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

The peculiar ion pairing elution behavior of anions with cations in electro static ion chromatography with water as eluent was investigated by using de tectors based on conductivity and inductively coupled plasma-atomic emissio n spectrometry, where a sulfobetaine-type amphoteric surfactant (C14SB) imm obilized on an octadecyl silica surface was employed as the stationary phas e. Some preferential combinations of anions with cations were experimentall y identified; Mg2+, for example, was preferentially paired with the anions in the order NO3- > Br- > NO2- > Cl- > SO42-. The preferential combination characteristics could be used for the control of ion pair formation of anal yte anions with counter cations. When MgSO4, used hen as a "decoy electroly te", in which Mg2+ and SO42- were the "decoy cation" and "decoy anion", res pectively, was added to the original sample solution (Cl-, NO2-, Br- and NO 3- as analyte anions; Na+ K+, Mg2+ and Ca2+ as counter cations) at a relati vely high concentration compared to the concentrations of analyte anions, a ll the analyte anions in the sample solution were eluted with the introduce d decoy cation (Mg2+), following the preferential combination. Consequently , all the analyte anions were practically separated as a "decoy cation" for m, and they could easily be determined from the calibration graphs of the a nions in the decoy cation salts by conductivity detection. The present ion chromatographic method involving the characteristic anion-cation recombinat ion technique was applied to the determination of anions (Cl-, Br-, and NO3 -) in a vegetable (radish). (C) 1999 Elsevier Science B.V. All rights reser ved.