Gas chromatography (GC) is a frequently employed chromatographic method to separate, identify, and quantify the constituent compounds in a sample by vaporization to the gas phase. It has recently been used to determine the concentrations of CBD and THC in cannabis products.
Cannabis. Image Credit: Yarygin/Shutterstock.com
Detectors used in GC-FID
Flame ionization detectors (FID) are among the most common detectors used in GC. They use a hydrogen-burning flame placed at the end of the chromatographic column, which decomposes gaseous organic compounds as they enter the flame. This causes organic compounds to decompose into their constituent cations and electrons, generating current between two electrodes placed on either side of the flame. The amount of current generated in this process is used to infer the number of carbons in the molecule.
Tetrahydrocannabinol (THC) and cannabidiol (CBD) are the major phytocannabinoids present in marijuana, among many others. THC is the primary psychoactive component, while CBD is the primary therapeutic component of marijuana, relieving nausea, pain, and even providing treatment for some types of childhood epilepsy.
With the recent trend of legalizing and making marijuana available for medical purposes, it is important to assess the quantity of THC and CBD present in marijuana, especially marijuana produced with the intention of relieving pain or reducing the symptoms of epilepsy.
How is marijuana prepared for GC-FID?
The majority of THC and CBD exist as tetrahydrocannabinol acid (THCA) and cannabidiol acid (CBDA), in their natural form within the plant. These molecules are thermally labile, and undergo decarboxylation to form THC and CBD when smoked, cooked, or vaporized within a gas chromatography machine.
Other types of analytical process, such as liquid chromatography, which does not vaporize samples, can detect both THC and THCA, and CBD and CBDA, which must be accounted for when calculating the total concentration of these components.
In order to ensure that the whole of a marijuana plant is tested for the concentration of THC/CBD representatively, random samples are collected from the flowers, stems, leaves, and buds of the plant and dried in a low-temperature oven for a few hours.
The resulting dry plant material is finely ground and mixed with an organic solvent. Organic solvents, such as methanol have been used to extract THC and CBD from marijuana and create an oily resin that can be injected directly into the GC-FID apparatus following a drying step.
Only minimum quantities of THC and CBD can be determined following this process, as not all of the chemicals present in cannabis will be recovered by this process. However, THC and CBD should be recovered in proportions representative of those present in the plant, particularly if suitable sampling methods have been employed.
How are THC and CBD distinguished by GC-FID?
The gas chromatography column separates molecules based largely on their polarity, with polar columns interacting more strongly with highly polar molecules. An intermediate polar column is recommended to successfully resolve CBD from THC and also to allow other less polar cannabinoids time to elute.
Cannabinoids, in general, are relatively non-polar, and an intermediate column means that the non-polar surface will be coated more quickly than the polar surface of the stationary phase. Thus, the most polar compounds will elute first.
CBD elutes more quickly than THC as it contains alcohol rather than an ether functional group at relatively the same position in the molecule, causing it to be slightly more polar.