Preparative Chromatography - Criteria for the Successful Operation of Preparative Chromatographs. > Page 73

Criteria for the Successful Operation of Preparative Chromatographs.

Preparative chromatography, or perhaps one should say semi-preparative chromatography, is not difficult to carry out providing some basic requirements are met most, of which have already been described in some detail. The following is a summary of the essential requisites for a successful preparative separations.

1. The stationary phase and the combined phase system must be very carefully chosen to provide the maximum separation between the solute of interest and its nearest neighbor.

The mechanism of retention has been discussed in detail in The Mechanism of Chromatographic Retention and reference should be made to the appropriate sections of that book. In general, the interactive character of the solute to be separated must be carefully considered, and the stationary phase chosen to have the appropriate interacting moieties that will enhance its interaction with the primary substance relative to that of the neighboring substance or impurity. In contrast the mobile phase will be chosen to have a quite different interactive character so that selectivity for the substance of interest will remain in the stationary phase. As a simple example, if the solute of interest is strongly polar, and the neighboring substance dispersive, then the stationary phase must be polar (e.g., silica gel in LC, or perhaps, polyethylene glycol in GC) and in LC the mobile phase would be made predominantly dispersive (e.g., n-heptane or methylene dichloride or mixtures of both) so that polar selectivity remains dominant in the stationary phase. Unfortunately, all substances are not simply polar or dispersive or ionic but can exhibit various combinations of all three interactive characteristics. It follows, that the stationary phase will also need to have the appropriate mixture of interactive properties to maximize selectivity and in contrast the interactive character of the mobile phase will need to oppositely balanced to ensure maximum selectivity still resides in the stationary phase. Under some circumstances the selectivity of the mobile phase can also be exploited to interact more strongly with the neighboring substance and thus elute it more quickly. It is clear, that some considerable effort must be made to identify the bests phase system for the isolation of a substance from a hitherto unknown mixture. This work can be carried out on an analytical column and its success will determine the maximum load that can be used, the purity of the products and time taken for the separation.