ELECTROSTATIC RETENTION MODEL FOR ION-EXCHANGE CHROMATOGRAPHY

A theoretical framework for the effect of eluting salt concentration on the capacity factor of small ions in ion-exchange and ion chromatography is described. The model is based on the Gouy-Chapman theory for the electrical double layer complemented with the possibility for specific adsorption of the counterions as well as the analyte ions to the chromatographic surface. Because of the complex dependence of the capacity factor on parameters such as net charge density of the stationary phase, eluent salt concentration, and electrostatic potential of the surface, numerical evaluation of the model is needed in the general case. The calculations show, in agreement with the general experimental observation, that a log k' vs log (eluent concentration of 1:1 salt) plot is Linear with a slope value close to -1 and -2 for mono- and dicharged analytes, respectively. It is also shown that the linearity as well as the slope is insensitive to the type of analyte ion, eluent counterion, and type of stationary phase and its charge density. The theory therefore offers a physically consistent approach to the analysis of retention data without resorting to the unrealistic stoichiometric models which has mainly been used so far in the ion-exchange chromatography of small ions.