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Retention model for ion-pair chromatography based on double-layer ionic adsorption and exchange

Previous work has demonstrated the validity of an electrical double-layer model for sorption of sample ions onto low capacity ion exchangers. In the present work it is shown how this model can be used in describing the chromatographic retention of sample ions in so-called reversed-phase ion-pair chromatography. The pairing ion added to the mobile phase is sorbed onto the reversed-phase sorbent, creating an electrical double layer and a surface electrical potential psi(o). Sample counterions undergo both ion exchange for electrolyte ions in the diffuse part of the double layer and surface adsorption. The latter depends on the magnitude of psi(o), which can be calculated from the Stern-Gouy-Chapman theory. The model predicts virtually all of the phenomena that have been described in the literature on ion-pair chromatography.