Determination of Δ9-tetrahydrocannabinol and 11-nor-9-carboxy-Δ9-tetrahydrocannabinol in water samples by solid-phase microextraction with on-fiber derivatization and gas chromatography–mass spectrometry

A new method for the determination of two cannabinoids, Δ9-tetrahydrocannabinol (THC) and 11-nor-9-carboxy-Δ9-tetrahydrocannabinol (THCCOOH), in wastewater samples is proposed using solid-phase microextraction (SPME) as extraction technique and gas chromatography coupled to mass spectrometry (GC–MS) as determination technique. Several parameters involved in SPME extraction were studied and optimized (time, temperature, volume, pH and ionic strength of sample, and type of fiber). Moreover two derivatization reagents, N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) and N-methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide (MTBSTFA), were studied in depth, providing MSTFA the best results. The use of internal standard calibration (with their corresponding deuterated analogs) allows avoiding standard addition calibration for the quantification of samples. Under optimal conditions, limits of detection at low nanograms per liter were achieved (1.0 and 2.5 ng L−1 for THC and THCCOOH respectively). Precision (RSD < 15%) and trueness (92–112% relative recovery) were also satisfactory. Analysis of several samples from a wastewater treatment plant (WWTP) shows the presence of cannabinoids at notable concentrations in raw wastewater (12–35 ng L−1 for THC and 50–153 ng L−1 for THCCOOH) and an incomplete removal, which translates into their detection in the receiving river.