Determination of gamma-hydroxybutyric acid in serum and urine by headspace solid-phase dynamic extraction combined with gas chromatography-positive chemical ionization mass spectrometry.

Gamma-hydroxybutyric acid is an emerging drug of abuse. Beside relaxation and euphoria it causes hypnosis and unconsciousness. Therefore the substance is misused as recreational drug and at drug-facilitated sexual assaults. An automated and effortless method for quantitation of gamma-hydroxybutyric acid in serum and urine was optimized and validated. Five hundred microliters sample volume are used for both matrices. The acid catalyzed conversion of gamma-hydroxybutyric acid to the corresponding gamma-butyrolactone is applied. Furthermore the method is based on headspace solid-phase dynamic extraction coupled with gas chromatography-mass spectrometry. The extraction process is performed by repeated aspiration and ejection of the headspace through a steel cannula which is coated on the inside with a polydimethylsiloxane sorbent. Thus absorption of analyte molecules by the sorbent is achieved. The influence of parameters as sorbent type, incubation temperature, number of extraction strokes, injection port temperature and injection flow speed on extraction recovery was investigated. The validation revealed good accuracy with a bias less than +/-5%. Intra- and interday precision determined at 10, 50 and 150 microg/ml for each matrix were in following ranges: 1.96-3.49% (intraday, serum), 2.38-4.31% (intraday, urine), 2.33-5.13% (interday, serum) and 2.53-5.64% (interday, urine). The method provided good linearity between 2 and 200 microg/ml yielding coefficients of determination R(2) > or = 0.9985. Limit of detection were determined at 0.16 microg/ml for serum and 0.17 microg/ml for urine, respectively. This method exhibits a fast, solvent-free and widely automated extraction process. It has been applied to toxicological routine analysis and therapeutic drug monitoring successfully.