Accurate Quantification of Cannabinoid Acids by GC – Is it Possible?

I think by now we’ve all heard that GC potency testing for cannabis or hemp has some drawbacks. That being said, GC is a popular, rugged, and cost-effective laboratory workhorse and is still employed in many cannabis laboratories. The major drawback of GC versus HPLC cannabinoid testing is the fact that the acidic cannabinoids convert to their neutral form in the GC inlet and cannot be separately quantified. This means that the amount of acidic cannabinoids in a sample cannot be reported by GC, and this result is becoming more and more important with the growing popularity of non-smoked cannabis products.

 

Cannabinoid Acids Convert to Their Neutral Forms in a Hot GC Inlet

 

While HPLC would be the more straightforward choice for cannabis or hemp potency analysis, some labs only have GC instruments. Joan Serdar at ARRO Laboratories, Inc. is one such analyst. Her lab is equipped with GC instruments, but she wants to accurately quantify both the neutral and acidic forms of cannabinoids in hemp and cannabis – good for you, Joan! In working with Joan, we were able to take a page from the toxicologists’ handbook and derivatize our cannabinoids. What is derivatization, you ask? In a nutshell, derivatization prior to GC analysis involves ‘sticking’ functional groups onto a molecule in order to make it more volatile, more stable, or to improve detectability.

In the case of our cannabinoids, we want to make them more stable so the acidic forms don’t convert to their neutral forms in our GC inlet (see figure above). The derivatization technique we decided to try for this analysis involves N,O-Bis(trimethylsilyl)trifluoracetamide + 1% trimethylchlorosilane (BSTFA + 1% TMCS). This derivatization reagent targets –OH groups and replaces the hydrogen in that group with a trimethylsilyl group, creating an ether that is easier to gas chromatograph than an acid or alcohol.

 

Hydroxyl (-OH) Groups on Cannabinoids are Derivatized Using BSTFA + 1% TMCS, Making Them Stable in the GC Injection Port

So did it work? The preliminary work performed by ARRO Laboratories and here at Restek indicates that this derivatization technique works for all of the cannabinoids of interest for most scientists, resulting in stable, chromatographically-resolved derivatization products. The chromatogram below is a derivatized high-level standard analyzed by my colleague Jack Cochran via GC-FID using the same 15m x 0.25mm x 0.25µm Rxi-35Sil MS column we recommend for underivatized cannabis potency work. The Rxi-35Sil MS had the selectivity needed to separate all of the derivatized cannabinoids. Jack also ran the sample by GC-TOFMS to verify peak IDs and ensure we were getting the derivatization products we hoped to get.

 

GC-FID Chromatogram of Derivatized Cannabinoid Acids and Neutrals – All Compounds are Resolved on Rxi-35Sil MS

So these results look pretty promising! While HPLC is a more straightforward way to measure cannabinoids, the derivatization used here was very simple and shows really good preliminary results. Stay tuned for more discussion of this method – does it work in the presence of matrix? Find out in our next installment (spoiler alert: it does).