Selective Determination of Methylmercury by Flow-Injection Fast-Scan Voltammetry

Abstract

A simple flow-injection system, suitable for solution flow rates at microliters-per-minute, has been combined with fast-scan voltammetry for selective determination of methylmercury. A thin Hg film was formed at a Pt microelectrode prior to the measurement. Detection of methylmercury is carried out by measuring the oxidation of methylmercury radicals that have been generated at the Hg microelectrode. At slow scan rates, the electrogenerated methylmercury radicals undergo a follow-up dimerization reaction to form dimethyldimercury (E r C i 2 mechanism). At fast scan rates, it was found that methylmercury radicals can be quantitatively reoxidized (reversible electron-transfer). Optimization of the experimental conditions of the system was performed based on studies of the relationship between the scan rate and the dimerization rate of the methylmercury radical. Under optimized conditions, detection level of subnanomole was obtained with a sample consumption of less than 10 μL and the concentration detection limit for methylmercury at 50 V/s was estimated to be about 0.56 μM. To demonstrate the applicability of this method to automatic analyses, repetitive fast-scan cyclic voltammetry was conducted in conjunction with multiple sample injections. Determination of methylmercury in the presence of excess inorganic mercury was also conducted. This new approach to methylmercury determination was successfully applied to the analysis of elevated dogfish muscle samples.

Voltammetric Studies on the Electrochemical Reduction of Methylmercury in HCl Aqueous Medium at a Carbon Microelectrode

“…Typical cyclic voltamograms of methylmercury chloride at a carbon microelectrode are shown in Figure 1. Data are very similar to those ones obtained by Heaton and Laitinen with the dropping mercury electrode [26] and by Lai et al [27].…”

“…The analysis of the calibration data leads to the analytical equation I (nA) ¼ À 0.585 þ 0.0098 C, with C expressed in mM and a regression coefficient of 0.99. The results are very similar to that obtained by Lai et al [27], who have used fast scan voltammetry conducted in a microflow system on a Pt-based Hg electrode prepared ex situ.…”

“…The voltammetric data of Lai et al [27] show one broad cathodic peak and a well-defined anodic peak. According to their conclusions, at fast scan rates, the methylmercury cation can be quantitatively oxidized.…”

“…These studies have been performed in various aqueous media: acid and basic solutions, complexing and noncomplexing media, in the presence and in the absence of Triton-X100. A more recent work on the electrochemical reduction of the methylmercury has been carried out by Lai et al [27], in a miniaturized electrochemical flow cell, at fast scan rates, with an ex situ plated mercury thin film platinum electrode.…”