Principles and Practice of Chromatography - Thin Layer Chromatography Apparatus > Thin Layer Chromatography Chambers > Page 63

Solvent must be prevented from evaporating from the plate surface during development as this would cause the composition of the developing solvent to change. In the simplest case, the developing chamber can consist of a round or square glass jar fitted with a glass cap. Sufficient solvent mixture is placed in the chamber to raise the level about a centimeter from the base. The plate is then placed in the chamber with the end where the samples have been placed dipping into the solvent and the cap replaced. The sample spots must not be at, or below, the surface of the solvent mixture or they will be washed from the plate. To improve the air space saturation with solvent, the walls, or part of the walls, of the chamber are sometimes covered with filter paper to act as a wick that soaks up the solvent and provides a greater surface area for evaporation. The use of a paper wick is depicted on the right-hand side of figure 29.

The saturated solvent vapor in the chamber not only prevents solvent evaporating from the plate surface but partly controls the retention mechanism by surface deactivation. The solvents 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. It should be emphasized, however, 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, for a binary mixture of solvents having concentrations of solvent in the gas phase of c1 and c2 respectively,


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

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