Mass Fragmentation Characteristics of Ketamine Analogues

As of October 2022, the United Nations Office on Drugs and Crime (UNODC) Early Warning Advisory (EWA) has monitored over 1 150 new psychoactive substances (NPS) that had appeared in 137 countries and territories, including 76 substances that were notified for the first time in 2022. The increasing popularity of NPS has become a policy challenge and a major international concern, and poses potential risks for public health. Ketamine and ketamine analogues belong to β-keto-arylcyclohexylamines, which are classified into dissociative NPS due to their “dissociative anesthetic” effect. According to the monitoring data of the National Narcotics Laboratory, as of August 2022, ten ketamine structural analogues have been detected in China, including 2-(2-chlorophenyl)-2-(ethylamino) cyclohexan-1-one (NENK), 2-(3-methoxyphenyl)-2-(ethylamino) cyclohexan-1-one (MXE), 2-phenyl-2-(methylamino) cyclohexan-1-one (DCK), and 2-(2-fluorophenyl)-2-(methylamino)cyclohexan-1-one (2-FDCK), 2-(ethylamino)-2-phenylcyclohexan-1-one (2-oxo-PCE), 2-(methylamino)-2-(2-methylphenyl)cyclohexan-1-one (2-MDCK), 2-(ethylamino)-2-(2-fluorophenyl)cyclohexan-1-one (2-FXE), 2-(2-bromophenyl)-2-(methylamino)cyclohexan-1-one (2-BDCK), and 2-(ethylamino)-2-(thiophen-2-yl)cyclohexan-1-one (tiletamine), and 2-(ethylamino)-2-(2-methylphenyl)cyclohexan-1-one (deoxymethoxetamine). According to the structures, these compounds can be seen as structural analogues of ketamine with benzene ring replaced by fluorine, bromine, methyl and methoxy groups, or with methyl group on the nitrogen atom replaced by ethyl group, or with benzene ring replaced by furan group. NENK, MXE, DCK and 2-FDCK have already been included in the annexes of the law in China, while 2-oxo-PCE, 2-MDCK, 2-FXE, 2-BDCK, and tiletamine have not been listed in the regulation. In order to evade the supervision of the law, new types of ketamine analogues have been continuously produced and offered for sale on the NPS market. These emerging new compounds have posed a great challenge to the testing and identification ability of forensic science laboratories around the world. The knowledge of MS fragmentation pathway characteristics of known structure NPS is essential for the structure elucidation of new types of NPS. In order to investigate the mass fragmentation characteristics of ketamine analogues, NENK, MXE, DCK, 2-FDCK, 2-oxo-PCE, 2-MDCK, 2-FXE, 2-BDCK, and tiletamine were analyzed using gas chromatography-Orbitrap mass spectrometry (GC-Orbitrap-MS) and ultra-high performance liquid chromatography-quadrupole time of flight-mass spectrometry (UPLC-QTOF-MS). High-resolution mass spectra were obtained by electron impact (EI) and electrospray ionization collision-induced dissociation (ESI-CID), the structures and fragmentation pathways of main ions were deduced. The product ions of ketamine analogues were mainly formed by the loss of CO, C₂H₅·, C₃H₇·, C₄H₉·, C₅H₁₁·/C₆H₁₃· in EI mode, and were mainly formed by the loss of H₂O, CH₃NH₂/C₂H₅NH₂, CO, C₄H₆, C₂H₄O in ESI-CID mode. The ketamine analogues distinguishing features were summarized by investigating the high-resolution mass spectrometric characteristics, which can provide a reference for the identification of new ketamine analogues with similar chemical structures.