Electrostatic Ion Chromatography of Common Anions and Cations with a Zwitterionic Surfactant-Modified Silica-C18 Column Using Water Eluent

Abstract

Electrostatic ion chromatography (EIC) of anions and cations with water eluent has been investigated for the development of water-monitoring systems in developing countries, which have the nature of simple, lower running cost, and non-chemical waste. For selecting the separation column, sulfobetaine-type zwitterionic surfactant (CHAPS)-modified silica C18 and silica C30 columns, and a zwitterionic stationary phased column HILIC were compared for anion separations. The retention order of the analyte anions was SO42−<Cl−<NO3−<I−<ClO4− without regard to the types of the columns, depending on the nature of EIC separation. However, the resolutions were different, because the anion separations by EIC were strongly affected by the hydrophobicity of the stationary phase. As a result, the CHAPS-modified silica C18 column was the most suitable as a separation column in EIC in terms of the peak resolution and the retention time. In contrast, cation separations using the CHAPS-modified silica C18 column with a water eluent were in the order of monovalent cations (Li+, Na+, K+ and NH4+) < divalent cations (Mg2+ and Ca2+). This fact means that the sulfobetain-type zwitterionic stationary phase has much higher selectivity for anions than for cations. Moreover, a pre-column (cation-exchange resin in the Li+-form for anion separations, and anion-exchange resin in the Cl−-form for cation separations) was connected in tandem before the separation column, in order to make uniform the counter ion for analyte ions and to apply this method to real water samples. Under the optimized conditions, the linearity of the calibration graph, the detection limit, and the reproducibility for the common anions were tested, and satisfactory results was obtained for all common anions. The potentiality of EIC was demonstrated in practical applications to the determination of common anions (SO42−, Cl−, NO3− and HCO3−) and hardness in river water.