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Don’t overestimate cannabidiol during medical cannabis potency determinations with gas
chromatography. Use stationary phase selectivity for accuracy and hydrogen for fast analysis.
Sunday, February 17th, 2013 by
Jack Cochran
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
Sky® 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.
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