Simultaneous amperometric detection of electroinactive anions and cations in ion chromatography

An amperometric detector with two working electrodes modified with polydiphenylamine (PDPA) and polydiphenylamine dodecyl sulfate (PDPA-DS) was successfully used for the simultaneous determination of electroinactive anions (SO₄^2-, Cl^-, NO₃^-) and cations (Na⁺, NH₄⁺ and K⁺) in single-column ion-exclusion chromatography-cation-exchange chromatography (IEC-CEC). The PDPA-DS chemically modified electrode (CME) was based on the incorporation of dodecyl sulfate (DS) into PDPA by electropolymerization of diphenylamine (DPA) in the presence of surfactant. The electrochemical responses against the anions and cations in differential pulse voltammetry were studied. A set of well-defined peaks of electroinactive anions and cations were obtained. The relative standard deviations (RSDs) of chromatographic peak height (nA) for anions were all <4.4% and those for cations were all <4.1%. The anions and cations were detected conveniently and reproducibly in the linear concentration range 6.0 x 10^-6-5.0 x 10^-3 mol L^-1 and their correlation coefficients were all >0.990. Their detection limits were investigated using 3s (where s represents the standard deviation of a blank solution, n = 11). They were 9.86 x 10^-7 mol L^-1 for SO₄^2-, 6.92 x 10^-7 mol L^-1 for Cl^-, 7.86 x 10^-7 mol L^-1 for NO₃^-, 7.86 x 10^-7 mol L^-1 for Na⁺, 9.79 x 10^-7 mol L^-1 for NH₄⁺ and 8.94 x 10^-7 mol L^-1 for K⁺. The recoveries of the anions and cations by this method were in the range 97.8-103%. The proposed method was rapid, sensitive and simple and was successfully applied to the analysis of rainwater samples. Both electrodes were stable during a 1 week period of operation with no evidence of chemical or mechanical deterioration.