Centre of Behavioural and Forensic Toxicology, Institute of Legal Medicine, University of Padova, Via Falloppio 50, I-35121 Padova, Italy

A new method of detecting toxic, therapeutic and sub-therapeutic levels of GHB in plasma and urine samples by headspace Solid-Phase Microextraction (SPME) and Gas Chromatography-Positive Ion Chemical Ionization-Mass Spectrometry (GC-PICI-MS) is reported. The method was carefully evaluated and optimized in terms of choice of SPME fiber, working pH, absorption and desorption times, absorption temperature, effect of salts, location of the fiber in the GC injector, and choice of CI reagent gas.

The main steps in SPME of 0.5-ml-aliquots of plasma and urine samples included GHB conversion to its lactonic form Gamma-butyrolactone (GBL) at 80°C in acid conditions in the presence of D₆-GBL as internal standard, addition of solid phosphate buffer to adjust pH to 6-7, and headspace adsorption with a 50-µm Carbowax/TPR-100 fiber for 15 min at 70°C. GC-PICI-MS analysis with splitless injection was carried out using a 25-m FFAP (acid-modified polyethylene glycol phase) capillary column, methane as CI reagent gas, and Selected Ion Monitoring (SIM) of ionic species at m/z 86, 87, 88 (GBL) and 92, 93, 94 (D₆-GBL).

The assay was linear over a plasma GHB range of 1-100 µg/ml (n = 5, r = 0.999) and a urine GHB range of 5-150 µg/ml (n = 5, r = 0.998). Intra- and inter-assay relative standard deviations (n=5) at 5 and 50 µg/ml were below 5%. Limits of detection were 0.05 and 0.1 µg/ml for plasma and urine respectively, and practically dictated by their endogenous GHB levels.

Compared with previous GC and GC/MS methods for GHB analysis, the method presented here is simpler, faster and more specific, and may be conveniently applied for emergency toxicology and therapeutic drug monitoring purposes.