Ion Exchange Chromatography (IEX) separates biomolecules on the basis of their net surface charge. Biomolecules will provide different interaction properties depending on the net charge, density and surface charge distribution. Biomolecules without a net charge at a pH equivalent to their isoelectric point (PI) will not interact with the resin.
Depending on the working pH, the biomolecule will exhibit interaction to anion or cation exchanger media. Therefore, at a pH above its pI, the biomolecule will interact with a positively charged resin (anion exchanger) and at a pH below its pI, the biomolecule will show affinity to a negative charged resin (cation exchanger).