Cathodic stripping voltammetry of glutathione in natural waters

Abstract

The electrochemistry of metal–glutathione complexes at a hanging mercury drop electrode was investigated and a procedure was optimized to determine glutathione in natural waters using cathodic stripping voltammetry (CSV). Adsorption of complexes of mercury(I) and copper(I) with glutathione can be utilized to preconcentrate the glutathione on the electrode prior to a voltammetric scan. The greatest sensitivity was obtained when the voltammetric scan was preceded by adsorptive deposition of a 1:1 complex of copper(I) with glutathione in the presence of added copper(II). The reduction current of the copper(II) in the adsorbed glutathione complex was utilized to quantify the concentration of glutathione in solution. The sensitivity of the CSV determination of glutathione in sea-water was high at pH 5 and 8.5 with optimum copper concentrations of 1 and 0.2 µmol l^–1, respectively. Adsorption was carried out at –0.225 V at pH 5 and –0.250 V at pH 8.5. The limit of detection was 1.2 nmol l^–1 of glutathione at pH 8.5 using a deposition time of 60 s. Interference by surface-active organic compounds in samples from the Mersey estuary was minimized by passing the samples through a Sep-Pak C₁₈ cartridge; interference by copper complexing compounds was minimized by omitting the addition of copper to the sample. A preliminary study of the Mersey estuary using the proposed procedure indicated the presence of glutathione at levels between 20 and 300 nmol l^–1; the glutathione behaved conservatively at salinities between 23 and 33 [of the practical salinity scale (psu)], decreasing with increasing salinity.