A novel ion chromatographic method using zwitterionic surfactants as the stationary phase and water as the mobile phase

Zwitterionic surfactants immobilized on the surfaces of octadecylsilica (ODS) are used for the stationary phase and water as the mobile phase for the ion chromatography (IC) of target analytes. The creation of an electrical double layer (EDL), when a zwitterionic stationary phase is in contact with the analyte ions, is proposed to explain the separation mechanism. When an EDL is created using a zwitterionic stationary phase (ZWEDL), its properties differ considerably to those of a single charge-fixed stationary phase created EDL. For a ZWEDL, (i) the electrostatic field is increased, resulting in the simultaneous retention and separation of both cations and anions; (ii) the electrostatic affinity between the analytes in the ZWEDL and the stationary phase is extremely weak. This results in the ''effective'' distribution of the analytes between the stationary phase and the mobile phase without the need for ion-exchange. Since only water is used for the mobile phase, the sensitivity of detection by conductivity is vastly improved and the direct determination (without pre-concentration) of inorganic ions at ultra low levels is possible. Furthermore, since both positive and negative electrostatic fields are produced simultaneously, both analyte cations and anions are retained and separated in a single stage of the stationary phase. This provides the basis for a simple ant! rapid chromatographic method for the simultaneous analysis of cations and anions.