Sensitive determination of paraquat and diquat at the sub-ng ml^–1 level by continuous amperometric flow methods

Two methodologies are described for the determination of paraquat and diquat. The first is based on the pre-treatment of an electrode with a surfactant solution, which improves the electrochemical determination of the herbicides. Linear calibration graphs were obtained in the ranges 10–80 and 10–100 ng ml^–1 for paraquat and diquat, respectively. The limits of detection were 6.32 for paraquat and 4.80 ng ml^–1 for diquat. The method was applied to the determination of the herbicides in synthetic water samples. The second methodology is based on the preconcentration of paraquat and diquat in a minicolumn packed with a cation-exchange material. The determination ranges and detection limits depend on the sample volume used (5–50 ml). Thus, 50 ml of sample provides limits of detection of 0.016 and 0.020 ng ml^–1 for paraquat and diquat, respectively. The applicability of the method was demonstrated with the determination of the herbicides in both synthetic and real water samples.