Direct and sensitive analysis of methamphetamine, ketamine, morphine and codeine in human urine by cation-selective exhaustive injection and sweeping micellar electrokinetic chromatography

Cation-selective exhaustive injection and sweeping micellar electrokinetic chromatography (CSEI-Sweep-MEKC) was directly used to test some abuse drugs in human urine, including morphine (M), codeine (C), ketamine (K) and methamphetamine (MA). First, phosphate buffer (50 mM, pH 2.5) containing 30% methanol was filled into uncoated fused silica capillary (40 cm, 50 μm I.D.), then high conductivity buffer (100 mM phosphate, 6.9 kPa for 99.9 s) was followed. Electrokinetic injection (10 kV, 500 s) was used to load samples and to enhance sensitivity. The stacking step and separation were performed at −20 kV and 200 nm using phosphate buffer (25 mM, pH 2.5) containing 20% methanol and 100 mM sodium dodecyl sulfate. Using CSEI-Sweep-MEKC, the analytes could be simultaneously analyzed and have a detection limit down to ppb level. It was unnecessary to have sample pretreatments. During method validation, calibration plots were linear (r ≥ 0.9982) over a range of 150–3000 ng/mL for M and C, 250–5000 ng/mL for MA, and 50–1000 ng/mL for K. The limits of detection were 15 ng/mL for M and C, and 5 ng/mL for MA and K (S/N = 3, sampling 500 s at 10 kV). Comparing with capillary zone electrophoresis, the results indicated that this stacking method could increase 6000-fold sensitivity for analysis of MA. Our method was applied for analysis of 28 real urine samples. The results showed good coincidence with immunoassay and GC–MS. This method was feasible for application to detect trace levels of abused drugs in forensic analysis.