Determining Decarboxylation in the Cannabis Lab
Even those who know very little about cannabis will have heard of THC and CBD.
The former, tetrahydrocannabinol, is the chemical compound responsible for marijuana’s intoxicating properties. Many strains of cannabis have been specially bred for years by expert cultivators to increase the potency of this compound in the plant. The latter, cannabidiol, is experiencing a breakthrough into markets such as skincare and beauty, food and drink, and even the pet care industry, as a specialty ingredient known for its anti-inflammatory and anti-oxidant effects.
Many of those who know of these compounds may still be surprised to learn that there is relatively little THC or CBD in freshly harvested raw cannabis.
The decarboxylation of THCA and CBDA
Every major cannabinoid in the cannabis plant begins as cannabigerolic acid (CBGA). From there, plant enzymes unique to each cannabis strain convert this CBGA into other acidic cannabinoid precursor compounds, including tetrahydrocannabinolic acid (THCA) and cannabidiolic acid (CBDA).
(The decarboxylation of CBGA into CBG, CBD, and THC)
THCA and CBDA themselves are non-intoxicating. In order to produce THC or CBD from these precursors, the plant material must be exposed to heat or some other force that is capable of triggering decarboxylation, the chemical reaction whereby the acidic carbonyl group is lost from the molecule.
Exactly how much THCA or CBDA is decarboxylated will expend on a number of external factors. Over time, some amount of these compounds will naturally degrade into THC and CBD. But the majority of this decarboxylation will occur during consumption, whether that is from burning during smoking, vaporization in vaping, or the heat involved in making baked edibles.
“The amount of heat introduced during the consumption method will determine the amount of decarboxylation,” explains Douglas Duncan, PhD, laboratory director of CannaSafe's Oregon location.
“Decarboxylation of THCA to THC begins at 220°F. So, the amount of decarboxylation of THCA will change drastically in a lit joint compared to an edible or tincture which are not heated.”
Calculating potency and studying decarboxylation
With there being relatively little THC naturally existing in raw cannabis, scientists cannot simply study the amount of THC in a sample to determine its potency. Instead, cannabis testing labs must also look at the THCA content of the samples they receive and use this to calculate the total THC or potency of a strain or a product sample.
Crucially, calculating the total THC of a sample is not as straightforward as simply summing the THC and THCA content of a sample.
“THCA breaks down/decarboxylates when introduced to heat. During the decarboxylation process, a THCA molecule releases a CO2 molecule,” Duncan explains.
Because of this, Duncan says, THC as a molecule is lighter than its acidic precursor. With THC’s exact molecular weight being just 87.7 percent that of THCA, this difference in weight must be incorporated when calculating total THC as a percentage of dry weight in the product.
“The 0.877 factor used in calculating total THC is simply the mass fraction of this decarboxylation process. Anyone can calculate this by dividing the molar mass of THC by THCA. If we assume perfect decarboxylation, 1 gram of THCA becomes 0.877 gram THC and 0.133 gram CO2.”
Realistically, the consumption of cannabis is unlikely to result in perfect decarboxylation, due to the reasons listed above with differences in combustion conditions. Analysis labs will use their own formulae to take this into account for each sample they test to provide the most accurate total THC figure possible.
This potency testing to determine total THC is normally done using some form of liquid chromatography (LC), in order to study the THC and THCA content of the sample with no external heat being applied. Because LC does not heat the sample, it will not contribute to the further decarboxylation of THC or any other cannabinoid precursors in the sample, allowing for the direct measurement of these compounds.
“HPLC [high-performance liquid chromatography] has two major advantages; first, the sample does not undergo decarboxylation. In the case of gas chromatography (GC), the sample is injected into a heated inlet capable of instantly converting THCA to THC. The second HPLC advantage is the ability to measure each compound independently by retention time,” says Duncan.
A note on CBDA
For THCA and THC, much of the focus is on maximizing conversion to THC in order to increase the intoxicating power of a given product. But when it comes to CBDA, there is growing evidence that this precursor compound may actually be powerful in its own right.
Research has shown that CBDA exhibits the same anti-inflammatory behavior as many common nonsteroidal anti-inflammatory drugs (NSAIDs). CBDA is also up to one thousand times more powerful than CBD in binding to a specific dopamine receptor that is known to produce anti-nausea and anti-anxiety effects.
In part because of this research, high-CBDA products have become a trend within the consumer market, with some cannabis users even taking to “raw juicing” their cannabis to minimize decarboxylation and preserve a higher CBDA level.
As evidenced by its ready conversion into CBD, CBDA is naturally quite an unstable compound. For products advertising a high CBDA content, it is important to have testing methods that will not further decarboxylate the compound, such as HPLC. For this reason, HPLC can also be used to study the amount of decarboxylation that as already occurred in a given sample.
This article originally appeared in Analytical Cannabis' Technologies and Techniques for Cannabis Testing eBook in September 2020.