Feasibility of Extraction and Quantitation of Δ9-Tetrahydrocannabinol in Body Fluids by Stir Bar Sorptive Extraction (SBSE) and GC/MS

Importance of the Topic

This application note addresses the growing need for sensitive and reliable confirmation of Δ9-tetrahydrocannabinol (THC) in oral fluid at low parts-per-billion levels. Proposed federal guidelines require confirmatory cutoff levels as low as 2.0 ng/mL, which challenges conventional GC-MS approaches developed for urine. The ability to extract and quantify THC efficiently from small-volume, complex biological matrices is critical for workplace drug testing and forensic toxicology.

Study Objectives and Overview

The study evaluates the feasibility of using Stir Bar Sorptive Extraction (SBSE) coupled with thermal desorption and standard single-quadrupole GC-MS to achieve detection limits below 1 ng/mL for THC in oral fluid. Key aims include optimizing extraction parameters, minimizing matrix interference and carryover, and demonstrating method precision and linearity across relevant concentration ranges.

Methodology and Used Instrumentation

Sample Preparation and Extraction:

• Negative oral fluid (1 g) spiked with known THC levels in 10% methanol/water.
• Internal standard: Δ9-THC-D3 at 1.0 ng/mL in final extract.
• SBSE using a PDMS-coated Twister stir bar, 90 min extraction at room temperature.

Thermal Desorption and GC-MS Conditions:

• GERSTEL TDS 2 or MPS 2 with TDU and CIS 4 PTV inlet.
• Desorption: splitless at 275 °C after ramping 60 °C/min.
• PTV inlet solvent vent (50 mL/min), then splitless transfer to column.
• Column: 30 m × 0.25 mm HP-5 MS, 0.25 µm film.
• Carrier gas: helium at 1.2 mL/min.
• Oven program: 60 °C hold, 30 °C/min to 175 °C, 25 °C/min to 300 °C.
• MSD scan: full scan 31–350 amu for method development; SIM at m/z 299 for quantitation.

Instrument Models:

• Agilent 6890 GC with 5973 MSD detector.
• GERSTEL thermal desorption unit (TDS-2 or MPS-2 with TDU) and cooled injection system (CIS-4).

Main Results and Discussion

Optimization:

• Methanol content: 10% (v/v) provided optimal extraction recovery.
• Desorption temperature: 275 °C eliminated THC carryover.
• Extraction time: 90 min achieved near-quantitative recovery.
• Trap temperature: 40 °C Peltier-cooled inlet sufficient; cryogen not needed.

Analytical Performance:

• Calibration range: 0.3–10 ng/mL in oral fluid, linear with r² > 0.99.
• Limit of quantitation: 0.3 ng/mL; signal-to-noise ≥ 5.
• Precision: RSD ≤ 3.2% at 0.5 and 2.0 ng/mL levels across multiple samples.
• Carryover: < 10% of LOQ response following a 40 ng/mL injection.
• Recovery: quantitative extraction confirmed by comparison of spiked blanks.

Benefits and Practical Applications

• Low detection limits (< 1 ng/mL) without MS/MS instrumentation.
• High extraction efficiency for nonpolar analytes in small volume fluids.
• Minimal sample cleanup and solvent consumption.
• Compatibility with existing single-quadrupole GC-MS systems in certified laboratories.
• Potential extension to other drugs of abuse, pharmaceuticals, and environmental contaminants.

Future Trends and Potential Applications

Further refinements may include integration with fast GC modules to reduce runtime, expansion to multiplexed analyte panels, and application to additional alternative matrices such as sweat or plasma. Advances in SBSE coating chemistries and automated thermal desorption systems could enhance throughput and selectivity, supporting broader adoption in forensic, clinical, and environmental laboratories.

Conclusion

SBSE coupled with thermal desorption and single-quadrupole GC-MS provides a cost-effective, sensitive, and robust approach for confirmatory quantitation of THC in oral fluid at regulatory cutoff levels. This workflow meets precision, linearity, and carryover requirements and can leverage existing laboratory infrastructure without the need for MS/MS.

References

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