It’s important to properly quantify cannabidiol in medical marijuana samples, as it is one of the chief cannabinoid compounds with pharmacological value, including relief against nausea, anxiety, and inflammation. However, on typically used “5 type” GC columns, it can coelute with cannabichromene, a compound that likely also has medical value and is more and more becoming part of potency testing. To determine both of these compounds accurately, a GC column with a different stationary phase can be used.
A good hint at what stationary phase might be best for cannabinoid analysis is seen in their structures below, in that all contain an aromatic piece. A non-aromatic stationary phase, like the Rxi-1ms, which is 100% dimethyl polysiloxane, shows a coelution for cannabichromene (CBC) and cannabidiol (CBD). While both the Rxi-5ms and Rxi-5Sil MS contain some phenyl (5%), they also show coelutions for CBC and CBD under the GC conditions used for this work. The higher phenyl-content columns, the Rxi-35Sil MS (35% phenyl type) and the Rxi-17Sil MS (50% phenyl type; should be the 50Sil MS, right, but that’s a story for another day), both resolve not only CBC and CBD, but also delta-8-THC (d8-THC), delta-9-THC (d9-THC), cannabigerol (CBG), and cannabinol (CBN). I prefer (and recommend) the Rxi-35Sil MS because of a slightly faster analysis time and more overall space between the peaks of interest.
A few comments on the analyses: I used the same format columns for all the work, 15m x 0.25mm x 0.25µm. A 15m column with the proper selectivity easily provides the separating power for this work and is about half the cost of its 30m cousin. The 0.25mm x 0.25µm version has good sample loading capacity and is robust, especially when a proper split injection is used with a Restek Premium Precision® split liner with wool. I also used hydrogen carrier gas, the fastest gas, and a good option if anyone is being impacted by helium availability and price.
If you are already using a “5 type” column for this work, don’t panic! If you vary your GC conditions, especially carrier flow and oven temperature program, you can likely separate CBC and CBD, just not as easy or quickly as I’ve done it with the Rxi-35Sil MS. Also, I used a relatively fast oven program rate in my work, since I had the stationary phase selectivity in my favor and wanted to reduce analysis time, but if your GC won’t heat that fast, you can still do the separation on the 35Sil MS, no problem. Contact me if you need help.
Restek continues to support the medical cannabis analysis community with GC and LC columns, accessories, and reference materials. Check out our Medical Marijuana web page.