Introduction

Thin layer chromatography (TLC) is a form of liquid/solid chromatography (LSC) where the stationary phase, instead of taking the form of a packing in an open tube, is formed as a thin layer on the surface of a suitable plate. The mobile phase is allowed to flow over the surface (normally driven by surface tension forces) eluting the solutes along the plate in the process. Although in thin layer chromatography the distribution systems (the phase systems) are basically similar to those used in liquid chromatography, the necessary apparatus is much simpler and, as a consequence, a great deal less expensive. In addition, by multiple spotting, many separations can be carried out simultaneously and analysis times can be much shorter. In certain multiple separations, there may be up to 60 samples per plate. In practice, the resolution obtainable from a TLC plate is far less than that obtainable from an LC column, nevertheless, as a result of the cost advantage, the technique is very popular. In fact, despite the sophistication of modern LC techniques, the use of TLC for routine analyses continues to grow.

There are, however, a number of disadvantages to TLC one being a restricted resolving power. The relatively poor resolving power is due, not merely to relatively low plate efficiency, but more often to be due to the restricted physical size of the plate which limits the number of sample spots that can be accommodated in a given sample. Thus, multi-component samples are not readily separated on a TLC plate due to the restricted plate capacity (this corresponds to the peak capacity obtainable from packed columns). It must be emphasized that in TLC all the solutes must be contained by the plate whereas in LC, as the solutes are eluted from the column, the component capacity (peak capacity) is much greater. The minimum detectable mass of solute in TLC (i.e. the sensitivity) is much larger than that in liquid chromatography and, consequently, the technique is not so suitable for trace analysis unless prior concentration is employed. Finally, the unique selectivity that is obtainable from reversed phases often cannot be comprehensively realized in TLC as the reversed phases are not wetted by solvent mixtures having high water contents and consequently, plate development becomes difficult if not impossible.