Anion-exchange chromatography uses a stationary phase bearing positively charged groups to reversibly retain negatively charged solutes. Strong anion exchangers usually contain permanently charged quaternary-ammonium ligands, whereas weak exchangers such as diethylaminoethyl phases have pH-dependent charge. A solute binds when the mobile-phase pH gives it sufficient net negative charge and when competing-ion concentration is low enough for electrostatic association with the exchanger ^[1]. Elution is produced by increasing salt concentration, changing counter-ion identity, or shifting pH to decrease solute charge or exchanger ionization ^[2]. Selectivity reflects charge number and distribution as well as nonideal secondary interactions, while capacity is set by the density and accessibility of cationic ligands. The mode is the anionic counterpart of cation-exchange chromatography and a subclass of ion-exchange chromatography. It is distinct from electrophoresis, reversed-phase chromatography, and affinity chromatography because repeated exchange with immobilized positive sites is its primary retention mechanism.