Supercritical Fluid Chromatography: The Future of Cannabis Refinement?
Chromatography plays a very important role within the cannabis space. For many applications, it is not enough to simply extract the oils from the cannabis plant and call it a day – the constituent cannabinoids and other compounds that make up these oils still need to be further separated from each other. This is true in testing, where you want to look at the presence of each compound present individually. But it is also relevant for product manufacturers looking to create CBD or THC isolates from broad-spectrum cannabis oils.
Traditionally, the cannabis industry has made use of high-performance liquid chromatography (HPLC) and flash chromatography for these applications. But these methods are time-consuming, utilize large amounts of chemicals, and generally are not well-suited to being scaled up for industrial use.
As the cannabis industry expands and more large-scale operations begin to emerge, the race is on to find new chromatography techniques to leverage for better results.
How it works
Supercritical fluid chromatography (SFC) is just one of those methods now beginning to be adopted by the cannabis community.
“The setup is, well, you can think of it as a hybrid between supercritical CO2 extraction and HPLC chromatography,” Dr Nikos Xynos, founder and managing director of the phytochemistry-inspired technology transfer firm Nomad Labs, tells Analytical Cannabis.
In SFC, like HPLC chromatography, the cannabis extract is passed through a chromatography column filled with a solid silica-based stationary phase, where it separates out into its constituent parts that are analyzed and collected. But unlike HPLC, the mobile phase that carries the extract through the apparatus is not a liquid solvent. Instead, it is supercritical CO2.
“We have the column and we have the co-solvent module, which is something we also have in supercritical CO2 extraction, and we utilize it for similar purposes, for modifying the polarity [of the solvent],” Xynos continues.
“So we have this module that brings in the cosolvent, typically ethanol, and then at the end of the column we have a [fraction] collector. And before collecting we have a split that goes to the detector. Usually this detector is a UV detector, as it is usually in other kinds of chromatography.”
The pros and cons of SFC
So, operationally, SFC is very similar to HPLC. But the SFC method comes with some very unique advantages.
As those familiar with CO2 extraction will already be aware, supercritical CO2 has some unique properties that make it a very powerful solvent. Supercritical fluids have the density of a liquid, the viscosity of a gas, and a diffusion coefficient that sits roughly between those two matter states – all of which are useful properties for supercritical fluids’ use as a solvent.
CO2 also comes with the advantages of being a chemically inert and non-flammable solvent, so it can be safely mixed with very small amounts of other co-solvents to create a solvent with an ideal polarity. Its gaseous state at room temperature also means that there is no risk of residual solvents being present in the separated cannabinoid fractions, as the CO2 will naturally evaporate off at room temperature.
“[SFC is] a much, much greener process,” Xynos adds. “Our main solvent is a pressurized liquid that, in ambient conditions, is a gas. And usually it's a gas that is being recovered from other industrial activities – so it's been recycled in order to reduce carbon footprint.”
Compared to HPLC and other novel chromatography technologies, such as centrifugal partition chromatography, SFC has some unique advantages in terms of longevity.
With HPLC and flash chromatography, the solid silica-based stationary phase will last for approximately 100-1,000 runs, depending on the technique being used, Xynos estimates. Centrifugal partition chromatography uniquely uses a liquid stationary phase, which can be mostly recycled and demonstrates an improvement on the longevity of HPLC. But due to the novel properties of the supercritical fluid solvent, SFC systems can run thousands of cycles before parts need to be changed.
“This is because basically, [in HPLC] it is the liquid solvents that gradually reduce the resolution capacity of your chromatography coil,” Xynos explains. “Here, because you are utilizing such small amounts of liquid solvent – maybe just two or three percent ethanol – your stationary phase will last much, much longer.”
So, what’s the catch? Operationally, the largest drawback to SFC is that it can only tolerate a certain volume of polarity-modifying co-solvent. But for cannabinoid purification and isolation, this is not hugely relevant; SFC on cannabis extracts generally only requires small amounts of co-solvent.
For the cannabis industry, the largest limitation at present is likely just the availability of SFC equipment. As Xynos explains, there are currently only a handful of technology suppliers producing SFC systems suitable for academic and analytical purposes, with just one really producing the kind of large-scale system needed for industrial use. And with the technique being so uncommon, this means that application notes and other documentation, as well as academic publications investigating the method’s effectiveness, do not quite have the same depth as other refinement techniques.
SFC for the cannabis industry
Despite only one equipment manufacturer currently supplying industrial-scale systems for SFC, the very existence of such a system is a positive check mark against SFC. This means the method is demonstrably robust and scalable, and when the cost of solvents and expendables are taken into consideration, it requires roughly the same investment as any other good HPLC system. If the technique can lift itself out of its relative obscurity within the cannabis sector, it has the potential to become a real heavy-hitter in the world of industrial-scale CBD isolate production.
“It remains a niche technology, but it is starting to be implemented in the cannabis world. I think that it will become a more and more popular system and technology to us,” says Xynos. “Scalability is another thing that is very, very important – we have a new scalability with SFC.”
“It's a very repetitive technique, and reliable, and robust. So I expect that SFC will eventually become very popular, especially at the industrial scale.”
This article originally appeared in Analytical Cannabis' Advances in Cannabis Extraction and Processing eBook in March 2021.