Characteristic conversion of ion pairs among anions and cations for determination of anions in electrostatic ion chromatography using water as a mobile phase Original Research Article

The peculiar ion pairing elution behavior of anions with cations in electrostatic ion chromatography with water as eluent was investigated by using detectors based on conductivity and inductively coupled plasma-atomic emission spectrometry, where a sulfobetaine-type amphoteric surfactant (C14SB) immobilized on an octadecyl silica surface was employed as the stationary phase. Some preferential combinations of anions with cations were experimentally identified; Mg2+, for example, was preferentially paired with the anions in the order NO−3>Br−>NO−2>Cl−>SO2−4. The preferential combination characteristics could be used for the control of ion pair formation of analyte anions with counter cations. When MgSO4, used here as a “decoy electrolyte”, in which Mg2+ and SO2−4 were the “decoy cation” and “decoy anion”, respectively, was added to the original sample solution (Cl−, NO−2, Br− and NO−3 as analyte anions; Na+ K+, Mg2+ and Ca2+ as counter cations) at a relatively high concentration compared to the concentrations of analyte anions, all the analyte anions in the sample solution were eluted with the introduced decoy cation (Mg2+), following the preferential combination. Consequently, all the analyte anions were practically separated as a “decoy cation” form, and they could easily be determined from the calibration graphs of the anions in the decoy cation salts by conductivity detection. The present ion chromatographic method involving the characteristic anion–cation recombination technique was applied to the determination of anions (Cl−, Br−, and NO−3) in a vegetable (radish).