Differentiating Δ ⁸ ‐THC and Δ ⁹ ‐THC Isomers: Mass Spectrometry Analysis and Computational Explanation

Rationale Isomerism is common in new psychoactive substances (NPSs). Due to insignificant differences in their structure and properties, NPS isomers are hard to separate and identify, posing challenges for forensic laboratories to accurately identify them. Delta‐8‐tetrahydrocannabinol (Δ 8 ‐THC), a double‐bond isomer of delta‐9‐tetrahydrocannabinol (Δ 9 ‐THC), is synthesized from cannabidiol (CBD). Methods To explore the MS fragmentation pattern of Δ 8 ‐THC and Δ 9 ‐THC, we systematically investigated them by gas chromatography–quadrupole time‐of‐flight mass spectrometry (GC‐Q‐TOF/MS) and liquid chromatography coupled to high‐resolution quadrupole Orbitrap mass spectrometry (LC‐Q‐Orbitrap/MS). Results For GC‐Q‐TOF/MS analysis, significant differences in the ion abundance of the two characteristic ions at m/z 314 and 299 between these two isomers (Δ 8 ‐THC and Δ 9 ‐THC) stem from the varied stability of the free radicals they generate after losing an electron. Moreover, the ratio of the ion abundance of the fragment ions at m/z 314 and 299 can be utilized to recognize these two isomers. For LC‐Q‐Orbitrap/MS analysis, the fragmentation patterns of Δ 8 ‐THC were similar to those of Δ 9 ‐THC. Additional investigations by computational study revealed that Δ 8 ‐THC cationic radical can undergo a stepwise process like a retro Diels–Alder reaction to release an isoprene molecule directly but Δ 9 ‐THC via a double‐hydrogen atoms transfer, shifting the location of carbon–carbon double bond, which indicates the different peaks at m/z 246 of Δ 8 ‐THC and 231 of Δ 9 ‐THC in EI‐MS fragmentation processes. Conclusions Distinguishing accurately between Δ 8 ‐THC and Δ 9 ‐THC assists drug laboratories with in‐depth analysis of drug sources, providing exact intelligence and scientific support to law enforcement.