On-line stacking and sweeping capillary electrophoresis for detecting heroin metabolites in human urine

Heroin metabolites including morphine, codeine, and 6-acetylmorphine were determined by cation-selective exhaustive injection and sweeping micellar electrokinetic chromatography (CSEI–sweep-MEKC). Liquid–liquid extraction was used for urine pretreatment. An uncoated fused silica capillary (Ld = 30 cm, 50 μm ID) was filled with phosphate buffer (50 mM, pH 2.5) containing 30% methanol, then high conductivity buffer (100 mM phosphate, 41.3 kPa for 18 s) was followed. Samples were injected electrokinetically (20 kV, 300 s). The sweeping and separation were performed at −25 kV using phosphate buffer (20 mM, pH 2.5) and 80 mM sodium dodecyl sulfate. The baseline separation was done within 10 min. During method validation, the calibration curves were linear over a range of 50–500 ng/mL (r ≧ 0.994). The RSD and RE values in intra-day and inter-day assays were all below 20%, which showed good precision and accuracy. Their detection limits were 10 ng/mL (S/N = 3). The optimized method was applied to determine real urine samples from addicts. These samples were confirmed by liquid chromatography/mass spectrometry.