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

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Abstract

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.

Introduction

THC is one of the major drugs of concern in police roadside testing programs as well as in workplace drug testing due to its high prevalence around the world. Oral fluid is an increasingly popular matrix to use in drug testing for a number of reasons, including its non-invasive collection, reduced risk of adulteration, a shorter window of detection and thus a more useful indicator of very recent ingestion than urine [1], [2], [3], [4], [5].

Many commercial oral fluid collection devices are available, several of them containing some form of stabilising buffer which dilutes the oral fluid collected. Previous studies have found that these devices often have difficulty collecting consistent volumes of oral fluid and accurate quantification of THC can be challenging [2], [3]. Expectoration is also problematic due to issues such as ‘dry mouth’ and foaming but it is still a viable collection technique, especially since it is the only way to analyse an accurate volume of oral fluid. Hence, it is important to know what interactions THC may have with the containers in which the samples are stored.

Sample containers are commonly made from polypropylene and such containers have been used in recent studies involving oral fluid [6], [7], [8], [9]. Polypropylene was chosen for this study to investigate the adsorptive properties of THC to plastic surfaces when in the oral fluid matrix and also to observe any losses that occur during storage for up to 4 weeks.

Section snippets

Materials

Capped polypropylene centrifuge tubes of 2-mL volume were obtained from Scientific Specialties Inc. (Lodi, CA, USA). THC-free oral fluid was provided from volunteers and used for the study on the day of collection. The absence of THC in collected oral fluid was confirmed by following the sample preparation and analysis procedures described in the following paragraphs.

All solvents and chemicals used were analytical grade or higher. Methanol (MeOH) and ethyl acetate was obtained from RCI Labscan

Method validation

Linearity of the GC–MS method was achieved over the range of 5–1000 ng/mL with a correlation coefficient of 0.9990. The intra- and inter-day precision and accuracy of the method were satisfactory and are summarised in Table 1. The RSD values were 2.52–8.57% and the MRE ranged from 1.38% to 6.48%.

The LOD was determined to be 1 ng/mL. The LOQ was found to be 5 ng/mL. Precision and accuracy at this concentration level was determined to be 3.14% and 9.73% respectively.

Plastic surface and THC recovery

When THC was spiked into 1.5 mL

Discussion

THC is known to be highly lipophilic and poorly water soluble, having a high octanol/water partition coefficient (log P = 6.97) [10]. It is therefore generally accepted that THC can interact with the non-polar plastic material via non-covalent interactions and adsorb to plastic container surfaces. It is also possible that THC degrades readily in oral fluid by way of metabolic action of microorganisms present in the matrix. Both these forces can be overcome by use of buffers containing surfactants

Conclusions

It was demonstrated that THC had the tendency to bind to polypropylene surfaces, leading to poor extraction recovery in neat oral fluid. The recovery of THC is dependent on the oral fluid volume to inner surface area ratio; smaller oral fluid volume in larger containers suffered from a higher loss of THC. Use of Triton® X-100 can significantly increase the THC recovery from polypropylene containers.

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This paper is part of the special issue entitled “The 50th Annual Meeting of the International Association of Forensic Toxicologists (TIAFT)”. June 3–8, 2012, Hamamatsu, Japan. Guest edited by Adjunct Professor Einosuke Tanaka and Associate Professor Masaru Terada.

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