Determination of cadmium(II) and zinc(II) in olive oils by derivative potentiometric stripping analysis

A method for the determination of cadmium(II) and zinc(II) in olive oils by derivative potentiometric stripping analysis after dry ashing of the sample is described. The metal ions were concentrated as their amalgams on the glassy carbon working electrode that was previously coated with a thin mercury film and then stripped by a suitable oxidant. Potential and time data were digitally converted into dt dE−1, and E was plotted vs. dt dE−1, thus increasing sensitivity of the method and improving resolution of the analysis. Quantitative analysis was carried out by the method of standard additions; a good linearity was obtained in the range of concentrations examined. Recoveries of 92–102% for cadmium(II) and of 89–99% for zinc(II) were obtained from an olive oil spiked at different levels. The detection limits were 5.1 ng g−1 for cadmium(II) and 7.6 ng g−1 for zinc(II) and the relative standard deviations (mean of nine determinations) were 4.1% and 5.2%, respectively. Results obtained on commercial olive oils were not significantly different from those obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES).