Modeling of ion-exclusion and vacancy ion-exclusion chromatography in analytical and concentration overload conditions

Abstract

Ion-exclusion chromatography (IEC) finds applications in various different analytical separations of weak acids. Pure, deionized water or a diluted, aqueous solution of a strong mineral acid (such as, e.g., sulphuric acid) is used as the mobile phase, whereas a typical stationary phase is a strongly acidic resin in the H+ form (e.g., the sulfonated polystyrene-divinylbenzene resin with a high ion-exchange capacity, provided by the sulfonic acid groups). When pure water is used as the mobile phase, then the characteristic leading (i.e., frontally tailing) peaks are obtained, and the retention depends mainly on the concentration of the analyte. An alternative technique is vacancy ion-exclusion chromatography (v-IEC), in which the column is equilibrated with the sample solution, flowing as the mobile phase through the system, and pure water is injected as the sample. In this case, the symmetrical vacant peaks are obtained. The aim of this paper is to describe the retention mechanism in IEC and v-IEC for the adsorptive and nonadsorptive acids in analytical and concentration overload conditions, with pure water and the diluted sulphuric acid solution as the two different mobile phases. The retention times and the peak shapes predicted by the derived equations remain in a good qualitative and quantitative agreement with the experimental data. The model proposed in this paper predicts the new features characteristic of IEC for the adsorptive acids. These are, namely, an increase in the retention time of the peak apexes (up to a certain level and concurring with an increase in the acid concentration), followed by a subsequent decrease of the retention time (with the further growth of the acid concentration in the eluent). Similar changes in the retention time observed for v-IEC in the specific adsorption conditions were also correctly predicted by the model.