Forensic detection of regular cannabis

Cannabis is a sedating and hallucinogenic drug most known for its euphoric, disinhibiting and relaxing properties. These effects are caused by the active substance 9-tetrahydrocannabinol (9-THC), which is metabolized in the body to an inactive metabolite, 9-tetrahydrocannabinoid acid (THC-COOH). A cannabis leaf is shown in Figure 1.

In order to prove regular cannabis consumption, hair analysis is preferred, as unlike other biological matrices such as urine and blood, many drugs are well preserved in hair. Hair analysis therefore provides long-term forensic information and can distinguish between casual or repeated use. As a result, hair analysis is used in criminal investigations and to monitor abstinence.

Different countries specify different recommended levels of detection of THC-COOH in forensic cases. According to the recommendations of the Society of Hair Testing for analysis in forensic cases, the limit of quantification for THC-COOH in hair should be less than 0.2 pg/mg. However, the United States and many countries in the European Union recommend an even lower detection limit of 0.05 pg/mg.

This study demonstrates the reliable and robust quantification of THC-COOH at 0.05 pg/mg in hair samples using gas chromatography mass spectrometry (GC-MS) in negative chemical ionization mode.

Detecting THC-COOH in hair using GC-MS

The identification of 9-THC in hair indicates exposure to marijuana, whether active or passive, whereas the presence of the THC-COOH metabolite indicates active consumption. In order to overcome constraints related to environmental contamination, detection of regular cannabis consumption therefore requires the analysis of THC-COOH.

The absorption of THC-COOH into hair is very low, making quantitation challenging due to the high sensitivity required to meet forensic regulations. To overcome these challenges a method has been developed using a triple quadrupole gas chromatography mass spectrometer, which demonstrated excellent robustness and sensitivity, meeting the levels of quantification outlined by United States forensic regulations.

Hair specimens were washed to remove unwanted decontaminates and interferents using isopropanol. The samples were then dried, weighed and mechanically shredded to obtain 25 mg of hair powder. After being incubated for 30 minutes in sodium hydroxide solution, the samples were then acidified. In order to obtain maximum extraction efficiency, extraction was performed twice. The samples were then dried and derivatized before being reconstituted with 100 µl of ethyl acetate 1 µl of the extract was then injected into the GC-MS in negative chemical ionization mode. The chromatographic and mass spectrometry conditions used are shown in Tables 1 and 2.

Figure 2 shows the chromatogram obtained for a hair extract spiked with 0.05 pg/mg of THC-COOH. This low level of quantification betters the recommended level of quantification set by the Society of Hair Testing for analysis in forensic cases. In addition, the overlay of 20 chromatograms of THC-COOH at 0.05 pg/mg in Figure 3 shows the robustness and reproducibility of the results at very low levels. The sensitivity and repeatability demonstrated easily enables the routine quantification of THC-COOH at 0.05 pg/mg, which is the limit of quantification set by the United States and many EU countries.

Conclusion

The analysis of THC-COOH in hair is the method preferred to prove regular cannabis consumption, providing long-term forensic information that distinguishes between occasional and frequent use. However, the analysis of THC-COOH in hair requires extremely sensitive instruments due to the very low levels present and strict forensic regulatory requirements.

This study has demonstrated the reliable and robust quantification of THC-COOH at 0.05 pg/mg in hair samples using gas chromatography mass spectrometry in negative chemical ionization mode.