Graphite-poly(tetrafluoroethylene) electrodes as electrochemical detectors in flowing systems

The use of graphite-poly(tetrafluoroethylene) composite electrodes as indicator electrodes in flowing systems is reported. Flow-injection methods with amperometric detection based on the electrochemical oxidation of the herbicides thiram and disulfiram on these electrodes are described. No cleaning or pretreatment of the electrode surface after successive injections of 5.0 × 10−6 mol l−1 herbicide in 0.1 mol 1−1 phosphate buffer of pH 7.4 was required. Detection limits of 4.3 × 10−8 and 2.0 × 10−8 mol l−1 were achieved for thiram and disulfiram, respectively. The effect of the presence of several species which may be present together with thiram and/or disulfiram in water samples such as ziram, phenol and Zn(II) on the amperometric signals for both herbicides was evaluated. The determination of thiram in spiked tap and well waters at a concentration level of 40 μg 1−1 thiram was carried out with recoveries of 103 ± 6% for tap water, and 97 ± 7% for well water, respectively. The possibility of performing adsorptive accumulation of the analytes by passing through the flow cell a stream of the herbicides was demonstrated. After accumulation, quantitation by a linear sweep anodic potential ramp was accomplished, as well as an electrochemical cleaning of the electrode surface. Linearity was achieved in a shorter range, and no lower detection limits were attained with respect to the direct flow-injection methods. Nevertheless, using adsorptive preconcentration the selectivity was improved. Finally, a continuous separation scheme of both herbicides was accomplished in order to carry out their simultaneous determination.