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The Challenge of Residual Solvent Testing for Cannabis Edibles

By Alexander Beadle

Published: Oct 23, 2019   
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The use of cannabis-infused edibles has skyrocketed over the past few years. Since 2016, the palatable products have gone from a relatively rare method of cannabis consumption to the most common form of cannabis used in the United States, according to the Global Drug Survey.

Unlike raw, dried cannabis, edibles and other processed cannabis products are likely to have been exposed to solvents and processing chemicals at some point during their production. As a safety precaution, these products then need to be tested to ensure no unsafe residual amounts of these chemicals have remained on or in the final cannabis product.


A brief history of cannabis edibles

While edibles are still a relatively recent addition for the North American cannabis consumer, edible cannabis products have a long history elsewhere in the world. In India, there are Sanskrit recipes from as early as 1,000 BCE that describe the preparation of a cannabis-infused ghee called Bhang that was used in spiritual and ceremonial events and given to soldiers before battle.

In 1954, Alice B. Toklas, an American-born champion of the Parisian avant-garde, published a memoir-cum-cookbook, “The Alice B. Toklas Cookbook” which featured a recipe for a Moroccan hashish fudge. While that recipe was censored out of the original American edition of the book, it was included in a second edition released in the early 1960s. The recipe, and the concept of cannabis edibles, quickly became adopted into the hippie culture of the US that was present at the time.

In modern times, the legalization of cannabis in some parts of the United States has led to the development of a huge variety of edible cannabis products. Some of the more simple edibles, like the culturally ubiquitous “pot brownie” or cannabis butters that can be made at home, are made directly from dried cannabis flower, by mixing cooked (“decarboxylated”) cannabis flower in with the other traditional recipe ingredients.

But for other more complex products (cannabis gummies) or for products made at an industrial level, it’s more common to infuse products with an oily or waxy cannabis concentrate made by extracting certain cannabinoids from cannabis flower using industrial solvents.


Residual solvents and the cannabis extraction process

These cannabis concentrates are made using the cannabis extraction process. While there are a number of different cannabis extraction techniques – the most common being alcohol extraction, hydrocarbon extraction, and supercritical carbon dioxide extraction – they are all essentially a process by which cannabis material is exposed to a volatile solvent which strips cannabinoid compounds such as tetrahydrocannabinol (THC) and cannabidiol (CBD) from the cannabis plant material.

The resultant solvent/cannabinoid mixture can then be gently heated to evaporate off the volatile solvent, leaving behind a high-purity cannabis oil or wax, which is the cannabis extract. In theory, the volatile solvent will be completely removed in this process, but in practice, residual amounts of potentially toxic solvents can remain.

As a result, some jurisdictions with legal cannabis markets require all edible products and concentrates made from cannabis extracts to be tested for the presence of common residual solvents, including the likes of acetone, ethanol, butane, benzene, and more.

This residual solvent analysis for cannabis and cannabis extracts is normally done using headspace analysis. In this method, researchers use a gas-tight syringe to obtain a sample of any volatile gases present in the “headspace” – the area above the cannabis sample – in a sealed sample vial. The captured vapors can then be identified and quantified using gas chromatography coupled with a flame ionization detector. This is an extremely sensitive test, able to detect the presence of residual solvents on the order of parts per million.


The unique challenges of residual solvent testing in edible products

In 2016, the US Drug Enforcement Administration (DEA) published an analysis of marijuana edibles. This review noted the testing difficulties edibles present; testing one corner of an edible brownie may not give the same potency or contaminant analysis results as another sample taken from the center of the product.

The DEA also noted that there is no single established method for the testing of cannabis edibles – the differences in matrix between different types of edible product have proved too challenging for this thus far.

For water soluble samples, such as a sugar-based gummy candy, samples can be dissolved in water, which can then be tested for the presence of residual solvents. More complex samples may have to be homogenized first to afford the cannabis extract, but this means further solvent use that could affect the results.

Developing a reliable test that can accurately analyze finished products such as cannabis edibles for impurities is the next greatest challenge for cannabis testing labs, says Holly Johnson, chief scientist at the American Herbal Products Association.

“Producers and testers [are facing] method validation or verification challenges to ensure accurate measurements in new product matrices, to prove that their gummy bear or fudge contains what the label claims,” said Johnson to Lab Manager.

Alexander Beadle

Science Writer

Alexander Beadle has been working as a freelance science writer since 2017 and has covered the cannabis industry for Analytical Cannabis since 2018. He has also written for our sister publication, Technology Networks, and the cannabis industry consultant firm Prohibition Partners, among others. Alexander holds a Master's in Materials Chemistry from the University of St. Andrews, where he won a Chemistry Purdie scholarship, and conducted research into zeolite crystal growth mechanisms and the action of single-molecule transistors.


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