Analytical vs preparative chromatography in cannabis processing

Chromatography is a method of separating mixtures in a laboratory setting using two distinct phases, a mobile phase and a stationary phase.  While analytical chromatography helps gather specific information on the compounds in a mixture, preparative chromatography isolates and purifies the sample.¹ In the cannabis industry, both analytical and preparative chromatography are important for the identification and production cannabis products.  Both types of chromatography are complementary. Not only do the methods ensure the compounds are correctly identified, but they also ensure that final products are safe and effective, as well as labeled accordingly.²

The basic process of chromatography is simple. It occurs when a mixture is dissolved in a fluid, which can be liquid or gas, and is then passed through another material, usually solid.  The fluid, or mobile phase, interacts with the solid, or stationary phase.  During this interaction, the mobile phase separates into components as it travels through the stationary phase.¹ There are many types of different chromatography that take advantage of the properties of the mobile and stationary phases to separate and/or identify a substance. Liquid or gas chromatography change the physical state of the mobile phase. Column or planar chromatography changes the shape of the stationary phase. Ion-exchange and size exclusion chromatography also chance the actual mechanism of separation. Chromatography is a powerful tool available to scientists to identify and purify mixtures.

Chromatography can either be used to identify or purify a mixture. In fact, most often, chromatography is used for both, isolating and purifying specific compounds for further research or commercial purposes. The process of identifying and analyzing compounds is analytical chromatography. In this type of analysis, the goal is to separate as many peaks as possible, using a baseline to help identify components of a mixture. For instance, in analytical HPLC, the sample volume is usually small, around 1 – 20 µl, as identification is the primary purpose.³ The column is small, only 1-4mm in diameter, and the flow rate is low, typically 1.0 mL/min. Detection is set up to be as sensitive as possible.  As well, there are no fraction collections or solvent recovery, as the mixture is just being analyzed, not collected.³ On the other hand, preparative chromatography isolates and purifies specific compounds to collect and form a substantial sample of the desired product. Therefore, with preparative chromatography, recovery is most important rather than identification.  For instance, in HPLC, the separation goal is to see the one desired peak.  The sample volume is as large as possible, to maximize collection while minimizing contamination or peak overlap.³ The column diameter is usually larger than analytical techniques, greater than 4mm. The flow rate is also higher, around 5 – 5000 mL/min, as volume is more desired for this preparation.³ Mostly importantly, the solvents and eluents are collected in preparative chromatography, as the end goal is to collect a purified specific substance from the column.²

Both analytical and preparative chromatography are extremely important to the budding cannabis industry.  One of the biggest concerns in the industry today is the accurate inclusion and reporting of cannabinoids and other cannabis compounds, such as terpenes, in medical and recreational commercial products. Analytical chromatography is used to identify the various components of a sample of cannabis.  A sample of cannabis plant material can have thousands of different compounds in it, including over 110 active cannabinoids.  THC and CBD are among the most commonly identified and used cannabis components.  Analytical chromatography can identify the THC and CBD in a sample as well as the other components as well.  Initial cannabis samples can be tested for the presence of illegal pesticides.⁴ As well, cannabis can be tested for the presence of illicit substances in their formulations. Also of importance, because chromatography is not heat dependent, cannabinoids and other cannabis components can be accurately identified by analytical chromatography, as decarboxylation or destruction by temperature is avoided.⁵ Preparative chromatography allows for these components to be separated out of the plant slurry and be refined.  In HPLC, silica resin columns are popular to use with lipophilic cannabis compounds. Cannabinoids are easily separated from the rest of the plant material using this process.² Unfortunately, stationary columns are expensive and run times are long, so scientists are exploring other types of preparative chromatography for cannabinoids such as centrifugal partition chromatography purification.²

Analytical and preparative chromatography use the same techniques of separating mixtures using a mobile phase and a stationary phase.  While analytical chromatography strives to qualitatively identify mixtures, preparative chromatography strives to separate mixtures.  In the cannabis industry, the analytical technique identifies cannabinoids as well as the many other components of the cannabis plant.  It can also identify contaminants, pesticides, or other illicit chemicals. Preparative chromatography can separate and collect cannabis components, such as the desirable compounds THC and CBD. It may, however, become expensive, based on column and solvent costs. In the big picture, the two different techniques of analytical and preparative chromatography, however, will remain complementary – one technique to identify and one technique to purify, especially in the cannabis industry.

References:

1. Analytical and Preparative Chromatography. News Medical Life Sciences website. https://www.news-medical.net/life-sciences/Analytical-and-Preparative-Chromatography.aspx#:~:text=In%20analytical%20chromatography%20the%20purpose,the%20components%20of%20the%20sample.&text=The%20purpose%20of%20preparative%20chromatography,specific%20substance%20from%20the%20sample. Accessed on October 28, 2020.
2. HPLC vs CPC for cannabis testing. Gilson website. https://www.gilson.com/default/hplc-vs-cpc-for-cannabis-testing. Accessed on October 28, 2020.
3. What is the difference between analytical and preparative HPLC? Knauer website. https://www.knauer.net/en/what-is-the-difference-between-analytical-and-preparative-hplc/f24135. Accessed on October 28, 2020.
4. Advancing chromatography methods for cannabis analysis. Cannabis Science and Technology website. https://www.cannabissciencetech.com/view/advancing-chromatography-methods-cannabis-analysis. October 28. 2020.
5. Determination of cannabinoids with analytical HPLC. https://www.knauer.net/en/Blog/Determination-of-Cannabinoids-with-Analytical-HPLC. Accessed October 28, 2020.