Ion exchange chromatography separates molecules based on differences in the net charge of the molecules. In anion exchange, the analyte is negatively charged (anion), while the chromatographic material is positively charged (cation). Ion exchange is commonly used for protein purification, but it may be used for purification of oligonucleotides, peptides, or other charged molecules. For interaction to occur, the protein of interest must have a charge opposite to that of the functional group of the sorbent particle. Because the interaction is ionic, binding must take place under low ionic strength conditions. Elution is achieved by increasing the ionic strength of the mobile phase to reduce ionic attractions, or by changing the pH of the mobile phase to alter the ionization state of the protein.