Electrokinetic supercharging in CE for the separation and preconcentration of barbiturate drugs in urine samples

Three barbiturate drugs, barbital, phenobarbital, and secobarbital were separated and analyzed by electrokinetic supercharging. The influence of different parameters on electrokinetic supercharging performance was evaluated using both univariated and multivariated optimization processes. The parameters studied were sample pH, concentration, and length of the leading and terminating electrolytes, electrokinetic injection of the sample and composition and hydrodynamic injection of the solvent plug. The leading electrolyte (50 mM NaCl) was hydrodynamically injected (50 mbar × 120 s) prior to the sample that was adjusted to pH 9.6 and electrokinetically injected at -8.5 kV for 300 s. The terminating electrolyte (100 mM of 2-(cyclohexylamino) ethanesulphonic acid) was then hydrodynamically injected (50 mbar × 140 s). The results showed that this strategy enhanced detection sensitivity around 1050-fold compared with normal hydrodynamic injection, providing detection limits ranging between 1.5 and 2.1 ng/mL for standard samples with good repeatability in terms of peak area (values of relative standard deviation, %RSD < 3). The applicability of the optimized method was demonstrated by the analysis of human urine samples spiked with the studied compounds at different concentration levels and further liquid-liquid extraction step. The estimated detection limits obtained in the urine samples extract ranged between 8 and 15 ng/mL.