Thin Layer Chromatography - Selection of the Mobile Phase 1
Selection of the Mobile Phase
The choice of the optimum mobile phase composition is often the most difficult and most time consuming procedure involved in the development of a TLC analytical method. Even though the mobile phase may be selected on a rational basis, in the end, the composition of the optimum solvent mixture will always need to be confirmed by careful experiment.
As a rule, the type of interactions that are chosen to dominate in the mobile phase (e.g. dispersive, polar or ionic) are made complimentary to those that have been chosen to control retention in the stationary phase. For instance, if dispersive interactions are chosen to control retention in the stationary phase (e.g. a reversed phase is chosen) then strong polar interactions might be chosen to dominate in the mobile phase (e.g. the mobile phase could comprise a mixture of methanol/water or, perhaps, a mixture of acetonitrile and water). On the other hand, if strong polar interactions are selected to control retention (e.g. silica gel is chosen as the stationary phase) then a strong dispersive solvent mixture might be selected for the mobile phase (e.g. an alkane or an alkane/methylene dichloride mixture).
To increase the elution rate of the solutes, the complementary interactions in the stationary phase must be reduced and those interactions that dominate in the mobile phase must be increased. Taking as another example a reversed phase separation (e.g. using an alkane bonded phase as the stationary phase), to reduce retention and, consequently. elute solutes more rapidly, the quantity of methanol or acetonitrile in the mobile phase would need to be increased relative to that of water. More solvent in the aqueous mixture would enhance the dispersive interactions in the mobile phase and would compete with the dispersive interactions between the solute and the alkane chains of the stationary phase. Coincidentally, and as a secondary effect, it would also reduce the polar interactions in the mobile phase as the quantity of water in the mobile phase would also be reduced. The net result, however, would be that the interactive forces between the solutes and the mobile phase would increase (relative to interactive forces with the stationary phase) and, as a consequence, the retention time of the solutes would be shortened. Although, the retention of all the solutes would be reduced, they would not necessarily be reduced proportionally and so the selectivity of the phase system may also be changed. Consequently, the resolution of some solutes may be improved, whereas the resolution of other solutes may be reduced.