Simultaneous Determination of 24 Abused Drugs in Sewage by Liquid Chromatography-Tandem Mass Spectrometry with Online Solid Phase Extraction

The filtered sewage sample （10 mL） was taken, and 200 μL of 1 mol·L⁻¹ phosphate buffer solution （pH 7.2） and 100 μL of 4 μg·L⁻¹ mixed isotope internal standard solution were added. The mixture was homogenized, and 24 abused drugs in the solution were determined by online solid phase extraction-liquid chromatography-tandem mass spectrometry. Online solid phase extraction was performed using Oasis HLB Direct Connect solid phase extraction column. Poroshell 120 EC C₁₈ column was used as the stationary phase and the mixed solution composed of 0.1% （volume fraction, the same below） formic acid solution and acetonitrile solution containing 0.1% formic acid at different volume ratios was used as the mobile phase for gradient elution. Electrospray ion （ESI） source was used for mass spectrometry. The targets were analyzed by multiple reaction monitoring （MRM） mode in positive ion and negative ion scanning mode. As shown by the results, the sewage sample could be stored at 4 ℃ and tested within 15 d, while repeated freeze-thaw cycles were avoided. The pretreated sample solution was demonstrated to have good stability within 24 h. Linear relationships between the ratios of the corresponding peak areas to the isotope internal standard peak areas and mass concentrations of 24 abused drugs were kept in definite ranges, with lower limits of determination （10S/N） in the range of 0.2‒5.0 ng·L⁻¹. Test for recovery was made by the standard addition method, giving results in the range of 82.8%‒110%, with RSDs （n=6） of the determined values less than 10%. This method was applied to the analysis of 6 actual sewage samples, and cotinine, methamphetamine, codeine, tramadol, and ephedrine were all detected.