Abstract: Graphite/PTFE composite electrodes (details given) were used as indicator electrodes for the flow injection amperometric detection of the herbicides, thiram (I) and disulfiram (II) at a potential of +1 V vs. Ag/AgCl/3 M KCl in a carrier stream (1.9 ml/min) of 0.1 M phosphate buffer at pH 7.4 and with an injection volume of 250 µL. Calibration graphs were linear up to 40 µM-I and -II and the detection limits were 0.043 and 0.02 µM, respectively. RSD (n = 10) were 7.7 and 5.7%, respectively, for 0.1 µM of I and II. Recoveries of 40 µg/l of I from spiked tap and well water were >97%. The adsorptive pre-concentration of the herbicides from flowing streams (2.7 ml/min) was carried out at 0.0 V. At the end of the pre-concentration period the phosphate buffer stream was passed for 30 s prior to the determination of the suface-bound herbicide by applying a linear sweep anodic potential ramp up to +1.2 V. Calibration graphs were linear from 0.4-1 and 0.2-1 µM-I and -II, respectively. The continuous-flow injection separation of I and II was carried out by inserting a 30-40 µm VYDAC SC-201 column (3 cm x 0.2 mm i.d.) into the FIA system and using acetonitrile/0.1 M phosphate buffer at pH 7.4 (1:3) as mobile phase (1.9 ml/min).

Abstract: The voltammetric behavior of two dithiocarbamic acid derivative herbicides, thiram and disulfiram, at a carbon paste electrode modified with the electron mediator cobalt phthalocyanine is described. Cyclic voltammograms showed a decrease of the overpotential of around 100 mV for the electrochemical oxidation of the herbicides, as well as a remarkable increase in the peak current, using the modified electrode with respect to the plain carbon paste electrode. The appropriate CoPC percentage in the carbon paste and the working medium used as background electrolyte were optimized by differential pulse voltammetry. Both thiram and disulfiram showed enhanced oxidation signals when open-circuit accumulation in stirred solutions was carried out, and, consequently, the adsorption process can be used as an effective pre-concentration step. The dependence of peak current on accumulation time was studied. Using differential pulse voltammetry in the stripping step, limits of detection of 7.1 x 10^-8 and 2.2 x 10^-8 mol L-1 were obtained for thiram and disulfiram, respectively. The effect of the presence of the herbicide ziram, phenol, and Zn(II) on the oxidation signals for thiram and disulfiram was evaluated. The developed using the catalytic-adsorptive processes of the herbicides at the CoPC modified electrode was applied to the determination of thiram in samples of spiked strawberries. Finally, the capabilities of the modified electrode as an amperometric indicator electrode for flow injection analysis with pulsed amperometric detection were considered. (19 References)