Differentiating Δ⁸-THC and Δ⁹-THC Isomers: Mass Spectrometry Analysis and Computational Explanation

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 (Δ⁸-THC), a double-bond isomer of delta-9-tetrahydrocannabinol (Δ⁹-THC), is synthesized from cannabidiol (CBD).

To explore the MS fragmentation pattern of Δ⁸-THC and Δ⁹-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).

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 (Δ⁸-THC and Δ⁹-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 Δ⁸-THC were similar to those of Δ⁹-THC. Additional investigations by computational study revealed that Δ⁸-THC cationic radical can undergo a stepwise process like a retro Diels-Alder reaction to release an isoprene molecule directly but Δ⁹-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 Δ⁸-THC and 231 of Δ⁹-THC in EI-MS fragmentation processes.

Distinguishing accurately between Δ⁸-THC and Δ⁹-THC assists drug laboratories with in-depth analysis of drug sources, providing exact intelligence and scientific support to law enforcement.