How does ion exchange chromatography work?

The chromatographic media contain charged groups attached to a solid support, creating either a positively charged (cation) or negatively charged (anion) environment. Polystyrene-divinylbenzene copolymers are commonly used as the support. Fixed ions attach to the surface of these supports by modifying the phenyl rings of the polystyrene. In cation exchange resin, the fixed sulfonate groups have cations associated with them as counterions. In anion exchange resin, the fixed quaternary amine groups have anions associated with them as counterions. A salt gradient is applied to the column, starting with low salt concentrations and gradually increasing. Proteins with fewer charged groups are eluted first at low salt concentrations, while those with more charged groups are eluted later at higher salt concentrations. The binding is reversible, and the compounds are eluted by changing the salt concentration or pH. The undesired proteins and impurities are then washed away from the column with a buffer solution.