Electrochemical behavior and voltammetric determination of the herbicide metribuzin at mercury electrodes

The electrochemical behavior of the herbicide metribuzin (4-amino-6-teat-bu tyl-4,5-dihydro-3-methylthio-1,2,4-triazin-5-one) at mercury electrodes was studied in aqueous solutions by direct current (DC) and tast polarography, differential pulse (DPV) and cyclic voltammetry (CV), and controlled-poten tial coulometry. The electrolysis products were separated and identified by chromatographic techniques combined with mass spectrometric detection. The reduction process in acid media includes two irreversible steps. In the fi rst four-electron step the N-NH2 and the 1,6-azomethine bonds are reduced. The second step leads to the formation of 5-tert-butyl-2,3,4,5-tetrahydroim idazol-4-one at the mercury-pool electrode. The first reduction step combin ed with adsorptive accumulation of the herbicide molecule at the mercury el ectrode surface was used for its determination by differential pulse adsorp tive stripping voltammetry (DPAdSV). Calibration curves were linear in the range 1-30 mug L-1 with a detection limit of 0.27 mug L-1 (1 nmol L-1) unde r the conditions used (buffer pH 4.5, E-acc = -0.45 V relative to Ag/AgCl a nd t(acc) = 10 s). Preconcentration on solid-phase extraction columns (SPE-phenyl) was used fo r the determination of very small amounts of metribuzin in river water samp les. Recovery was approximately 97%. The reproducibility of the analytical procedure including SPE treatment and DPV determination was expressed as re lative standard deviations of 2.53 and 3.66% for 2 and 6 mug L-1 metribuzin , respectively.