Stable isostopes(δ¹³C): A proposed means of identifying the source of gamma-hydroxybuturic acid (GHB)

AIMS: GHB is produced naturally in the human body and is also a Class C controlled substance under the Misuse of Drugs Act 1971. It is notorious because of its association with drug facilitated sexual assaults (DFSA). Studies have indicated that large variations in urinary and blood concentrations of endogenous GHB occur across population groups (1). At present the recognised cut off values of 10 µg/mL in urine and 5 µg/mL in blood may only provide reliable evidence if the sample was taken <12 hours after ingestion because GHB is rapidly metabolised and excreted from the body (2). A recent study, using gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS), found a significant difference (> 13.5%) in the values of ∂13C found in endogenous GHB in five postmortem blood samples (range: 13.8 - 86.3 µg/mL) compared to synthetically produced GHB with the implication that stable isotope measurements could significantly increase time frames of detection in reported DFSA (3). The aim of this study is to determine the ∂13C values of GHB at concentrations below the recognised cut off values in urine samples. METHODS: GHB can be derivatised or converted to gamma-butyrolactone (GBL) for GC analysis. We have derivatised GHB using N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) with 1% trimethylchlorosilane (TMCS) (linear response: 0.05 - 25 µg/mL), and converted GHB to GBL (linear response: 0.5 - 50 µg/mL) based upon on a method developed ‘in-house’ by the Forensic Science Service Ltd. RESULTS AND CONCLUSIONS: Preliminary ∂13C (%) values for synthetic GBL at 50 µg/mL (mean -26.5%; σn-1 0.06; n = 3) and GHB-TMS derivatives at 200 µg/mL (mean -34.2%; σn-1 0.21; n = 3) have been obtained using a Thermo Finnigan MAT 253 IR-MS coupled to a Trace Ultra GC/Combustion III Interface. However, we found that extraction of GHB from urine using solid phase extraction (CLEAN SCREEN® GHB and Oasis® MAX cartridges) did not remove interfering compounds sufficiently to be able to precisely determine ∂13C values at levels less than 10 µg/mL. Therefore, we propose synthesising an immunising antigen to make an antibody to specifically target GHB using immunoaffinity column extraction. (1) A.A. Elian (2002).Forensic Science International 128, 120-122. (2) P.V. Kavanagh, P. Kenny and J. Feely (2001). Journal of Pharmacy and Pharmacology 53, 399-402. (3) C. Saudan, M. Augsburger, P. Kintz, M. Saugy, and P. Mangin (2005). Journal of Analytical Toxicology 29, 777-781