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 octadecylsilic a (ODS) are used for the stationary phase and water as the mobile phas e 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 separ ation mechanism. When an EDL is created using a zwitterionic stationar y phase (ZWEDL), its properties differ considerably to those of a sing le charge-fixed stationary phase created EDL. For a ZWEDL, (i) the ele ctrostatic field is increased, resulting in the simultaneous retention and separation of both cations and anions; (ii) the electrostatic aff inity between the analytes in the ZWEDL and the stationary phase is ex tremely weak. This results in the ''effective'' distribution of the an alytes between the stationary phase and the mobile phase without the n eed for ion-exchange. Since only water is used for the mobile phase, t he 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 ne gative 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 chr omatographic method for the simultaneous analysis of cations and anion s.