Principles and Practice of Chromatography - The Development Process > Elution Development > Page 10

 

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). 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. 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.

The complexity of the system increases with the number of solvents used and, of course, their relative concentrations. The process can be simplified considerably by pre-conditioning the plate with solvent vapor from the mobile phase before the separation is started. Unfortunately, this only partly reduces the adsorption effect, as the equilibrium between the solvent vapor and the adsorbent surface will not be the same as that between the liquid solvent and the surface. It is clear that by forming a gradient by the frontal analysis of the mobile phase and carefully choosing the solvent mixture, very delicate pseudo-gradients can be created, which, in no small measure, accounts for the great versatility, popularity, and success of TLC.