Elucidation of Ion-Pairing Elution Behavior of Anions and Cations in Electrostatic Ion Chromatography Using Water as a Mobile Phase

Peculiar ion-pairing elution behavior of analyte anions with counter cations in electrostatic ion chromatography (EIC) using water as a mobile phase was theoretically elucidated based on the Donnan membrane equilibrium principle and charge balance condition. New parameters, f anion and f cation , containing a fluctuating coefficient, x , which corresponded to capacity factors, were derived from the Donnan equation. The f anion and f cation of the constituent anion and cation in ion-pairing elution were connected with each other by the x , and the capacity factor, k ion pair (of co-eluting anion and cation) could be predicted by solving the relation of f anion = f cation for x . The elution volumes for various ion pairs thus semiempirically calculated were in good agreement with those experimentally obtained. In addition, the selective ion-pairing formation in EIC could reasonably be explained by applying the parameters to a modified classical plate theory.