Square-wave anodic-stripping voltammetric determination of Cd, Pb and Cu in wine: Set-up and optimization of sample pre-treatment and instrumental parameters

Abstract

For the first time, square-wave anodic-stripping voltammetry (SWASV) was set up and optimized for the determination of Cd, Pb and Cu in white wine after UV photo-oxidative digestion of the sample. The best procedure for the sample pre-treatment consisted in a 6-h UV irradiation of diluted, acidified wine, with the addition of ultrapure H2O2 (three sequential additions during the irradiation). Due to metal concentration differences, separate measurements were carried out for Cd (deposition potential −950mV vs. Ag/AgCl/3M KCl deposition time 15min) and simultaneously for Pb and Cu (Ed −750mV, td 30s). The optimum set-up of the main instrumental parameters, evaluated also in terms of the signal-to-noise ratio, were as follows: ESW 20mV, f 100Hz, ΔEstep 8mV, tstep 100ms, twait 60ms, tdelay 2ms, tmeas 3ms. The electrochemical behaviour was reversible bielectronic for Cd and Pb, and kinetically controlled monoelectronic for Cu. Good accuracy was found both when the recovery procedure was used and when the results were compared with data obtained by differential pulse anodic stripping voltammetry. The linearity of the response was verified up to ∼4μgL−1 for Cd and Pb and ∼15μgL−1 for Cu. The detection limits for td=5min in the 10 times diluted, UV digested sample were (ngL−1): Cd 7.0, Pb 1.2 and Cu 6.6, which are well below currently applied methods. Application to a Verdicchio dei Castelli di Jesi white wine revealed concentration levels of Cd ∼0.2, Pb ∼10, Cu ∼30μgL−1 with repeatabilities of (±RSD%) Cd ±6%, Pb ±5%, Cu ±10%.