High-sensitivity low-cost methods for determination of cocaine in hair: high-performance liquid chromatography and...

Free solution capillary electrophoresis and high performance liquid chromatography were applied to the analysis of cocaine in hair. Capillary electrophoresis conditions were as follows. Background electrolyte: 0.050 M borate, pH 9.2; capillary: bare silica, 40 cm long, 50 micrograms i.d.; potential: 15 kV; detection: UV at 238 nm wavelength. In addition, the separation was accomplished in a 50 cm x 75 microns capillary with an instrument with a photodiode array detector, recording on-line UV spectra of peaks from 200 to 400 nm. The isocratic high performance liquid chromatographic method with fluorimetric detection used 230 nm (ex.) and 315 nm (em.) wavelengths. Cocaine separation was carried out under conditions summarized as follows: column packed with spherical polystyrene-divinylbenzene 5 microns particles, mobile phase 0.1 M potassium phosphate (pH 3.0)/methanol/THF (70:25:5) at a flow rate of 0.5 ml/min and at room temperature. Liquid-liquid and solid-liquid sample preparation methods were used. Typically, levels of cocaine in hair as low as 0.15-0.30 ng/mg were detected by capillary electrophoresis, while HPLC allowed the determination of concentrations lower by one order of magnitude (0.015 ng/mg). Intra- and inter-assay precision data of the two techniques were comparable and characterized by relative standard deviations in the range from 3 to 7%. On the other hand, the sample throughput of capillary electrophoresis (7-10 injections/h) was higher than high performance liquid chromatography (2 injections/h). A good correlation of the results (r2 = 0.92) between the two techniques was observed in the assay of real cases.