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Analyzing Residual Solvents in Cannabis Concentrates: A Sticky Situation

12 May 2014

Along with the increasing demand for various forms of cannabis concentrates comes increased concern regarding residual solvents in these products. In many cases, cannabis concentrates are prepared by extracting either the acidic or decarboxylated forms of cannabinoids from plant material using organic solvents. Some of the solvents used for extraction can have detrimental health effects, which nobody wants.

After extraction, the solvents can be purged from the final product, but the effectiveness of the purge needs to be verified by analyzing the final product to determine if any solvent remains. The most common way to do this is through headspace-gas chromatography (HS-GC), which is perfectly suited to this purpose. Instead of injecting a liquid sample like we do for traditional GC analyses, the headspace instrument heats the sample in a closed vial, releasing the volatile solvents for introduction into the GC, but leaving the less volatile material behind. This results in a very clean chromatogram with very little sample preparation. It also helps keep our injection port and column free of non-volatile material, resulting in very little maintenance for the GC system.

For traditional HS-GC analyses, the sample is dissolved in a less-volatile solvent (most commonly DMSO, DMF, or water), then placed into the headspace instrument. The dissolved sample is heated, establishing an equilibrium between the liquid and gas phases, then the gas phase in the vial (referred to as headspace) is injected onto the GC and analyzed. Sample preparation involving dissolving the sample into a solvent is often referred to as the solution approach. Figure 1 shows the general setup of a HS-GC instrument.


HS-GC Instrument Diagram


So how do we analyze our sample if it won’t dissolve? One way to do that is to do away with the sample solvent altogether. This is called the Full Evaporation Technique (FET), and it is useful for difficult and varied sample matrices like cannabis concentrates. FET depends on using a very small sample size (about 20mg) and establishing essentially a single-phase equilibrium in the headspace vial. Not only does this make sample preparation very simple, it also cuts down on the sample amount required to perform an analysis, and we no longer have to worry about partition coefficients at equilibrium in the headspace vial. There are some caveats for the success of this technique, but I won’t get into them here. Please feel free to contact me for more information on this technique. Figure 2 below shows stylized representations of the solution and FET approaches for HS-GC. Note the very small sample size in the headspace vial. It really looks like that, and believe it or not, it’s enough to analyze.


FET vs Solution Approach


For quantification, very small volumes of standards are used to mimic the small sample amounts used during analysis and to maintain a single-phase system for our standards. The chromatogram in Figure 3 below shows a standard prepared representing 20mg of a cannabis concentrate sample containing 50ppm residual solvents.


50ppm FET Cann RS


Currently, we’ve finalized the chromatographic conditions (shown above), and we’re working on proving the concept for quantification using this technique for cannabis concentrates. Stay tuned to the blog for the latest developments in our work for this and other applications. We’re always looking for opportunities for collaboration, so if you’re interested, please let me know and I’ll get in contact with you.