Thin Layer Chromatography - Choice of a Specific Dispersive Stationary Phase.
Choice of a Specific Dispersive Stationary Phase.
The choice of the dispersive stationary phase is a exceptional as dispersive interactions are the only interactions that can take place in exclusion to all others. Polar interactions are always accompanied by some dispersive interactions and ionic interactions are always accompanied by dispersive interactions and usually by polar interactions as well. However, it is not easy to obtain a solid stationary phase (particularly if silica based) that exhibits solely dispersive interactions. The choice of dispersive stationary phases is summarized in table 5.
Table 5. Choice of Dispersive Stationary Phase
|Polymer Beads||Bonded Reversed Phase|
|Fairly strongly dispersive matrix contains polar groups exhibiting some polar character||Strongly dispersiveIf well produced very minimal polar characteristics|
There are a number of polymers used to make dispersive stationary phases, one of the more popular being the methacrylate polymers. The matrix, thus, contains a considerable number of ester groups that will give the material some significant polar character. Consequently, such stationary phases would exhibit a mixture of polar and dispersive selectivity. Reversed phases in contrast, if well made and efficiently "capped" can provide a uniquely dispersive selectivity that will separate mixtures exclusively on the basis of dispersive interactions. Perhaps, a little surprisingly, such materials are excellent for separating peptides that, as already mentioned, are so polar that they exhibit very similar polar interactions with all types of polar stationary phases. Another important property of the reversed phase is the effect of the chain length of the bonded alkane on retention. In general, and for obvious reasons, the longer the chains, the stronger the dispersive interactions with the solutes and the greater the retention. Unfortunately, if used to separate proteins, the strong dispewrsive interactions can also denature the protein. For this reason reversed phases having alkane chain lengths of only 2 or 3 carbon atoms may be preferred for the separation of such materials.