Recovery of spiked Δ9-tetrahydrocannabinol in oral fluid from polypropylene containers

Oral fluid is currently used by Australian and international law enforcement agencies and employers to detect recent use of cannabis and other drugs of abuse. The main psychoactive constituent of cannabis, Δ9-tetrahydrocannabinol (THC), is highly lipophilic and losses occur when in contact with plastic, possibly due to its adsorption onto the plastic surface. This study aims to investigate factors governing the interaction of THC with plastic and search for ways of overcoming such interaction so to improve THC recovery. As polypropylene is one of the most common types of plastic used in collection devices, it was the focus of this study. All experiments were done by preparing neat oral fluid samples spiked with THC in 2-mL polypropylene centrifuge tubes. Samples were transferred with or without prior addition of Triton® X-100 (0.25%) to glass tubes containing d3-THC as internal standard and 0.1 M phosphate buffer was then added. Samples were extracted by liquid–liquid extraction using hexane/ethyl acetate (9:1, v/v), dried and analysed by gas chromatography–mass spectrometry (GC–MS) after derivatisation. No significant difference was found in terms of THC loss to plastic when the concentration ranged from 25 to 1000 ng/mL in the same volume of oral fluid. Varying the oral fluid volume (0.5–1.5 mL) while keeping THC at a constant concentration showed an upward trend with more loss associated with lower volumes. The use of Triton® X-100 significantly decreased the adherence of THC to the plastic tubes and increased the THC transfer (>96%) at all volumes tested. Degradation of THC during storage was also studied over a 4-week period and it was found that azide did not seem to play a significant role in preserving THC in oral fluid.