Biomolecules generally have charged groups on their surfaces, which change with the pH of the solution. This is the basis for Ion Exchange Chromatography (IEC), in which molecules reversibly bind to an oppositely charged group of the packing material.
Molecules with a higher charge density bind more strongly to the resin. The bound sample may be selectively removed from the stationary phase by changing pH or salt concentrations of the mobile phase. The higher the charge of the molecules and the stronger the binding to the stationary phase, the greater the change in the salt concentration required.
IEC is a powerful separation tool because of its high selectivity and capacity. The technique is used for a variety of samples and is particularly effective for proteins.