Analysis of the herbicides paraquat, diquat and difenzoquat in drinking water by micellar electrokinetic chromatography using sweeping and cation selective exhaustive injection

Optimum conditions for the determination of the herbicides paraquat, diquat and difenzoquat by micellar electrokinetic chromatography (MEKC) using sweeping and cation-selective exhaustive injection (CSEI) as on-line concentration methods were developed. Sodium dodecyl sulfate (80 mM) in 50 mM phosphate buffer (pH 2.5) with 20% acetonitrile was used as a background electrolyte for the methods studied. The limits of detection, based on a signal-to-noise ratio of 3:1, were about 2.6–5.1 mg l−1 in purified water when MEKC was applied for the standards. By using an on-line preconcentration method known as sweeping–MEKC, up to a 500-fold increase in detection sensitivity was obtained whereas up to a 50 000-fold increase for CSEI–sweeping–MEKC was achieved. The limits of detection using optimum CSEI–sweeping–MEKC were lower than 1 μg l−1 and the method was validated obtaining good reproducibility (relative standard deviation lower than 22%) and linearity. CSEI–sweeping–MEKC was successfully applied to the determination of the three herbicides in spiked tap water below the levels established by the US Environmental Protection Agency.