Novel cation selective exhaustive injection-sweeping procedure for 5-nitroimidazole determination in waters by micellar electrokinetic chromatography using dispersive liquid–liquid microextraction

A novel method consisting of cation-selective exhaustive injection and sweeping (CSEI-sweeping) as on-line preconcentration followed by a micellar electrokinetic chromatography (MEKC) separation has been developed for the determination of 5-nitroimidazoles (5-NDZ) in environmental waters. Moreover, dispersive liquid–liquid microextraction (DLLME) has been proposed for first time as sample treatment technique prior to CSEI-sweeping-MEKC. DLLME was applied to 5 mL of sample. Dibromomethane (1156 μL) and 2-butanol (1363 μL) were employed as extractant and dispersive solvents, respectively. Salting-out effect was achieved by the addition of 16% (w/v) NaCl to the samples. After DLLME and organic solvent evaporation, the residue was redissolved in a low conductivity solvent (5 mM phosphoric acid with 5% of methanol) and electrokinetically injected at 9.8 kV for 632 s in a bare fused-silica capillary (57.2 cm, 50 μm I.D.). Prior to the injection, the capillary was rinsed with 50 mM phosphate buffer pH 2.5, followed by a plug of a higher conductivity buffer (100 mM phosphate pH 2.5, 50 mbar, 264 s) and a plug of water (50 mbar, 2 s). Separation was carried out applying −30 kV at 20 °C in 44 mM phosphate buffer pH 2.5, containing 8% tetrahydrofuran and 123 mM sodium dodecyl sulfate. Analytical signals were monitored at 276 nm. Validation was performed in river and well waters, obtaining satisfactory results in terms of linearity, precision (% RSD generally lower than 10%) and trueness (recoveries higher than 70% in almost all cases). LODs ranged from 0.61 to 2.44 ng/mL. The combination of this microextraction technique with the proposed capillary electrophoresis methodology supposes a simple, sensitive and cheap alternative for 5-NDZ analyses, in accordance with the aims of green chemistry.