Using the cationic surfactants N-cetyl-N-methylpyrrolidinium bromide and 1-cetyl-3-methylimidazolium bromide for sweeping–micellar electrokinetic chromatography

This paper describes a sweeping–micellar electrokinetic chromatography (sweeping–MEKC) technique for the determination of seven benzodiazepines, using, as sweeping carriers, the ionic liquid-type cationic surfactants 1-cetyl-3-methylimidazolium bromide (C16MIMBr) and N-cetyl-N-methylpyrrolidinium bromide (C16MPYB). These surfactants resemble the commonly employed cationic surfactant cetyltrimethylammonium bromide (CTAB), but they provide different separation efficiencies. We optimized the separation and sweeping conditions, including the pH, the concentrations of organic modifier and surfactant, and the sample injection volume. Adding C16MIMBr or C16MPYB to the background electrolyte enhanced the separation efficiency and detection sensitivity during the sweeping–MEKC analyses of the benzodiazepines. C16MIMBr enhanced the sensitivity for each benzodiazepine 31–59-fold; C16MPYB, 86–165-fold. In the presence of C16MPYB, the limits of detection for the seven analytes ranged from 4.68 to 9.75 ng/mL. We adopted the sweeping–MEKC conditions optimized for C16MPYB to satisfactorily analyze a human urine sample spiked with the seven benzodiazepines. To minimize the matrix effects, we subjected this urine sample to off-line solid phase extraction (SPE) prior to analysis. The recoveries of the analytes after SPE were satisfactory (ca. 77.0–88.3%). Our experimental results reveal that the cationic surfactant C16MPYB exhibits superior sweeping power relative to those of C16MIMBr and CTAB and that it can be applied in sweeping–MEKC analyses for the on-line concentrating and analyzing of benzodiazepines present in real samples at nanogram-per-milliliter concentrations.