Journal of Chromatography A, 1032 (2004) 237–242 Capillary electrophoresis as a useful tool for the analysis of chemical tracers applied to hydrological systems V. Mart´ ınez a , N. Garc´ ıa a , I. Antigüedad b , R.M. Alonso a,∗ , R.M. Jiménez a a Departamento de Qu´ ımica Anal´ ıtica, Facultad de Ciencias, Universidad del Pa´ ıs Vasco/EHU, Apdo. 664, 48080 Bilbao, Spain b Grupo Hidrogeolog´ ıa, Departamento de Geodinámica, Facultad de Ciencias, Universidad del Pa´ ıs Vasco/EHU, Apdo. 664, 48080 Bilbao, Spain Abstract 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 1mM 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. © 2003 Elsevier B.V. All rights reserved. Keywords: Water analysis; Iodide; Sulphite; Inorganic anions 1. Introduction In environmental studies, determination of common in- organic anions, such as fluoride, chloride, bromide, nitrite, nitrate and sulphate is a significant component in the char- acterisation of the quality and extent of pollution in water. It is also a fundamental analytical task required to address many diverse questions in science studies. Accurate and reliable analytical data of chemical species present in such water samples are essential to understand the physical and chemical processes in water systems. Hydro- geologists promote the use of chemicals as tracers for the quantification of flow and transport processes in hydrologi- cal systems. Potassium iodide (KI) is the iodine-containing compound, most commonly used as tracer in hydrogeology. It shows a solubility of 1.2 kg l -1 in cold water and it is not toxic at the ∗ Corresponding author. E-mail address: qapalror@lg.ehu.es (R.M. Alonso). concentration used. Its determination is commonly interfere with S 2- , CN - , Br - , and Cl - (cited in order of interference strength). Iodide can be chemically and microbiologically converted [1]. Behrens [2] added sulphite to the samples to be investigated to prevent oxidation to iodate. He also de- scribed the incorporation of iodide to organisms. Therefore, it follows that iodide is only suitable as tracer for short flow distances. In the literature, 1 g Na 2 SO 3 in 100 ml of water is the concentration recommended [3]. The recovery rates resulting from the few tracing test us- ing KI carried out up to now in the Basque Country, have always been very low. Recovery rates below 30% have been reported in a karstic aquifer whereas other tracers assayed simultaneously, such as LiCl, provided recovery rates above 90%. The improvement in the analytical technique for the separation and determination of iodide should allow a better knowledge of the recovery rates and hence, the identifica- tion of the flow and transport processes that occur. The determination of iodide is very important in a vari- ety of fields, such as food, clinical and biological science [4], industrial [5], and also environmental [6] applications. 0021-9673/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2003.11.015