Potency testing: GC vs. LC

Alex-Edwards-at-LP-Analytical-3At The Good Lab, we use High-performance liquid chromatography (HPLC) for potency testing. Why is that important? Here’s a great explanation of the difference between gas chromatography (GC) and liquid chromatography (LC) from Lift Cannabis News Magazine (Canada).

Potency Testing

Marijuana is a complex matrix. Like many natural products it contains thousands of compounds, many of which have yet to be discovered or understood. Separating out the THC and CBD from the diverse soup of compounds contained in your marijuana is essentially like looking for a few needles in a big haystack.

Chromatography is the chemist’s way of sorting out that haystack. In the cannabis world, potency is tested by liquid chromatography or gas chromatography. Although there are some promising techniques for testing potency using spectroscopy instead of chromatography, chromatography is currently the industry standard method used in Cannabis testing labs from Colorado to Uruguay.

How it works

To get an understanding of how chromatography works and what the “liquid” and “gas” terminologies mean, one first needs to look at the “column”. This is where the separation occurs in both liquid and gas chromatography.

A column is basically a tube that contains a chemical phase or material (the stationary phase) and a “mobile phase” which is what keeps everything moving through the column (this is where the “liquid” and “gas” part comes in). In gas chromatography the mobile phase is a gas (an inert gas like helium, hydrogen or nitrogen) and in liquid chromatography the mobile phase is a liquid (like methanol, acetonitrile and water).

For the separation to take place, the marijuana extract gets put onto the column, and will be transitioned through the column from one end to the other by the mobile phase. The components inside the extract (the THC and CBD) will have varying affinity to stay in the stationary phase that lines the column. Some will stay in longer than others, so they become separated from each other.

gc vs lc liftcannabis caAt the end of the column, once each of the components have been separated, they will be detected or “counted”. There are differences between the way the THC and CBD are detected between liquid or gas chromatography. In gas chromatography (GC) you often see that the analysis is called “GCFID”. The FID part stands for “flame ionization detector” which is essentially like it sounds – a flame! The FID will burn the compounds as they exit the column and an electrical signal is measured. This signal is directly proportional to the amount of the compound present.

In this type of liquid chromatography, the measurement is by UV (ultraviolet). A light in the wavelength range of the ultraviolet (yes, the same UV that your Oakley sunglasses are blocking from your eyes) is directed through the compound as it exits the column. The helpful characteristic of the THC and CBD molecules is that they absorb UV light, and this absorption can be measured and is directly proportional to the amount of the compound present.

You may have noticed that “LC” is often referred to “HPLC”. This was just the chemists adding more letters to something that was already concise and sensible. The “HP” stands for high pressure, or sometimes “high performance”, which varies from manufacturer to manufacturer. Some even call their systems UHPLC or ULTRA high pressure liquid chromatography. A higher pressure system will do the analysis faster, but that’s it.

So what about the other cannabinoids? Chances are, if you are reading this blog you are aware that there are a lot of other important cannabinoids in marijuana; CBN (cannabinol), CBG (cannabigerol), CBC (cannabichromene), delta-8-THC and many, many more! Why are these not reported? Especially as there is an indication that these compounds have important medical properties. Well, for now, it is not required. But many labs and licensed producers are already testing for these other cannabinoids. And what about the acid forms of the compounds, THC-acid and CBD-acid? This brings out an important discussion that relates directly back to the analysis by GC versus LC.

The effect of the main psychoactive compound (THC) is greatly reduced unless the THC-A is converted to THC. When cannabis is heated, the acid forms of the cannabinoids will readily convert to their neutral forms. Without heating (and not waiting for a long time) THC will remain in acid form, and this structure of the molecule doesn’t bind to the receptors in our brain that happily accept the neutral THC (THC with the acid removed).

Why does this affect the choice of analytical instrumentation? Well, in order for GC (gas chromatography) to work, the marijuana extract has to be converted to gas form, which means it is heated before entering the column. Any acid cannabinoid compounds, such as THC-A will be converted to their neutral forms, and no acid compounds will be detected, Conversely, in LC, the extract in liquid state can be injected onto the column as is, and therefore, you can quantify all the acid and neutral forms of your cannabinoids.

Some argue that the GC better mimics the state in which marijuana is typically consumed; by smoking or vaporizing. However, many patients may choose to vaporize at lower temperatures, and they may in fact be consuming some of the cannabinoids in acid form. And what about the potential medical benefits of the acid cannabinoids and the other unreported cannabinoids such as CBC? At this early phase in our industry, we at Signoto believe it’s better to provide as much information as possible to medical practitioners and patients. It’s time to move quickly away from the mystery era of cannabis.

~Emily Kirkham

VP of Laboratory Operations at Signoto.

Thank you Lift Cannabis News Magazine for allowing us to share this! Please visit their site for more information on all things cannabis in Canada.

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