Proposed revisions to the DHHS Mandatory Guidelines for Federal Workplace Drug Testing Programs [1] (Federal Register, April 13, 2004) include a confi rmatory cutoff level of 2.0 ng/mL for THC (parent) in oral fl uid. The relatively low cutoff level and small specimen volume (versus the corresponding parameters for urine drug testing) present a challenge to workplace drug testing laboratories. The GCMS confi rmatory procedures for THCA commonly used in certified workplace urine drug testing laboratories lack the sensitivity needed for this application.
We have completed preliminary studies that demonstrate the feasibility of extracting THC from an aqueous matrix using Stir Bar Sorptive Extraction (SBSE) followed by thermal desorption into the injection port of a standard single quadrupole electron impact GC-MS to achieve detection limits less than 1.0 ng/mL. The specimen is diluted with water + methanol and extracted with a stir bar coated with a nonpolar polydimethylsiloxane phase. THC is extracted with high efficiency and minimal matrix interference with no additional sample cleanup needed. THC is introduced onto the GC column by thermal desorption followed by fast GC/MS and selected ion monitoring detection to achieve detection limits in the low ng/mL range.
This technique appears to offer significant advantages of low cost, high extraction efficiency, minimal sample preparation, and compatibility with standard electron impact GC-MS instruments already in common use in workplace drug testing laboratories. With further refinements and optimization for the oral fluid matrix, it may be possible to satisfy the routine confirmatory testing requirements for oral fluid without having to rely on GC-MS-MS or LC-MS-MS techniques to achieve the necessary sensitivity and avoid matrix interference.
