Integration of the free liquid membrane into electrokinetic supercharging – capillary electrophoresis for the determination of cationic herbicides in environmental water samples

Abstract

A new approach based on the integration of the free liquid membrane (FLM) into electrokinetic supercharging (EKS) was demonstrated to be a new powerful tool used in order to enhance online preconcentration efficiency in capillary electrophoresis (CE). A small plug of water immiscible organic solvent was used as a membrane interface during the electrokinetic sample injection step in EKS in order to significantly enhance the analyte stacking efficiency. The new online preconcentration strategy was evaluated for the determination of paraquat and diquat present in the environmental water samples. The optimised FLM-EKS conditions employed were as follows: hydrodynamic injection (HI) of 20 mM potassium chloride as leading electrolyte at 50 mbar for 75 s (3% of the total capillary volume) followed by the HI of tris(2-ethylhexyl) phosphate (TEHP) as FLM at a 1 mm length (0.1% of the capillary volume). The sample was injected at 10 kV for 360 s, followed by the HI of 20 mM cetyl trimethylammonium bromide (CTAB) as terminating electrolyte at 50 mbar for 50 s (2% of the total capillary volume). The separation was performed in 12 mM ammonium acetate and 30 mM NaCl containing 20% MeOH at +25 kV with UV detection at 205 nm. Under optimised conditions, the sensitivity was enhanced between 1500- and 1866-fold when compared with the typical HI at 50 mbar for 50 s. The detection limit of the method for paraquat and diquat was 0.15–0.20 ng/mL, with RSDs below 5.5%. Relative recoveries in spiked river water were in the range of 95.4–97.5%. A comparison was also made between the proposed approach with sole preconcentration of the field-enhanced sample injection (FASI) and EKS in the absence of the FLM.