti, A. Preparative liquid chromatography. Chromatographia 1987, 24,165-189. 

Schihtt, H. andGeiss,F., Thin-layer chromatography as a pilot technique for rapid column chromatography, J. Chromatogr., 67 476,1972. 

Compared to paper chromatography, thin layer is more versatile, faster and more reproducible. It is often used as pilot technique to quickly determine the complexity of a mixture. It may otherwise be used as an aid in order to find out the best conditions for large-scale chromatography. Because of its speed and simplicity, it is often used to follow the course of reactions. Thin layer technique has often been used to identify drugs, contaminants and adulterants. It has also been widely used to resolve plant extracts and many other biochemical preparations. 

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