Screening method for 11-nor-Δ9-tetrahydrocannabinol-9-carboxylic acid in urine using hollow fiber membrane solvent microextraction with in-tube derivatization

An efficient and inexpensive screening technique for the simultaneous clean-up, extraction, and derivatization of 11-nor-Δ9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH) from urine has been developed. Using the principles of solvent microextraction in the form of a 20-μl volume of solvent placed inside a permeable hydrophobic polypropylene hollow fiber membrane, the analyte of interest is preconcentrated inside this tubing as the bulk sample solution is stirred for a given extraction time. The pH of the sample is raised by adding buffer after which the charged moiety is extracted as an ion pair with tetramethylammonium hydrogen sulfate. Using a mixture of N,O-bis(trimethylsilyl)trifluoroacetamide and octane as the extraction solvent allows the GC-unstable carboxylic acid metabolite to be derivatized during the extraction without prior sample clean-up steps such as filtration of the urine. After an 8-min extraction, a 6-μl portion is drawn up with a syringe and directly injected into a gas chromatograph for separation and analysis. Samples as low as 10 ng ml−1 were analyzed successfully and the limit of detection was estimated at 1.0 ng ml−1 with relative standard deviations lower than 10% in the final protocol.