Secondary chemical equilibria in reversed-phase liquid chromatography

Abstract The addition of reagents to a reversed-phase liquid chromatography (RPLC) mobile phase enables the separation of ionizable compounds, inorganic anions, and metal ions, using conventional instrumentation, silica-based materials, and hydro-organic mixtures, thanks to a variety of secondary equilibria. This gives rise to several chromatographic modes, the main features of which are outlined in this chapter. The effect of the mobile phase pH on the retention of ionizable compounds is described, together with the recommended experimental practice. The mechanism of adsorption of amphiphilic anions or cations on the stationary phase to attract analytes with opposite charge, or suppress the silanol activity, is discussed. Different reagents such as alkylammonium salts, surfactants (below and above the critical micelle concentration or forming microemulsions), perfluorinated carboxylate anions, chaotropic ions, and ionic liquids are considered. The potential of metal chelation and redox reactions for the determination of metal ions and organic compounds are also summarized.