In my post, “Don’t overestimate cannabidiol during medical cannabis potency determinations with gas chromatography. Use stationary phase selectivity for accuracy and hydrogen for fast analysis.”, I showed how an Rxi-35Sil MS GC column provides excellent separation for cannabichromene (CBC), cannabidiol (CBD), delta-8-THC, delta-9-THC, cannabigerol (CBG), and cannabinol (CBN). I focused on the separation of cannabichromene from cannabidiol, in particular, which is difficult on 5% phenyl-type columns. In fact, these compounds coeluted under my conditions on our 15m x 0.25mm x 0.25µm Rxi-5ms and Rxi-5Sil MS columns.
Given that SELECTIVITY, provided by the stationary phase for the compounds analyzed, is the most influential force on resolution of compounds, I again suggest that the Rxi-35Sil MS is the column of choice for this work, but for those of you that already have “5” columns, I provide the following chromatograms and conditions below to show that with some effort you can separate (at least partially) CBC and CBD. Remember as you review this work that the 0.25mm x 0.25µm format used here offers better efficiency than wider bore columns, e.g. 0.32mm and 0.53mm IDs, which means that you may not get CBC and CBD separated on those columns under any operational conditions.
One more thing on selectivity… Did you notice that CBC and CBD changed elution orders depending on the type of “5” column and the conditions used? This happens because of two things. The first is that the 5ms and 5Sil MS are actually slightly different columns (“Selectivity differences in arylene GC phases…pesticides and zebras.”). The second reason is that the GC oven programs are different for the columns, which means that the compounds are eluting at different temperatures. You may be able to do better at separating these compounds than I did on 5-type columns, but I didn’t try very hard, preferring instead to relax and take advantage of the good cannabinoid separations afforded by the Rxi-35Sil MS.
Restek continues to support the medical cannabis analysis community with GC and LC columns, accessories, and reference materials. Check out the Restek Medical Marijuana web page.