Abstract

In gas chromatographic-combustion-isotope ratio mass spectrometry (GC-C-IRMS) doping control analysis, endogenous androgenic anabolic steroids and their metabolites are commonly acetylated using acetic anhydride reagent, thus incorporating exogenous carbon that contributes to the measured isotope ratio. Comparison of the endogenous δ13C of free, mono-, and di-acetylated steroids requires application of corrections, typically through straightforward use of the mass balance equation. Variability in kinetic isotope effects (KIE) due to steroid structures could cause fractionation of endogenous steroid carbon, resulting in inaccurate results. To test for possible KIE influence on δ13C, acetic anhydride of graded isotope ratio within the natural abundance range was used under normal derivatization conditions to test for linearity. In all cases, plots of measured steroid acetate δ13C versus acetic anhydride δ13C were linear and slopes were not significantly different. Regression analysis of the Δδ13C of enriched acetic anhydrides versus Δδ13C of derivatized steroids shows that KIE are similar in all cases. We conclude that δ13C calculated from the mass balance equation is independent of the δ13C of the acetic anhydride reagent, and that net KIE under normal derivatization conditions do not bias the final reported steroid δ13C. Verf.-Referat