Determination of benzodiazepines, related pharmaceuticals and metabolites in water by solid-phase extraction and liquid-chromatography–tandem mass spectrometry

This work presents a method for the simultaneous determination of 23 benzodiazepines, metabolites and related pharmaceuticals (zolpidem, methylphenidate and ritalinic acid) by solid-phase extraction (SPE) followed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Different SPE cartridges were considered: hydrophilic modified reversed-phase (Oasis HLB) and their modified versions containing also a cationic-exchange group (Oasis MCX) or anionic-exchange (Oasis MAX) funcionalities. Stability of analytes and the impact of the final eluate volume on the matrix effects on LC-MS/MS were also considered. In the final method, 100-200 mL of sample are extracted with Oasis MCX (60 mg), eluted with 5 mL of methanol (1.25% NH3) and the eluate concentrated and analyzed by LC-MS/MS. Under these conditions, LOQs were established between 0.1 and 18 ng L(-1) for influent wastewater. The use of surrogated deuterated internal standards allows obtaining recoveries in the 84-122% range. Finally the method was applied to determinate the analytes in wastewater and surface water and 10 compounds were detected in the range of 0.5-170 ng L(-1), being the ritalinic acid (the main metabolite of methylphenidate) the analyte detected in the highest concentrations.