Soap chromatography—a new high-performance liquid chromatographic technique for separation of ionizable materials

Abstract A new form of high-performance ion-pair chromatography is described in which a detergent (cetyltrimethylammonium bromide) is added at around the 1% level to a propanol-water eluent. The column packing may either be a reversed-phase material (e.g., SAS silica) or a silica gel (e.g., Partisil). The method allows high-resolution separations of a wide range of sulphonic acids and derived dyestuffs containing one to three −SO3H groups. The degree of retention can be varied by changing cetrimide concentration, acidity of eluent, concentrations, and nature of additives. In the reversed-phase system it is probable that cetrimide-sulphonate ion pairs are extracted from the water-rich eluent into an adsorbed layer rich in propanol and cetrimide. In the silica gel system ion pairs are probably present in the eluent phase and are adsorbed onto the silica gel surface with propanol acting as a protective agent for both ion pairs and surface. Column performance in terms of plate height is equivalent to that obtainable in normal adsorption chromatography. Specific separations are shown of naphthylaminesulphonic acid isomers, including J-acid and γ-acid, and of sunset yellow from impurities.