Residual solvent testing is important in non-medicated food types, but is especially pertinent to the safety of cannabis products. All processed product types must be tested for residual solvents. This test is required for pre-rolled products, concentrates, beverages, infused oils, food products and other similar matrix types. Overall, we must test anything that is not flower material only. This is because the concentrates used to infuse products (or, for use as is) often contain trace solvents from extraction processes. Though heat and low pressure vacuum is used to purge these products of their solvents, it may not be enough. We need to know when products are ready to be safely available to the public.
Sample preparation is a simple task: approximately half a gram of homogenized product is suspended in an organic and aqueous mixture for extraction. Both solvent types are necessary to both solvate and extract the organic, residual solvents in the homogenized material. Often, the method used for testing is HS-GC-FID (headspace gas chromatography flame ionization detection). This is because the solvents tested for are volatile and can be easily evaporated in the machine, at high temperature, into a vial’s headspace. This headspace is then sampled and an aliquot is injected onto the GC column for separation. There is no significant dilution step necessary, due to the fact that the split ratio can be controlled. After separation, the solvents are combusted by the flame ionization detector. Scientists identify and quantify each solvent using retention time and peak area, respectively.
Common challenges to this method include the detection of chlorinated solvents and overlapping peaks. Chlorinated solvents, such as chloroform and dichloromethane, have a high halogen to carbon ratio, making them less combustible than hydrocarbon solvents like butane, propane, and benzene. To address this challenge, the mass of product extracted from can be increased. This provides for an appropriate signal to noise ratio for less-responsive compounds.
Additionally, scientists can confirm surprising hits on an alternative method. Confirming potential positive hits on a secondary method also addresses issues with overlapping peaks. A change in ramp rate, temperature, and other variables will create a different environment, allowing for separation of potential positive category I (chloroform, 1,2-dichloroethane, benzene, ethylene oxide, dichloromethane, trichloroethylene) hits from compounds that behaved similarly to category I solvents during the initial chromatographic run. These hits can also be confirmed with a mass spectrometer. Overlapping peaks are most often a challenge with terpene-high sample types. Terpenes are also volatile compounds, often hydrocarbons, with similar boiling points to common solvents.
Overall, most organic solvents are easily extracted, separated, and detected from cannabis-infused matrix types using HS-GC-FID methodology. These tests are necessary and useful for all cannabis-product types intended for public availability and consumption.
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