Covalently Bonded Polymeric Zwitterionic Stationary Phase for Simultaneous Separation of Inorganic Cations and Anions

A novel polymer-based zwitterionic separation material was synthesized by a two-step reaction, in which 2-hydroxyethyl methacrylate−ethylene dimethacrylate copolymer beads were first activated with epichlorohydrin, whereafter (2-dimethylamino)ethanesulfonic acid inner salt was coupled to the epoxide groups on the activated beads in a quaternizing reaction. The resulting material carried strong/strong charge zwitterionic pendant groups, whose charge properties did not change over a wide pH range. It was capable of separating inorganic anions and cations both independently and simultaneously using aqueous solutions of perchloric acid or perchlorate salts as eluent. It was found that the retention of cations and anions depended on the individual ions in the analyte, not on type of salt injected, as reported for ODS columns dynamically coated with zwitterionic detergents using water only as eluent. This study also revealed that both the concentration and the water structure related properties of the eluent ions were important for the retention of ionic species. The mechanism appears to involve the interaction of the chaotropic anion component with the quaternary ammonium group, resulting in an increasing cation-exchange capacity of the zwitterion up to a perchloric acid eluent concentration of ∼10 mM. Above this concentration the material appears to become saturated, and the retention times of both cations and anions become practically independent of the HClO4 concentration.