Disposable Pipette Extraction (DPX) has been shown to be a rapid, efficient and reproducible technique for performing solid-phase extraction (SPE) of drugs from biological specimens.
DPX is based on pipette tips that incorporate loosely contained sorbent material. The sorbent is suspended in the sample solution using turbulent air bubble mixing for efficient contact between the phases, resulting in higher recovery and faster extraction. Elution can be performed with small volumes of solvent to minimize extract dilution. DPX is readily automated using the GERSTEL MultiPurpose Sampler (MPS).
The analyzed 0.5 mL blood specimens were protein precipitated with acetonitrile and centrifuged. The rest of the analysis was completely automated using a dual rail GERSTEL MPS 2 combined with a Cooled Injection System (CIS) inlet for large volume injection (LVI). Automated extractions were performed using 1 mL DPX-RP (reversed phase) tips, and the total time required per sample, was approximately 6 minutes.
The CIS inlet was used for sample introduction, solvent evaporation (analyte concentration), and chemical derivatization. A 50 μL aliquot of sample extract and 20 μL of derivatizing reagent were introduced directly into the CIS. The combined system provided excellent throughput for analysis of THC and metabolites.
Derivatization of THC, OH-THC and COOH-THC was performed by using a mixture of BSTFA and MTBSTFA. Limits of detection were determined to be 0.5 ng/mL for THC and OH-THC and 2 ng/mL for COOH-THC. Coeffi cients of variation were below 5 % for all 3 analytes. Extraction effi ciencies were found to depend mostly on the initial protein precipitation step. Recoveries for the DPX extraction (post precipitation) were close to 80 % for THC.
