Reversed-phase chromatography (RPC). The most common HPLC mode. Uses hydrophobic packings such as octadecyl- or octylsilane phases bonded to silica or neutral polymeric beads. Mobile phase is usually water and a water-miscible organic solvent such as methanol or acetonitrile. There are many variations of RPC in which various mobile phase additives are used to get a different selectivity. For example, for the RPC of anions, the addition of a buffer and tetra-alkyl-ammonium salt would allow ion pairing to occur and effect separations that rival ionexchange chromatography.