Electrostatic ion chromatography using hydroxide solutions as mobile phase with suppressed conductivity detection

An electrostatic ion chromatography (EIC) method for the separation of inorganic anions with detection by suppressed conductivity has been developed, in which analyte retention times can be manipulated by variation of the composition of the mobile phase. A stationary phase prepared by coating silica-based octadecyl material with a sulfobetaine zwitterionic surfactant (Zwittergent 3-14) has been used in conjunction with aqueous hydroxide solutions as the mobile phase. Inorganic anions were eluted in the order SO42– < F– < Cl– < NO2– < Br– < NO3– < ClO3– < I–, with retention times increasing with increasing concentration of hydroxide in the mobile phase. Retention times were also dependent on the nature of the counter-cation in the mobile phase, with divalent cations such as Ca2+ and Ba2+ showing longer retention times than monovalent cations such as Li+ and Na+. A retention mechanism involving formation of a binary electrical double layer is proposed, with the thickness of the double layer (and hence analyte retention) being dependent on the concentration of the mobile phase. The EIC system showed high sensitivity for the analyte ions due to the efficiency of the suppression reaction. Detection limits for SO42–, F–, Cl–, NO2–, Br– and NO3– were less than 1.0 × 10–7 mol L–1, whilst those for ClO3– and I– were 3.0 × 10–7 mol L–1 and 7.0 × 10–7 mol L–1, respectively, for a sample injection volume of 100 µL.