Structural Characterization and Identification of a New Psychoactive Etomidate Analogue 2-Mercapto-1-(1-phenylethyl)-1H-imidazole-5-carboxylic Acid Methyl Ester

In this study, a method combining gas chromatography-mass spectrometry (GC-MS), liquid chromatography-high resolution mass spectrometry (LC-HRMS) and nuclear magnetic resonance (NMR) spectroscopy was developed for the identification of new psychoactive substances (NPS) of etomidate class in suspected seized powder samples. Initial analysis was performed using GC-MS equipped with an electron ionization (EI) source to obtain characteristic fragment ions and fragmentation patterns of the unknown compound. The results indicated that the unknown compound produced characteristic fragment ions at m/z 262.0, 229.0, 158.0, 126.0, 105.1 and 77.1. A critical observation was that the fragment ions at m/z 262.0 and 231.0 exhibited an approximate mass increase of 32 u compared with the corresponding fragment ions of metomidate, which were observed at m/z 230.1 and 199.1, respectively. Based on empirical knowledge, the unknown compound was preliminarily hypothesized to contain an additional sulfur atom (S: 32 u) compared with metomidate. LC-HRMS provided accurate mass values of the quasi-molecular ion and fragment ions. It revealed that the quasi-molecular ion peak M+H⁺ of the compound was at m/z 263.0851, and its elemental composition was inferred to be C₁₃H₁₅N₂O₂S. Compared with metomidate M+H⁺ at m/z 231.1124, the mass difference was 31.9727 u, indicating the introduction of a sulfur atom into the structure. Further comparison of fragment ions showed that m/z 159.0233, 145.0067 and 126.9983 increased by approximately 31.9700 u relative to the corresponding fragments of metomidate, while the fragment ions at m/z 105.0698 and 103.0542 were well consistent with those of metomidate. Based on these results, it was speculated that the compound was a thiol-substituted structural analogue of metomidate. Finally, ¹H-NMR and ¹³C-NMR characterized the chemical environments of hydrogen and carbon atoms and provided direct evidence regarding the exact position of the substituent group. The integration of spectral data from all three complementary techniques—GC-MS for fragmentation patterns, LC-HRMS for exact mass and elemental composition, and NMR for atomic connectivity enabled the final and unambiguous identification of the unknown compound as 2-mercapto-1-(1-phenylethyl)-1H-imidazole-5-carboxylate. This study elucidates the mass spectrometry fragmentation patterns of this new psychoactive substance, providing reliable data support and analytical method references for the identification and characterization of such substances involved in related cases.