Zwitterionic ion chromatography

This work presents a comprehensive study on Zwitterionic Ion Chromatography (ZIC) for the separation of inorganic ions. The elution characteristics of anions on a sulfobetaine-type stationary phase are examined and it is shown that the Hofineister series could be used to predict the elution order, with retention increasing with increasing polarisability, as well as the effect of the mobile-phase anion on retention of anions. A mobile-phase anion with a greater polarisability than the analyte anion reduced retention, and vice versa. An increase in anion retention was also observed in changing the mobile-phase cation from Na+ to Mg2+ to Ce3+ . Electro-osmotic flow measurements in an analogous capillary electrophoresis system revealed a zeta potential on the stationary phase that could be modulated from positive (+40.2 mV for CeCI3) to negative (-53.4 mV for NaCI04) depending on the mobilephase anion and cation. Thus the zwitterionic stationary phase is only neutral for mobile phases where the mobile-phase anion and cation equally shield the charges on the stationary phase. A new retention mechanism for ZIC was developed, based the ability of an analyte to penetrate the repulsion effects of a Donnan membrane (established by the zeta potential) and to interact directly with the inner charge of the zwitterion. This mechanism was established on the basis of experimental data obtained for the anion system, and an analogous mechanism was proposed for a cation system that utilised a phosphocholine-type stationary phase, where a systematic study of the elution effects of this system was found to mirror that of the anion system. Manipulation of the separation selectivity of inorganic anions in ZIC was achieved by controlling the ratio of cationic and zwitterionic surfactants in the stationary phase coating solution. Even at a ratio of 2:8 cationic:zwitterionic surfactant, a large contribution of an ion-exchange mechanism from the cationic surfactant occurred, in particular for small, highly-charged analytes. This was evident in the slopes of log k' versus log [mobile phase] plots. For example, the slope for the analyte SO42- changed from 0.23 to ‚ÄövÑvÆ1.22 when the ratio was changed from 1:10 to 2:8. Hard and soft mathematical models were investigated to quantitatively describe the mechanism of ZIC. The non-stoichiometric hard model was successful in describing general experimental trends, but further work is required to achieve accurate predictions of retention factors. The experimental data obtained for the anion system were used successfully to train an artificial neural network capable of predicting retention factors for a wide range of mobile phases. Plots of calculated versus experimental retention factors for a test set of analytes gave an r2 of 0.985.