Determination of cannabinoids (THC) in plasma and serum samples with GC-MS

Significance of the Topic

Accurate quantification of delta-9-tetrahydrocannabinol (THC) and its primary metabolites in plasma and serum is essential for pharmacokinetic investigations, impaired driving cases, and monitoring of cannabis exposure. Rapid, sensitive assays enable forensic and clinical laboratories to determine consumption timing and intensity by measuring THC, THC-OH and THC-COOH in biological fluids.

Study Aims and Overview

This application note presents a streamlined gas chromatography–mass spectrometry (GC-MS) approach to simultaneously extract, derivatize, and quantify THC and its hydroxylated and carboxylated metabolites from small volumes of plasma or serum. The protocol employs liquid–liquid extraction, trimethylsilyl derivatization, and selective ion monitoring (SIM) to achieve low-nanogram per milliliter detection with excellent linearity.

Methodology and Instrumentation

Sample Preparation:

• Aliquot 0.5 mL plasma or serum into a safe-lock tube.
• Add deuterated internal standards and calibrators according to a defined pipetting scheme.
• Acidify with 25 µL of 25 % acetic acid and extract with 1 mL ethyl acetate/n-hexane (9+1, v/v).
• Vigorously shake, centrifuge, and transfer 500 µL organic phase to a vial.
• Evaporate to dryness at 40 °C under nitrogen.
• Derivatize with 50 µL ethyl acetate and 50 µL MSTFA at 80 °C for 30 min.

Chromatography and Detection:

• Column: Optima® 5 HT, 30 m × 0.25 mm ID, 0.25 µm film.
• Oven: 70 °C (2 min) → 250 °C at 20 °C/min (4 min) → 300 °C at 20 °C/min (17 min).
• Injection: 1 µL splitless at 250 °C.
• Carrier gas: Helium at 1.31 mL/min.
• MS: Shimadzu GCMS-QP2010plus, EI source (200 °C), interface 250 °C, SIM mode targeting characteristic m/z for each analyte and D₃-labeled analogue.

Used Instrumentation

• GCMS-QP2010plus mass spectrometer (Shimadzu).
• Optima® 5 HT capillary column (Macherey-Nagel, REF 726106.30).
• MSTFA derivatization reagent (REF 701270.201).
• Screw-neck vials and caps (REF 702293, REF 702107).

Main Results and Discussion

Chromatograms of standards, spiked plasma, and serum demonstrated clear resolution of THC, THC-OH and THC-COOH with their respective deuterated internal standards. Calibration curves prepared in solvent and biological matrix over 25–250 ng/mL exhibited strong linearity (R² ≥ 0.9945). Signal-to-noise ratios and retention time stability confirmed method reproducibility. The derivatization and extraction steps provided clean extracts and consistent recovery across concentration levels.

Benefits and Practical Applications

This GC-MS method offers:

• Minimal sample volume requirement (0.5 mL), ideal for clinical settings.
• Straightforward liquid–liquid extraction and single-step derivatization.
• High sensitivity and specificity through SIM acquisition.
• Robust quantitation aided by deuterated internal standards.

Applications span forensic toxicology, therapeutic drug monitoring, and clinical research on cannabinoid metabolism.

Future Trends and Opportunities

Enhancements could include solid-phase extraction to improve selectivity and lower detection limits. Integration of tandem mass spectrometry (GC-MS/MS or LC-MS/MS) may further increase sensitivity and throughput. Automation of sample preparation and data processing will support higher laboratory workload and improved turnaround times.

Conclusion

The described GC-MS procedure enables reliable, high-throughput measurement of THC and its main metabolites in plasma and serum. Combining simple LLE extraction, MSTFA derivatization, and SIM detection yields linear calibration, reproducible retention times, and adequate sensitivity for forensic and clinical applications.

Reference

1. Constituents of Cannabis sativa. Georg Thieme Verlag, Stuttgart · New York.
2. Grotenhermen F., Journal of Cannabis Therapeutics, Vol. 3(1) 2003, Clinical Pharmacokinetics of Cannabinoids.
3. Nadulski T., Sporkert F., Schnelle M., Stadelmann A. M., Roser P., Schefter T., Pragst F., Journal of Analytical Toxicology, Vol. 29, No. 8, 2005, pp. 782–789.