Complementary approach for accurate determination of carbon isotopic compositions in gamma-hydroxybutyric acid using gas chromatography/combustion-isotope ratio mass spectrometry

Abstract

Rationale gamma-Hydroxybutyric acid (GHB) is a naturally endogenous neurotransmitter that is popular as a recreational drug due to its sedative, hypnotic, and euphoric effects. GHB derived from endogenous production or exogenous ingestion has been effectively discriminated by carbon isotopic compositions (delta C-13 values) through gas chromatography/combustion-isotope ratio mass spectrometry (GC/C-IRMS). However, an unintended uncertainty of isotopic signatures caused by a wide range of GHB quantities remains unsolved when using only single-isotope corrections of the di-TMS derivative. Methods The delta C-13 values of the original GHB standard were first determined by elemental analyzer/isotope ratio mass spectrometry (EA/IRMS). The delta C-13 values of silylated GHB in concentrations from 10 to 500 ppm were determined by GC/C-IRMS. With respect to the silylated reaction products, the correction of delta C-13 values for the introduced carbons was calculated from a stoichiometric mass balance equation. Results The results showed a significant quantity-dependent trend in delta C-13 values of introduced carbon (delta C-13(di-TMS) values) with increased GHB standard concentrations (r(2) = 0.70, p ˂0.05). We applied a logarithmic equation to determine isotopic data in low-GHB urine specimens from five healthy female volunteers. The delta C-13(GHB) values in urine samples corrected with quantity-dependent delta C-13(di-TMS) values were different by an average of 2.7 parts per thousand from those corrected with single delta C-13(di-TMS) values (p ˂0.05). Conclusions Our results suggest that the overall residual amount-dependent isotope fractionation should be mathematically corrected by the logarithmic function and this may improve the reliability of isotopic analysis to evaluate the origin of GHB before applying the approach to routine toxicological and forensic studies.