An improved transient isotachophoresis (tITP) procedure for the preconcentration of iodide from highly saline matrices was developed with the objective to quantify iodide in seawater by capillary electrophoresis (CE). The procedure takes advantage of introducing cetyltrimethylammonium chloride into the high-sodium chloride background electrolyte, which due to a specific interaction with iodide amended placement of the analyte at a large distance from the matrix chloride (the latter performed the role of a leading anion). Computer simulation showed that 2-(N-morpholino)ethanesulfonate could be adopted as a suitable terminating ion to enable isotachophoretic focusing at the beginning of the CE run. Under optimized tITP conditions, the sensitivity response of iodide was improved by a factor of 140 over normal CE mode. This allowed for direct UV detection of as low as 0.6 microg/L iodide and made feasible CE analysis of undiluted surface seawater samples where iodide was found at a 30 microg/L level. The applicability of the proposed tITP-CE method could apparently be extended to the determination of other trace seawater anions (e.g., iodate).

While cationic surfactants are usually included in the separation electrolyte to reverse the electroosmotic flow, the presence of the surfactant may also offer a means of capillary electrophoresis (CE) separation selectivity control over the anionic analytes, especially those that are prone to ion-pairing interaction. For one such analyte anion, iodide, the formation of several ion-association/partition products with cetyltrimethylammonium chloride (CTAC) was first discovered when optimizing (decelerating) iodide mobility (in order to achieve effective transient isotachophoretic stacking). At comparatively high concentrations of iodide (≥0.01 mM) and the cationic surfactant well above the critical micelle concentration (25 mM), an additional peak due to interactions with the CTAC micelle was recorded, with a UV absorption spectrum fairly different from those of both interacting partners and also the iodide–monomeric surfactant ion pair. Never observed before in normal CE mode, this phenomenon is believed to have occurred due to the enrichment effect of the initial isotachophoresis state.

Usefulness of capillary electrophoresis (CE) in combination with transient isotachophoresis (tITP) as on-line preconcentration technique was examined for routine analysis of trace iodide in seawater. The combined method was based on an electrolyte system where 0.5moll−1 sodium chloride and 25mmoll−1 cetyltrimethylammonium chloride at an acidic pH and 0.5moll−1 2-(N-morpholino)ethanesulfonate (pH 6.0) served as carrier and terminating electrolyte, respectively. Under optimized tITP-CE conditions (a carrier electrolyte pH of 2.4), the peak of 1μgl−1 iodide in artificial seawater can be clearly detected at 226nm, with the detection limit as low as 0.2μgl−1. Calibration curve was linear in the range of 0–40μgl−1 (n=10; regression coefficient = 0.99992). Relative standard deviation values of the migration time, peak height and area of surface seawater iodide (0.6, 3.1 and 1.5%, respectively) proved an excellent run-to-run reproducibility performance of the proposed method. The method was applied to seawater samples containing sub- and low-μgl−1 levels of iodide, and the results obtained agreed well with ion chromatographic data. Detection of nitrate and nitrite was also possible in the same tITP-CE run (limits of detection 25 and 20μgl−1, respectively, at 210nm). The analytical characteristics of the method were discussed in comparison with an ion chromatographic procedure previously developed in authors’ laboratory.