On-line weak cationic mixed-mode solid-phase extraction coupled to liquid chromatography–mass spectrometry to determine illicit drugs at low concentration levels from environmental waters

This study presents a fully automated method based on on-line solid-phase extraction coupled to liquid chromatography with mass spectrometry detection (on-line-SPE-LC–MS) to determine illicit drugs in environmental water samples. The mixed-mode Oasis WCX sorbent used in an optimised protocol allows the addition of an effective washing step with 0.5 mL of methanol, which washed out all the interferences retained by reversed-phase interactions and helped to reduce the matrix effect, while the cationic target analytes remained bound and could then be selectively eluted with recovery values near to 100%. This method was successfully applied to the analysis of 10 mL of environmental water (river and sewage) spiked at low ng L−1 levels of the analytes, with recoveries from 50 to 100% for all studied analytes. The method was also validated with river water samples with figures of merit comparable to those of the off-line SPE-LC–MS/MS method.