Principles and Practice of Chromatography - Thin Layer Chromatography Apparatus > Thin Layer Chromatography Chambers > Page 64

In fact, Raoult's Law applies and

and (P1) and (P2) are vapor pressures of pure solvents (1) and (2) respectively. One form of apparatus that can be used for the pre-equilibrium of a thin layer plate is shown in figure 30.


Figure 30 Apparatus for Establishing the Pre-Equilibrium of a TLC Plate

The apparatus is very 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. 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. There are two main effects resulting from the pre-saturation of a TLC plate. These affects are depicted in figure 31. Firstly the velocity of the solvent front is increased relative to that of the unsaturated plate. Secondly, as a significant amount of the solvent at the front is adsorbed onto the silica surface of the unsaturated plate, the separated components are much closer to the solvent front. Consequently, the apparent advantage resulting from the faster moving front of the pre-saturated plate on the analysis time is offset by the closer proximity of the spots to the solvent front in the unsaturated plate. In most cases, the effect of pre-saturation on the actual separation is small but can be important for special mixtures where subtle changes in retention can make the necessary difference.