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Screening and confirmation of drugs of abuse in body fluids are important for the medicinal treatment and form the legal basis of court judgments. A fast and precise identification of toxic substances is necessary. Morphine was determined in human plasma by capillary zone electrophoresis (CZE) and micellar electrokinetic capillary chromatography (MECC) using sample stacking mode. The electrophoretic separation was performed in an uncoated fused-silica capillary, 70 cm long to the detector, with an additional 10 cm to the cathode (75 µm i.d. and 360 µm o.d.). The UV absorbance detection was set at 190 nm. The electrophoretic buffers were prepared from 60 to 300 mm disodium tetraborate decahydrate, pH 10.5. Sodium dodecyl sulfate was added to the final solution in a concentration of 60 mm for MECC. All electrophoretic separations were carried out at 10 kV and the capillary temperature was ambient (25°C). A linear calibration graph was obtained in the concentration range studied (50–5000 ng/mL). Several samples of drug-free plasma were checked for potential endogenous interference and the results showed no interference from the endogenous components, which co-migrated with morphine. As little as 50 ng/mL of morphine could be successfully analyzed by MECC in the concentration mode with acceptable precision. It is possible to determine morphine directly in plasma at therapeutic concentrations. Copyright © 2003 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]