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 (C(16)MIMBr) and N-cetyl-N-methylpyrrolidinium bromide (C(16)MPYB). 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 C(16)MIMBr or C(16)MPYB to the background electrolyte enhanced the separation efficiency and detection sensitivity during the sweeping-MEKC analyses of the benzodiazepines. C(16)MIMBr enhanced the sensitivity for each benzodiazepine 31-59-fold; C(16)MPYB, 86-165-fold. In the presence of C(16)MPYB, 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 C(16)MPYB 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 C(16)MPYB exhibits superior sweeping power relative to those of C(16)MIMBr 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.

Chaotropic salts: novel modifiers for the capillary electrophoretic analysis of benzodiazepines.

This paper describes a CE method for analyzing benzodiazepines using the chaotropic salts lithium trifluoromethanesulfonate (LiOTf), lithium hexafluorophosphate (LiPF(6)), and lithium bis(trifluoromethanesulfonyl)imide (LiNTf(2)) as modifiers in the running buffer. Although adequate resolution of seven benzodiazepine analytes occurred under the influence of each of the chaotropic anions, the separation efficiency was highest when bis(trifluoromethanesulfonyl)imide (Tf(2)N(-)) was the modifier. We applied affinity CE in conjunction with linear analysis to determine the association constants for the formation of complexes between the Tf(2)N(-) anion and the benzodiazepines. According to the estimated Gibbs free energies, the interactions between this chaotropic anion and the benzodiazepines were either ion-dipole or ion-induced dipole interactions. Adding chaotropic salts as modifiers into CE buffers is a simple and reproducible technique for separating benzodiazepines.