Supercritical chromatography and chemometrics capture cannabis compounds

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  • Published: Feb 15, 2017
  • Author: Ryan De Vooght-Johnson
  • Channels: Laboratory Informatics / Chemometrics & Informatics
thumbnail image: Supercritical chromatography and chemometrics capture cannabis compounds

Cannabis analysis remains difficult

A new, rapid UHPSFC method for determining the different compounds in cannabis has been developed by a team from the University of Mississippi, who went on to group different cannabis samples using chemometric methods.

Cannabis, both in the form of buds and leaves (marijuana) and a resin from the plant’s hair-like trichomes (hashish), is an extremely common illegal drug. The main psychoactive component is Δ9-tetrahydrocannabinol, or Δ9-THC, while the non-psychoactive cannabidiol, CBD, can also be a major component in some strains. A number of pharmaceuticals based on cannabis compounds (cannabinoids), such as dronabinol (Marinol™, Δ9-THC) and nabiximols (Sativex™, Δ9-THC and CBD), are marketed by pharmaceutical companies to combat various conditions, such as nausea, weight loss and spasticity. Some states in the USA have legalised cannabis for medical uses. The analysis of cannabis is thus of importance both to law enforcement agencies and the pharmaceutical industry. Many cannabinoids can be hard to separate, and they are also often susceptible to oxidation. Much of the Δ9-THC in smoked cannabis arises from a precursor, Δ9-tetrahydrocannabinolic acid (Δ9-THCA), which is decarboxylated to Δ9-THC by the heat of smoking. GC is often employed as a rapid method of cannabis analysis, but fails to capture the Δ9-THCA or other analogous acidic species, although a varying amount of decarboxylation may occur in the GC injection port. For accurate determination of cannabinoids, HPLC methods are normally used, some of which will detect the acid components. However, the runs are often lengthy, and multiple runs require large amounts of toxic solvents, such as acetonitrile. The Mississippi researchers attempted to develop an ultra-high performance supercritical fluid chromatography (UHPSFC) method that would be fast and use mainly non-toxic carbon dioxide as the solvent. They also aimed to use chemometric methods to group various samples of cannabis into different groups with similar properties, thus aiding the task of law enforcement agencies.

UHPSFC successfully used to detect the cannabinoids

The Mississippi researchers optimised UHPSFC conditions for cannabis analysis. They used a Waters ACQUITY UPC2 system with a high-pressure binary pump and a BEH 2-EP (2-ethylpyridine) column. The output was connected to a Waters ACQUITY single quadrupole mass spectrometer. The optimised conditions used pressurised carbon dioxide as the main mobile phase, with the second mobile phase being isopropanol:acetonitrile (80:20 + 1% water). The amount of the second mobile phase was increased on a gradient from 4% to 30%. After optimisation, the total run time was only 10 minutes and all the 11 main cannabinoid peaks (from characterised samples) were clearly separated, including the acidic compounds. Interestingly, the order of elution was the reverse to that seen in HPLC. Method validation was successfully carried out. The limit of detection (LOD) ranged from 1.0 to 3.0 μg/ml, while the limit of quantification was 5.0 μg/ml for acidic compounds, and 10.0 μg/ml for neutral compounds.

A total of 30 samples of cannabis were examined; these were mainly from material seized by enforcement agencies, and consisted of buds, leaves and hashish. The ground plant material was extracted with acetonitrile:methanol (80:20) using sonication, the liquid extract being injected into the UHPSFC system.

Only nine significant cannabinoid peaks were seen from the samples, and the amounts of these compounds present were used to assign the 30 samples to different groups using PCA (principal component analysis). Four groups were found: samples from leaves, samples from hashish, flower bud samples from the DEA (Drug Enforcement Administration), and flower bud samples from the University. Partial least squares discriminant analysis (PLS-DA) was used to further refine the data, giving ratios of the various compounds typical for particular groups.

UHPSFC and chemometrics are useful methods for cannabis analysis

It has been shown that UHPSFC is a fast and useful method for cannabis analysis. The grouping of samples using chemometrics has clear uses in law enforcement, giving clues as to the likely source of samples. It will be interesting to see to what extent the novel method of UHPSFC can be ‘rolled out’ at different sites for routine analysis.

Related Links

Journal of Forensic Sciences, 2016, Early View paper. Wang et al. Quantitative determination of cannabinoids in cannabis and cannabis products using ultra-high-performance supercritical fluid chromatography and diode array/mass spectrometric detection.

Phytochemical Analysis, 2004, 15, 79-94. Raharjo et al. Methods for the analysis of cannabinoids in biological material: A review.

Wikipedia, Tetrahydrocannabinol

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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