Thin Layer Chromatography - Elution Development in Thin Layer Chromatography 1

 

Elution Development in Thin Layer Chromatography

 

The development processes that take place on a thin layer plate using mixed solvents, compared with that in an LC column is, greatly complicated by the frontal analysis of the mobile phase itself, although the process is basically still an elution technique. The mobile phases used to elute the solutes in TLC are commonly multi-component, containing at least three different solvents, if not more. If the plate is not pre-conditioned with solvent, on elution there is an elaborate modification of the plate surface that is depicted for a ternary solvent mixture in figure 2. It is interesting to note that the separation is achieved (unless a single solvent is used as the mobile phase) by a complicated form of gradient elution, The edge of the plate is dipped into a tray of the solvent mixture which 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 2. 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) in the diagram. 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 has resulted from the frontal analysis of the three components of the mobile phase, is now quite complex. Consider now the distribution systems with which the solutes will interact during the separation process. In the first section, (X), solutes will be distributed between the ternary solvent mixture (A), (B) and (C) and the surface covered (and consequently deactivated) 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 (and consequently deactivated) with solvent (B).

 

Figure 2. The Development of a Thin Layer Plate