Hydrocarbon solvents are used in the extraction process of some cannabinoid extracts and distillates. Most commonly butane, ethanol, or supercritical carbon dioxide are used to process cannabis biomass into distillate. This distillate is then used as an inhalable product in vape pens or to produce edible marijuana products. Resin or shatter exclusively uses hydrocarbon extraction in their preparatory process because hydrocarbons allow for a “gentler” extraction of cannabinoids without loss of terpenes or decarboxylation of the carboxylic acid cannabinoids.
Certain hydrocarbons are more harmful than others. Benzene is particularly harmful and Oklahoma mandates that cannabis extracts only have a maximum level less than 2 ppm (parts per million). Other solvents are not as dangerous and have a higher ppm limit. Oklahoma currently mandates the following standards for cannabis products to pass residual solvent analysis.
- Acetone < 1,000 ppm
- Benzene < 2 ppm
- Butanes/ Heptanes < 1,000 ppm
- Hexane < 60 ppm
- Isopropyl Alcohol < 1,000 ppm
- Pentane < 1,000 ppm
- Propane < 1,000 ppm
- Toluene < 180 ppm
- Total Xylenes (m, p, o-xylenes) < 430 ppm
Residual solvent analysis is performed with headspace GC-MS. Headspace sampling involves transferring a known quantity of cannabis product into a headspace vial that is sealed and placed in an oven heated to a specific temperature. Volatile solvents come off the cannabis product, if they exist, and occupy the gaseous phase in vial. A known volume of gas in the vial is then injected directly into a GC column that separates the gaseous compounds by applying a temperature gradient.
The column is used for gas chromatography is typically a capillary column that is composed of a glass silica tube with a small inner diameter of usually less than 0.5 micrometers and has length between 10 and 30 meters. The inside of this glass capillary tube is coated with one of many liquid polymer stationary phases. The most common stationary phase is probably a predominantly non-polar (5%-Phenyl)-methylpolysiloxane polymer, although different ones are selected for different applications. This liquid coating provides a surface for compounds to interact and has different affinities for different compounds based on polarity. Separation of compounds is achieved based on the selective distribution of analyte molecules between the stationary phase and inert gas mobile phase. An increasing temperature gradient is produced during the analytic run causing compound volatility to be more of a factor in molecules favoring the inert mobile phase as the run progresses.
Any targeted analyte in the sample comes off at a specific different retention time. Once they come off the column, these compounds go through a “hard” ionization process of electron ionization (EI) that causes fragmentation, whereas “soft” ionization methods such as ESI used with LC-MS/MS generally doesn’t produce fragmentation of small molecules at optimized conditions. This ionization process involves the compounds crossing an electron path between and a wire filament and the entrance to the ion source block that has a standardized potential of 70eV. Electrons collide with the sample producing a characteristic and reproducible fragmentation pattern. In a qualitative situation this fragmentation pattern can be cross referenced with an NIST database to give insight into what the analyte could be based on a percentage match. However, with quantitative analyses such as this one, a mode called selective ion monitoring (SIM) is utilized to target specific molecules that elute in a given region to optimize sensitivity and specificity. As a result, it is impossible to retroactively look at a previous chromatogram and determine if a specific analyte was present if it wasn’t on the targeted test menu. These ions then get selectively transported through a quadropole based mass selector and finally come into contact with an electron multiplier that converts the analyte into an electronic signal to be quantified based on a calibration curve previous formed.