Comparison of silver, gold and modified platinum electrodes for the electrochemical detection of iodide in urine samples following ion chromatography

The electrochemical (EC) detection of iodide at gold, silver and platinum electrodes under similar experimental conditions was evaluated. To achieve optimal amperometric detection, the electrode sensitivity, selectivity, and stability was compared. Isocratic separation of iodide was attained by ion chromatography (IC) using an anion-exchange column with nitrate as an eluent ion (25 mM HNO3 + 50 mM NaNO3). Although the Ag electrode showed the highest selectivity due to the relatively low applied potential (+0.10 V versus Ag|AgCl), it requires continuous surface polishing upon injection of standard solutions or real samples; in addition, the chromatographic peak of iodide exhibited a pronounced dip-tailing. The limit of detection (LoD) of iodide was estimated to be 3.5 μg/L (S/N = 3) with an injection volume of 50 μL. Likewise, pulsed electrochemical detection at the silver electrode did not demonstrate the expected results in terms of peak shape and low detection limit. Using the same chromatographic conditions, iodide detection at the Au electrode (Eapp = +0.80 V versus Ag|AgCl) exhibited a regular peak shape accompanied by a sensitivity comparable to the silver one. Yet, upon continuous injections the signal intensity displayed a progressive lowering up to ca. 40% in 6 h. Best results in terms of signal stability, peak shape and analytical response were obtained with a modified platinum electrode which allowed to achieve a LoD of 0.5 μg/L (S/N = 3). The present IC-EC detection method using a modified Pt electrode (Eapp = +0.85 V versus Ag|AgCl) was successfully applied to determine low contents of iodide in human urine with solid phase extraction as pretreatment. Such a developed method correlated very well with the reference colorimetric method in urine (r = 0.95273), and it is specifically suggested when the iodide content is relatively low, i.e., <20 μg/L.