In this mode of chromatography, the separation depends upon the exchange of ions between the mobile phase and the ionic sites of the packing (cationic or anionic). The stationary phase matrix has a functional group with a fixed ionic charge covalently attached to it . An exchangeable counterion from the mobile phase buffer preserves charge neutrality. The mobile phase usually contains a large number of counterions opposite in charge to the surface ionic group.
The counterions are in equilibrium with the matrix charged group in form of an ion-pair. The presence of a sample ion of the same ionic charge as the counterion sets up another equilibrium. The sample ion can exchange with the counterion to form an ion-pair with the matrix. The retention of the sample ion is based on the affinity of the different ions for the site on the matrix and on a number of other solution parameters such as, pH, ionic strength, counterion type, etc. For example, sodium chloride is used in the mobile phase buffer, the counterion is Na+ (in the case of cation exchange process) and Cl- (in the case of anion exchange process).
