Capillary electrophoresis as a useful tool for the analysis of chemical tracers applied to hydrological systems

A capillary electrophoretic method was optimised for the separation and determination of iodide used as artificial tracer in hydrology. The influence of the buffer concentration and pH, electroosmotic flow modifier concentration (cetyltrimethylammonium bromide (CTAB)), the injection time and voltage applied, on the electrophoretic separation was studied. A running buffer of 20 mM phosphate (pH 8) containing 1 mM CTAB was found to provide the optimum separation of iodide with respect to resolution, migration time and selectivity. The water samples were injected hydrostatically at 10 cm for 110 s, the voltage applied was −20 kV and a detection wavelength of 214 nm. The influence of the sulphite added to water samples in order to prevent the oxidation of iodide to iodate was also studied. This method can be applied to the determination of iodide free of sulphite interference up to at least a ratio of 1:1000 (I −:SO 3 2−). The other inorganic anions, which are present in the water samples (mainly chloride, sulphate, nitrate, carbonate), do not interfere with the determination of iodide. This method allows the simultaneous determination of bromide, nitrite, and nitrate together with iodide. The electrophoretic method showed to be linear from 0.5 to 5 mg l −1 of iodide (the migration time was 2.6 min) with a quantitation limit of 0.45 mg l −1 and a intraday repeatability lower than 4% of R.S.D. at different concentration levels.