Thin Layer Chromatography - Thin Layer Chromatography Chambers 2

 

 

The air saturated with solvent vapor contained in the chamber not only prevents solvent evaporation from the plate surface, but also allows the surface of the plate to adsorb solvent and, thus, moderate the activity of the surface coating that controls the retention of the solutes. The components of the solvent are selectively adsorbed on the surface of the stationary phase causing the solutes to interact, not with the native silica surface, but with the silica surface covered with the most strongly interacting solvent component. However, it should be emphasized that the equilibrium between the solvent vapor and the plate will not be the same as the equilibrium between the solvent and the plate. For example, if a binary mixture of solvents is considered and the concentrations of solvent in the gas phase is c1 and c2 respectively,

 

 

where (x1) is the molar fraction of solvent (1)

and (x2) is the molar fraction of solvent (2)

 

In fact, Raoult's Law applies and

 

 

and (P1) and (P2) are vapor pressures of pure solvents (1) and (2) respectively. The apparatus necessary for the pre-equilibrium of a thin layer plate is shown in figure 4.

 

The apparatus is similar to that used for normal development but a separate reservoir contains the developing solvent. The plate is placed in the enclosure and allowed to come into equilibrium with the solvent vapor for a few minutes (as already discussed this will not be the same equilibrium that would be established with the actual solvent mixture). The plate is then lifted and placed so that the end now dips into the developing solvent and the separation processed in the usual way.