Elution Development in Thin Layer Chromatography

The development processes that take place on a thin layer plate is complicated by the frontal analysis of the mobile phase itself. The mobile phases used to elute the solutes in TLC are usually multi-component, containing at least three individual solvents. If the plate is not pre-conditioned with solvent, there is an elaborate modification of the plate surface that is depicted, for a ternary solvent mixture, in Figure 3.

The edge of the plate is dipped into a tray of the solvent mixture that begins to migrate along the plate, driven by surface tension forces. The different solvents array themselves on the surface in the manner shown in Figure 3. The solvent that interacts most strongly with the stationary phase is extracted from the mixture and forms an adsorbed layer on the surface that corresponds to the area (X) in the diagram. The now binary mixture continues to migrate along the plate and the next solvent component that interacts most strongly with the stationary phase (solvent B) is adsorbed as a layer on the surface corresponding to the area (Y).

Figure 3. The Development of a Thin Layer Plate

Finally, the remaining solvent (C) with the weakest interactions with the stationary phase continues to migrate and cover the surface with a layer of solvent (C) in the area (Z). It is seen that the distribution system, which results from the frontal analysis of the three mobile phase components is now quite complex. The solutes will interact during the separation process with all three surfaces. In the first section (X) solutes will be distributed between the ternary solvent mixture (A), (B) and (C) and the surface covered with solvent (A). In the next section (Y) the solutes will be distributed between a binary solvent mixture of (B) and (C) and a surface covered with solvent (B). Finally, distribution will take place in section (Z) between pure solvent (C) and a surface covered with solvent (C). Even this is an over-simplification, as the composition of the mobile phase in each section will not be constant but will decrease along the plate. Furthermore, as the separation progresses, the lengths of sections (X), (Y) and (Z) will continually increase. Such a system is extremely difficult to treat theoretically particularly as the boundaries are not as sharp as those depicted in Figure 3. In fact, the overall effect is as though the separation was carried out sequentially on three separate sections of a plate, each section having a different stationary phase and mobile phase. In each section, the separation will then be achieved by elution development, but the overall effect will be a form of gradient elution.