Analytical voltammetry in low-permitivity organic solvents using disk and cylindrical microelectrodes. Determination of thiram in ethyl acetate

Abstract

The electrochemical behavior of the N, N-dialkyldithiocarbamate pesticide thiram at a Au microdisk electrode (10 μm φ) and at cylindrical carbon fiber microelectrodes (CFMEs) (8 μm φ, 8 mm length) in low-permitivity organic solvents such as toluene and ethyl acetate is reported. The obtained cyclic voltammograms were very different in shape depending on the microelectrode and the solvent used, as well as on thiram concentration. Analytically useless responses were obtained with the gold microelectrode. However, a good fitting of the voltammetric signals at cylindrical CFMEs with the theoretical model proposed by Aoki was found. The influence of variables such as the concentration of thiram and of the background electrolyte, the length of the CFMEs and the potential scan rate was tested by CV. The oxidation responses obtained by square-wave voltammetry at carbon fiber microelectrodes in ethyl acetate were found to be analytically suitable to develop a method for the determination of thiram at low concentration levels. A linear calibration graph in the 1.0×10 −6–6.0×10 −4 mol l −1 concentration range was obtained, with a slope of (1.2±0.1)×10 4 μA l mol −1. The limit of detection was 4.3×10 −7 mol l −1 thiram, and the relative standard deviation at a 5.0×10 −5 mol l −1 concentration level ( n=10) was of 1.6%. A good reproducibility of the electroanalytical measurements was found with no need of cleaning or pretreatment procedures of the CFMEs surface. The SWV method was applied with good results to the determination of thiram in spiked grapes, by measuring directly the net current in the analyte extract in ethyl acetate. Commercial fungicides and pharmaceutical preparations containing thiram were also analysed.