Detection of exogenous GHB using Isotopic Ratio Mass Spectrometry

Objectives

Development of a method to determine carbon isotopic ratio in order to detect GHB present at low concentrations in the urine matrix. This method increases the range of detection of exogenous GHB and may be used in investigating sex crimes.

Introduction

Even though GHB (gamma-hydroxybutyrate) is considered to be an illicit drug, it has been used as a hypnotic and anesthetic starting in 1960. GHB is a metabolite of gamma-aminobutyric acid (GABA) and acts as the main neurotransmitter inhibitor of the encephalic nervous system in general, and of the central nervous system in particular. Because GHB is produced inside the organism, forensic toxicologists must be able to determine whether the GHB levels in the biological matrices reflect endogenous production or some intoxication. In practice, the forensic laboratories take into account the upper limits in endogenous GHB concentrations in urine and blood to determine whether exogenous GHB is also present.

Methods

GHB was isolated from urine in several purification steps and is eventually converted into GBL through esterification (formation of a lactone). This step prevents the introduction of additional carbon atoms into the molecule. The ¹³C/¹²C isotopic ratio of GBL was then determined by Gas Chromatography/Combustion/ Isotopic Ratio Mass Spectrometry (GC/C/IRMS).

Results

When applied to samples containing GHB at a concentration of 10 mg/L, this method yielded results with 0.3 ‰ precision. The ¹³C/¹²C ratio of GHB varied between - 32.1 and -42.1 ‰ in samples collected from individuals exposed to the drug. In contrast, GHB appeared to be ¹³C-enriched when its origin was endogenous and its concentrations was below 10 mg/L as seen in ante mortem and post mortem samples of Caucasian individuals (ratios ranging between -23.5 and -27.0 ‰.).

Conclusion

The preliminary results obtained in this study demonstrate that the determination of carbon isotopic ratio may be used to differentiate between an endogenous and an exogenous origin of GHB in a biological sample.

Publications

1. Saudan C, Augsburger M, Kintz P, Saugy M, Mangin P. Detection of exogenous GHB in blood by gas chromatography-combustion-isotope ratio mass spectrometry: implications in postmortem toxicology. J Anal Toxicol. 2005 Nov-Dec;29(8):777-81
2. Saudan C, Augsburger M, Mangin P, Saugy M. Carbon isotopic ratio analysis by gas chromatography/combustion/isotope ratio mass spectrometry for the detection of gamma-hydroxybutyric acid (GHB) administration to humans. Rapid Commun Mass Spectrom. 2007;21(24):3956-62

^{GHB conversion in GBL}

Last Update on 07.10.2008