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Marijuana Potency Testing—Quick and Easy by GC or LC
By Amanda Rigdon and Jack Cochran
• Single extraction for both GC and LC.
• Fast results on Rxi®-5Sil MS GC or Ultra Aqueous C18 LC columns.
• Convenient standards for potency testing.
Although marijuana is illegal at the federal level in the United States,
the use of medicinal marijuana is currently legal in many states. In
some areas, it is widely used, and demand is rising for potency data
for medicinal products purchased at dispensaries. Potency testing
is more straightforward than impurity testing because the active
compounds are present in much higher concentrations relative to
matrix. Currently, GC is the most popular method for potency testing
due to its ease of use and the availability of relatively inexpensive
instrumentation. However, LC is also a viable technique for medical
cannabis potency testing. As shown in this article, the same straight-
forward sample preparation technique can be used for cannabis
potency testing by either GC or LC.
Simple Sample Prep
Cannabinoids were extracted from 7 different marijuana samples
under the supervision of local law enforcement personnel. The extrac-
tion procedure consisted of weighing 0.2 g of sample into a 40 mL
VOA vial, adding 40 mL of isopropyl alcohol, shaking for 5 minutes, and
then allowing the sample to settle. The procedure was very quick and
produced extracts that were compatible with both GC and LC analysis.
GC Analysis
The 3 compounds of interest for GC potency testing are
Δ9-tetrahydrocannabinol (THC), cannabinol (CBN), and cannabidiol
(CBD). While THC is primarily responsible for the hypnotic effects of
marijuana, CBD acts to attenuate these effects. Since CBD has been
shown to have medicinal properties, it is desired at higher concentra-
tions in medical marijuana. Because the samples that were extracted
were illicit samples seized by local law enforcement, the CBD levels
were very low. In general, higher CBD levels are observed in medici-
nal marijuana strains. CBN is an indicator of sample breakdown due
to age or poor storage conditions.
For GC potency testing, 1 µL of prepared extract was manually
injected onto a 5890 GC equipped with a flame ionization detector
and analyzed on a 15 m Rxi®-5Sil MS column (cat.# 13620). To ensure
accurate and reproducible manual injections, a Merlin Microshot
injector (cat.# 22229) was used. Figure 1 shows an overlay of a can-
nabinoid standard (cat.# 34014) that contains the 3 target analytes
(blue trace) and a representative chromatogram of a marijuana sam-
ple (red trace). The use of a narrow-bore, thin-film analytical column
resulted in sharp peaks, which improve sensitivity and allow a split
injection to be used to reduce column contamination.
LC Analysis
LC potency testing requires the analysis of the 3 components
discussed above, but also includes Δ9- tetrahydrocannabolic acid
(THCA). While THCA is not hallucinogenic, all THC in the marijuana
plant exists as THCA, and only converts to THC upon heating (i.e.,
smoking, vaporizing, cooking, or injecting into a hot GC inlet). Since
the sample extraction and LC analysis employ no heat, potency must
be determined based on THCA when using LC, rather than with THC
as is used in GC analysis.
For LC potency testing, extracts were diluted 10x with isopropyl
alcohol, and 10 µL of extract was injected onto a 3 µm Ultra Aqueous
C18 column (cat.# 9178312). Figure 2 shows an overlay of the can-
nabinoid standard described above with the addition of THCA (blue
trace) and a representative chromatogram of the same marijuana
sample (red trace).
Summary
Both the GC and LC methods shown here for determining medi-
cal marijuana potency employ a straightforward and cost-effective
extraction procedure and fast analysis times. This allows reliable
potency analyses at a reasonable cost per sample.
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