Extension of the electrostatic model of reversed-phase ion-pair high-performance liquid chromatography to include the effect of the eluent pH

Abstract

The electrostatic retention model is extended to predict the retention changes of monoprotic weak acids and bases as the eluent pH and the mobile phase concentration of the ion-pairing reagent are varied simultaneously in reversed-phase high-performance liquid chromatography. At constant ionic strength and organic modifier concentration, the magnitude of solute retention shifts can be predicted from the model equations using a very limited set of experimental data. The effect of the eluent pH on the adsorption of the sodium octylsulphonate pairing ion and the surface potential was studied. Predictions from the model were compared with experimental retention data and good agreement was found for both oppositely and similarly charged solute ion — pairing ion combinations. The advantages and limitations of applying the electrostatic retention model in the optimization of ion-pair chromatographic separations are discussed in detail.