Thin Layer Chromatography - Choice of a Specific Polar Stationary Phase.
Choice of a Specific Polar Stationary Phase.
The choice of a specific polar stationary phase can also be arrived at on a rational basis. The selection of polar stationary phases offered in table 3 does not encompass the same variety of interactions as the ion exchange stationary phases or the dispersive phases for that matter. The polar stationary phases available are summarized in table 4. It is seen that silica gel can only offer significant polar selectivity but it must be born in mind that the mobile phase will also control selectivity. In silica gel distribution systems it is commonto exploit dispersive forces in the mobile phase by employing strong dispersive solvents. There appears little advantage in selecting alumina or any other inorganic polar phase as an alternative to silica and this is why the other stationary phases are not particularly popular. Furthermore, in most instances alumina TLC plates are not as efficient as silica plates. The polymer stationary phase does, however, offer a useful alternative as it contains a large number of aliphatic chains and, thus, can contribute to dispersive selectivity. If the solutes are insoluble in dispersive solvents and so the solvent cannot be used to influence selectivity, then the polymer stationary phase might provide the necessary dispersive selectivity. This would allow a very polar solvent to be used to maintain solute solubility.
Table 4. Choice of Polar Stationary Phase
|Silica Gel||Alumina||Other Inorganic Stationary Phase||Polymer Beads|
|Strongly Polar matrix of Siloxyl bonds exhibiting little dispersive character.||Strongly Polar (not as strong as silica) matrix of Aloxyl bonds exhibiting little dispersive character.||Medium Polarity inorganic matrix exhibiting little dispersive character.||Weak to Medium Polarity with polymer matrix exhibiting significant dispersive character.|