TD-IMS Holds Promise for Portable Preliminary Cannabis Screening

The cannabis plant is a highly versatile crop and exists in many different varieties, each with different uses and characteristics. One of the most common ways of classifying and dividing up cannabis strains with varying characteristics is to study the chemical composition of the cannabis sample in question.

Cannabis contains over 500 different chemicals, including approximately 100 different compounds known as cannabinoids which are responsible for many of cannabis’ distinctive properties. 

Classifying cannabis strains

The ratio in which Δ9-THC, CBD, and other major cannabinoids are present in a cannabis strain can greatly influence its properties. The United Nations currently states that it is possible to broadly characterize cannabis strains as either drug-type or fiber-type based on the Δ9-THC/CBD ratio. Strains that are predominantly CBD-based with only low levels of Δ9-THC can be classed as fiber-type and have little-to-no psychotropic effects on humans. Conversely, Δ9-THC dominant strains have significant psychotropic effects and are classed as drug-type. 

Currently, the study of cannabinoids such as Δ9-THC and CBD typically relies on the use of gas chromatography (GC) or liquid chromatography (LC) based techniques. One point of note is that GC analysis does suffer from the complication that under the heating involved in the analysis, some of the acidic cannabinoids can be decarboxylated into their neutral counterparts. This can affect the amounts of each cannabinoid present and represents a source of inaccuracy. In order to prevent this thermal degradation, a derivatization step must be introduced prior to analysis. The addition of this step lengthens the overall time of analysis and the complexity of the method. LC analysis avoids this challenge as no heating is required.  Typical LC and GC systems are large, complicated and come with the inherent drawback that they are only suited for laboratory use and cannot be practically used for in-field measurements.

Portable analysis of cannabinoids

Ion mobility spectroscopy (IMS) has the potential to be a suitable portable analysis technique that could be used in the field as itis small, portable, and simple to use. IMS has also been shown to have good sensitivity but can suffer from poor selectivity and false-positive readings if used as a standalone technique. IMS can be done in conjunction with time-of-flight mass spectroscopy (TOF-MS) to improve its selectivity, but this removes the benefit of portability that makes IMS so attractive.

With the aim of preserving this portability, a multidisciplinary team of researchers from Spain recently investigated the feasibility of thermal-desorption ion mobility spectroscopy (TD-IMS) for the in-field cannabinoid profiling. 

The study assessed the spectral fingerprints of 33 different cannabis samples in both the positive and negative ionization modes of the IMS apparatus., Principal component analysis (PCA) and linear discriminant analysis (LDA) was applied to the data in order to chemotype the cannabis samples. Separately, GC-MS analysis was also performed on the samples to give the TD-IMS a baseline for comparison against.

The TD-IMS output is presented as a plot of peak intensity versus the reduced ion mobilities, K0. Compounds will often have characteristic K0 values, which allow them to be identified in a sample containing many other chemicals. The TD-IMS method used in the study also used a nicotinamide internal calibrant. Nicotinamide has a high proton affinity and so restricts the protonation and detection of molecules in the TD-IMS analysis to only molecules that also have a high proton affinity, which increases the overall selectivity of the technique. 

The GC-MS data from the cannabis samples were used to define cannabis chemotypes that could then be correlated against the TD-IMS data. In the positive mode, TD-IMS was able to distinguish between different samples as the resultant spectra had different overall profiles, but there were many shared K0 signals. It was, however, possible to assign some signals to specific cannabinoids, such as CBD, Δ9-THC, and their acidic forms. Unfortunately, many chemotypes could only be discriminated between by examining relative intensities, and the low peak resolution of TD-IMS means that similar chemotypes often resulted in broad or shouldered peaks that hampered the ability to assign signals. The negative ionization mode was found to suffer many of the same challenges as the positive ionization mode, although overall, the signal peaks had a lower intensity in the negative mode.

Positively, the TD-IMS technique appears to return false positive results rarely. Other plant material (horsetail weed, chamomile, marigold, poppy, oregano, tobacco) containing terpenes was tested with positive and negative ionization mode TD-IMS and all were found to be clearly distinct from cannabis plant material.

As the original TD-IMS spectra could be hard to differentiate, PCA-LDA was performed on the spectral fingerprint data. This showed clustering of the cannabis samples according to the chemotypes and cannabinoid content of each sample across both ionization modes. Importantly, the non-cannabis plant material formed a distinct cluster to the cannabis material, which is not seen with other IMS methods.

The practicality of TD-IMS analysis

While TD-IMS cannot provide detailed data in the same way that GC-MS analysis can, it remains a promising method for cannabis analysis. The PCA-LDA data shows that it is certainly effective at discriminating between cannabis and non-cannabis plant material, which combined with the portability of TD-IMS, makes the method a promising candidate as a preliminary screening method in the field, or as a complementary system to GC-MS analysis.