Separations Based on Ionic Interactions

As ionic materials are not volatile under the conditions normally employed in GC, ionic interactions cannot be exploited in GC stationary phases to control retention. However, they are extremely important in LC, and ion exchange chromatography (the name given to LC separations that employ ionic interactions to control retention) are widely used in industry and university to analyze ion mixtures.

Parameters that Control the Chromatographically Available Stationary Phase (V_s)

The other parameter controlling retention in a chromatographic system is the volume of stationary phase that is made available to the solutes that can be controlled, or limited, in a number of ways. Firstly, the stationary phase loading on the column can be increased or reduced, which, from the Plate theory, will proportionally increase or decrease the retention as required. A specific stationary phase loading may be selected, to improve the resolution, or to reduce the analysis time. In addition, a particular amount of stationary phase may be selected to increase the sample load and, thus, the capacity of the column. In some instances (mostly in liquid chromatography) the stationary phase loading may be reduced to render the column more compatible to certain types of compounds (e.g. to prevent proteins from being denatured on the stationary phase surface).

Secondly, the stationary phase can consist of molecules of a particular shape that can only come in close contact with molecules of a complementary shape. Other molecules will be unable to interact so closely with the stationary phase and, thus, the amount of stationary phase available to them will be restricted. This situation occurs in chiral chromatography where the stationary phase often consists largely of a specific enantiomer that confers chiral selectivity on the chromatographic system by virtue of its special conformity.