Modes of chromatographic separation

Before proceeding with a discussion on the individual modes of separation it is worth reviewing a brief glossary of terms in association with the stationary phase properties. A simple strategy for selection of the stationary phase will naturally follow, in order to help the reader to choose a supplier wisely. 

There are two main classes of polymeric support utilized in the preparation of stationary phases for all modes of chromatography, namely silica and polystyrene-divinylbenzene copolymer. For simplicity, from this point on, the former will be referred to as silica and the latter class as polymeric. 

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It is important for the analyst to be able to select the best stationary phase to use for a particular chromatographic analysis. Silica gel can be used in two modes of chromatographic separations as a stationary phase in normal elution development or as a stationary phase in exclusion chromatography. 

Successful application of chromatographic techniques relies on resolution, or the resolving power of the particular technique used. Resolution is defined by the relation of selectivity and efficiency of the chromatographic gel media (i). Selectivity is a function of the mode of separation of the gel (i.e., gel filtration, ion exchange, etc.) and efficiency is a function of the support matrix (Le., particle shape, size distribution, mechanical stability, density of interactive chemical groups, etc.). Each of the various modes of chromatographic separation have unique advantages that dictate where and when in a purification process these techniques should be used. 

Too many chromatographic texts dwell heavily on a theoretical and mathematical complexity that bears little relevance to what you actually need to do in order to practice preparative HPLC. Hopefully this book will describe how to practically go about a preparative separation. It is designed to guide the reader through the choice of equipment and chromatographic modes with minimal fuss and with reference to only relevant formulae. Much of the black art will be removed by the hints and tips of a practitioner with over 20 year s experience in many modes of chromatographic separation. 

Figure 7.1 Major modes of chromatographic separation (a) partition, (b) adsorption, (c) size-exclusion and (d) ion-exchange.

Antia, F. Horvath, Cs. Operational modes of chromatographic separation processes. Ber. Bimsenges. Phys. Chem. 1989, 93, 968. 

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