Electrochemical detection of the herbicide paraquat in natural water and citric fruit juices using microelectrodes

A novel electroanalytical procedure for detecting the paraquat herbicide in natural water and citric fruit juice samples using gold microelectrodes and square wave voltammetry at high frequencies is proposed. The results obtained showed two reversible peaks for the reduction of paraquat, the first peak associated with the reduction of the paraquat molecule in solution, with subsequent adsorption of the intermediate on the electrode surface. This adsorbed species was shown to undergo electroreduction in a reaction associated to the second voltammetric peak. The variation in pH and square wave parameters indicated that the best conditions under which paraquat could be reduced were a pH of 5.0, a frequency of 1000 s(-1), a scan increment of 2 mV and a square wave amplitude of 50 mV. Under these conditions, the variation of the concentrations of paraquat from 1.00 x 10(-6) to 1.66 x 10(-4) mol L-1 presented, for peak 1, detection and quantification limits of 4.51 and 15.05 mu g L-1 respectively in pure electrolyte with a recovery factor of 99.50%. The proposed analytical procedure was also applied to natural water samples giving recovery factors of 95.00, 89.50 and 92.50% in three water samples collected from an urban stream. The recovery factor was observed to depend on the content of organic matter which was determined by the biochemical and chemical oxygen demand. In lemon and orange juice samples that were spiked with 5.70 x 10(-5) mol L-1 of paraquat, the recovery factors obtained were 94.30 and 92.70% respectively. (c) 2005 Elsevier B.V. All rights reserved.