Ion exchange chromatography separates proteins or peptides based on charge characteristics. The net surface charge of a protein or peptide determines its adsorption to oppositely charged groups immobilized on the ion-exchange medium. Proteins are multivalent anions or cations, and the charge of a protein depends on the pH of the environment. When the pH is greater than the isoelectric point (pI) of the protein (number of positive charges equals the number of negative charges), the protein will have a net negative charge and will bind to an cation (+) resin. When the pH is less than the isoelectric point, the protein will have a net positive charge and will bind to an anion (-) resin. Once the sample is bound to the medium, unbound components are washed away and bound samples selectively eluted and collected.

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