Of more than 700 different compounds present in cannabis plants, around 100 are classified as cannabinoids which are capable of exerting various therapeutic effects including anti-inflammatory, antipsychotic, analgesic, antispasmodic, antiproliferative, and anticonvulsant activities. 
Several cannabinoids have a 21-carbon atom scaffold with variations in length and connectivity of side chains on the aromatic ring (see table). Constitutional isomers are molecules with the same chemical formula but different structural connections. Examples include delta-9-THC (Δ9-THC, -trans or -cis), delta-8-THC (Δ8-THC), and cannabicyclol (CBC). Another example is cannabidiolic acid (CBDA), delta-9-tetrahydrocannabinolic acid A (THCA A), and THCA B (see table).
Many studies aim to detect structural isomers in cannabis plants, but obtaining a good resolution of structurally similar molecules can be difficult.
Reprinted from: dos Santos NA, et al. Analysis of isomeric cannabinoid standards and cannabis products by UPLC ESI TWIM-MS: a comparison with GC MS and GC × GC-QMS. Journal of the Brazilian Chemical Society. 2019;30(1):60-70. doi:10.21577/0103-5053.20180152. License: CC BY 4.0
The detection method applied to plants and extracts can affect the final constituent’s identification; for this reason, the evolution and implementation of different analytical methodologies is crucial to delineate a more precise plant profile. For cannabis chemovars, the accurate analysis of constituents means more clarity about the therapeutic effects of the particular chemovar. It is also important for forensic analysis to trace possible traffic routes depending on the composition of each cultivar.
Gas chromatography coupled with mass spectrometry (GC-MS) allows the separation and identification of cannabinoids by comparing analysis outcomes with the National Institute of Standards and Technology (NIST) library. This technique is not suitable for acidic cannabinoids (unless you derivatize them) due to the instability of the carboxylic groups on GC columns; because of thermal degradation, the cannabinoid acids will convert into neutral cannabinoids or other degradation products.
Two-dimensional gas chromatography (GCxGC) is capable of enhancing the chromatographic resolution when coupled with quadrupole MS (QMS), allowing the identification of complex matrices including cannabinoid isomers.
When analysing cannabis extracts with liquid chromatography (LC)-MS, it is possible to have a complete overview of all cannabinoids in their neutral and acidic forms. To implement LC-MS, ion mobility spectrometry (IMS) and the recent travelling wave ion mobility mass spectrometry (TWIM-MS) techniques were developed, allowing the separation of molecules by their sizes and spatial conformations.
The technique used to produce ions in MS can increase the resolution of the analysis as in the case of electrospray ionization (ESI) mode (positive + or negative –); this method can permit the distinction of isomers bearing substituents in ortho-, metha- and para- positions.
Dos Santos et al  compared the outcome of GC-MS, GCxGC-QMS, ultra-performance liquid chromatography (UPLC)-ESI-TWIM-MS, and UPLC-ESI-quadrupole-time of flight (ESI-QTOF)-MS techniques in the characterization of the constituents of cannabis plants and derivatives (i.e., hashish). They used five neutral cannabinoid standards and two standards of acidic cannabinoids.
While GC-MS resulted in close retention time of components, GCxGC-QMS increased the resolution, providing a better identification and distinction of cannabinoids’ constitutional isomers.
Complete chemical information was obtained using UPLC-ESI-QTOF-MS and UPLC-ESI-TWIM-MS. In positive mode, ESI allowed the detection of seven constitutional isomers of Δ9-THC; in negative mode (–), ESI allowed the detection of four isomers of Δ9-THCA A.  Thus, the advanced methods proved superior for identifying cannabinoid isomers.
References dos Santos NA, et al. Analysis of isomeric cannabinoid standards and cannabis products by UPLC‑ESI‑TWIM-MS: a comparison with GC‑MS and GC × GC-QMS. Journal of the Brazilian Chemical Society. 2019;30(1):60-70. doi:10.21577/0103-5053.20180152 [Times Cited = 10] [Journal Impact Factor = 1.838]
Image: Cannabis testing _ Picture Pulone Sabina