Quantification of metals released by metallothionein adsorbates at mercury film electrodes by electrochemical ICP-atomic emission spectrometry

Electrochemistry combined on-line with inductively coupled plasma-atomic emission spectrometry (EC/ICP-AES) was used to study the electrode reactions of rabbit liver metallothionein (MT) adsorbates at thin mercury films (TMFs). Three major oxidation peaks, with peak potentials at −1.20, −0.75 and −0.32 V, were observed. Through the correlation of the DPV characteristics to the elution of the Zn2+ and Cd2+ ions monitored by time-resolved ICP-AES responses, the peak at −1.20 V was assigned to the anodic stripping of Zn(Hg). For the first time, both Zn2+ and Cd2+ were found to be associated with the initial oxidation at −0.75 V by EC/ICP-AES. In the subsequent voltammetric scans, the peak at −0.75 V decreased but remained steady and did not result in the elution of additional metals. The initial decrease in current is suggested to originate from the anodic stripping of the Cd(Hg) amalgam and the Zn–Cd intermetallic compound in the TMF. The anodic peak remaining at −0.75 V was attributed to the formation of the cystine analog from the electrodimerization of the uncomplexed cysteine residues in MT molecules. The quantities of Cd2+ and Zn2+ under the oxidation peaks were measured by EC/ICP-AES. Our work demonstrates that EC/ICP-AES is a viable technique for elucidating relatively complicated electrode reactions that involve metal release and transfer.