Isotope fractionation during precipitation of methamphetamine HCl and discrimination of seized forensic samples

Methamphetamine is a widely abused illicit drug. Increasingly, studies have focused on stable isotope analysis by isotope ratio mass spectrometry (IRMS) to link samples of methamphetamine synthesized together or from the same source of precursor. For this reason, it is important to understand potential sources of isotope fractionation that could cause variability in forensic data sets. In this study, methamphetamine free base samples were synthesized from (−)-ephedrine HCl using the HI/red phosphorus synthetic route and then precipitated as HCl salts by bubbling with HCl gas. The entire sample did not precipitate when first bubbled with gas, and multiple precipitation steps were required. Fractions of precipitate were individually analyzed for δ13C, δ15N and δ2H by IRMS. Both δ15N and δ2H were found to become more negative, with the heavy isotopes depleted, in successive fractions of precipitate. Homogenizing the precipitate fractions together could eliminate this fractionation. However, in a clandestine situation this fractionation could lead to greater than expected δ15N and δ2H variability between illicit samples of methamphetamine HCl that have been synthesized from the same sample of ephedrine. This needs to be taken into account when interpreting forensic IRMS data. We also present an analysis of four separate seized case lots of methamphetamine HCl, taking into account the possible sources of fractionation and available intelligence information.