Ion Exchange Chromatography (IEC or IEX) is driven by interactions between charged target molecules and charged immobilized ligands on the chromatography resin. IEX can be divided into two different sub types. Cation exchange chromatography, in which positively charged species in the mobile phase bind to a negatively charged ligand on the resin; and anion exchange chromatography, in which the binding species in the mobile phase are negatively charged, and the immobilized ligand is positively charged.
After the molecule of interest has been adsorbed, the column is washed to remove any residual unbound species from the solid phase. The bound molecules are then desorbed using a gradient of a second, higher ionic strength mobile phase to steadily increase the ionic strength of the eluent solution. An alternative desorption method involves using a mobile phase of greater or lesser pH than the equilibration mobile phase to give your molecule of interest or the ligand a charge at which they will not interact and your molecule of interest elutes from the resin.