Ion-Exchange HPLC (IEX-HPLC) separates analytes based on differences in their surface charge. The basic principle governing IEX-HPLC separations is that analytes with an electrical charge are attracted to ligands on the stationary-phase particles which bear the opposite charge. The mobile phase typically consists of a pair of aqueous buffers, with the A phase containing a low concentration of salt and the B phase containing much more salt. Two types of ion-exchange chromatography exist - anion exchange separates negatively-charged particles, and cation exchange separates positively-charged particles. In ion-exchange HPLC the stationary phase is composed of small stationary-phase particles to which charged ligands are attached on the surface. These particles are packed into a dense three-dimensional matrix within a column.