What is Residual Solvent Analysis?
In the manufacturing of cannabis concentrates and extracts, solvents (water, butane, carbon dioxide, ethanol, etc.) are used to extract the active ingredients from plant material to produce a resinous, sometimes glassy product. Steep Hill regularly tests solvent extractions for residual solvents, helping manufacturers ensure the quality of their product, and empowering consumers with greater awareness of the composition of the products they are consuming.
Common solvents for such extractions include ethanol, water, butane, propane, hexane and many others. Commercially available solvents can be of varying purity. Butane, propane, and other hydrocarbons (pentane, hexane, etc.) in particular, are all derived from petroleum. Petroleum extracted from the ground is a mixture of thousands of compounds. Refineries separate them into groups according to their boiling points. It gets expensive to separate the compounds that have similar boiling points into high purity, so gas mixtures are sold as a single product. As the sources of crude oil change, the impurities present in the mixtures will change significantly. Some solvents have odorants, such as mercaptan, added to the mix. These compounds can be very toxic.
Commercial “butane” is actually a mixture of many different isomers of butanes and pentanes and sometimes also propanes and hexanes.
Neopentane and Methylbutane both have the formula C5H12 and both are common contaminants. Because of their higher molecular weight, both are less volatile than butane. Because of this, it is more difficult to purge these substances than butane.
The process of making solvent extracts from raw plant material is typically solvent intensive. Large quantities of solvent are often needed to increase extraction efficiency and to achieve higher levels of purity. After the extraction process is complete, skill and discipline are required to ensure that residual solvents are removed. It is close to impossible to remove ALL residual solvents and the varying experience levels among concentrate manufacturers can significantly impact the efficiency of the purging process. Residual solvents are found in a large majority of solvent extracts.
Among the many factors affecting purging is the rate at which residual solvents will diffuse to the surface of the samples and evaporate. Sample viscosity and molecular size are particularly important factors. In more viscous samples (variables include heat, atmospheric pressure, etc), solvent molecules have a harder time escaping to the surface. Smaller molecules like butane and propane diffuse more quickly, which is why they are popular solvent choices. Commercial products are available in a wide range purity levels and prices. Various contaminants may be present and these compounds may be more difficult to remove from the sample material. Some solvents, such as ethanol, are comparatively less volatile solvents and require significantly different purging conditions.
Residual solvent amounts are commonly expressed in parts per million (ppm). When using ppm as a unit of concentration, it is useful to remember this analogy to percent: 1% can also be referred to as 10 parts per thousand (ppt), 1ppt can also be referred to as 1,000 ppm; 1 ppm can also be referred to as 1,000 parts per billion (ppb).
Why headspace analysis?
The analysis of residual solvents is best performed using a process called headspace sampling. This is especially true for fixed gases (substances that are normally gases at room temperature, such as butane and propane). In headspace analysis, a gas-tight syringe is used to obtain a small sample of the gases in the headspace of a sealed vial with the sample preparation. The headspace sample will include vapors of all volatile components in the vial. Headspace analysis used in combination with a GC/MS is an extremely sensitive test for most of the relevant volatile compounds.