Sigmoidal retention changes caused by crown ether complexation with eluent cations in ion-exchange chromatography

Abstract

Addition of a crown ether to the mobile phase in cation-exchange chromatography causes specific sigmoidal changes in probe retention. An electrostatic model, including surface ion-pair formation as a retention mechanism, is developed to explain this unusual retention change. In most cases, electrostatic terms play minor roles in the determination of overall retention changes, and thus can be neglected for the discussions outlined here. This simplification allows us to estimate the order of ion-pair formation constants, their ratios, and the stoichiometry of crown ether complexation. In aprotic solvents, this approximation gives a sufficient description of the interactions taking place at the surface of an ion-exchange resin. In contrast, it is inapplicable to some systems, e.g. Na⁺ eluents in methanol, implying significant contributions from electrostatic effects.