Simultaneous determination of oxalate, thiosulfate, and thiocyanate in urine by ion-exclusion chromatography with electrochemical detection

A sensitive ion-exclusion chromatographic method has been developed for determination of oxalate, thiosulfate, and thiocyanate. The method is based on separation of these anions on a polymethacrylate-based, weakly acidic cation-exchange resin (TSKgel OApak-A) and detection by means of a glassy carbon (GC) electrode electrochemically modified with polyvinylpyridine (PVP), palladium, and iridium oxide (PVP/Pd/IrO₂). The electrochemical behavior of oxalate, thiosulfate, and thiocyanate at this chemically modified electrode (CME) have been investigated by cyclic voltammetry. The results indicated that electrocatalytic oxidation of these anions by the electrode was efficient and that the sensitivity, stability, and lifetime of the electrode were relatively high. Combined with ion-exclusion chromatography the PVP/Pd/IrO₂ electrode was used as the working electrode for amperometric detection of these anions. All linear ranges were over two orders of magnitude and detection limits, defined as S/N = 3, were 9.0 × 10^-7 mol L^-1 for oxalate, 6.7 × 10^-7 mol L^-1 for thiosulfate, and 5.6 × 10^-7 mol L^-1 for thiocyanate. Correlation coefficients were all >0.998. Coupled with microdialysis sampling the method has been successfully applied to the determination of oxalate, thiosulfate, and thiocyanate in urine.