In gas chromatography the sample is vaporised in a heated inlet and carried by an inert gas through a long, thin column. Compounds separate by volatility and their interaction with the column coating, then reach a detector: a flame ionisation detector (GC-FID) for quantification, or a mass spectrometer (GC-MS) for identification.
In practice
GC is the standard for terpene profiles and residual solvent testing, often with headspace sampling (analysing the vapour above a sealed sample). Specialised setups, such as two-dimensional GC with sulfur detection, found the prenylated thiols behind skunk aroma. For cannabinoids, GC has a built-in problem: the hot inlet decarboxylates THCA to THC, and the conversion is not always complete or consistent. Without a derivatisation step, a GC potency result reports mostly neutral THC and cannot separate the original THCA and THC. That's why potency is normally done by HPLC.
Don't confuse with
- HPLC: liquid-phase, room temperature, measures acids directly.
- GC-MS for pesticides: the same technique family, but a different method and panel.
A terpene profile is the measured mix of terpenes in a sample, reported as each compound's % w/w and the total, used to describe and compare cultivar aroma chemistry.Homogenisation is grinding and mixing a sample so every portion the lab tests represents the whole; together with representative sampling it often matters more than the instrument.Volatile sulfur compounds (VSCs) are trace sulfur-containing aroma molecules, especially prenylated thiols, that give cannabis its skunky, gassy smell at concentrations far below terpenes.A cannabinoid is any compound that binds the body's cannabinoid receptors or shares the typical cannabis terpenophenolic structure; the plant-made ones are phytocannabinoids such as THCA and CBDA.Decarboxylation is the loss of a carboxyl group as CO2 that turns acid cannabinoids such as THCA and CBDA into neutral THC and CBD, quickly with heat, slowly with time.THCA (tetrahydrocannabinolic acid) is the acid cannabinoid that drug-type cannabis makes in its trichomes; heat or time decarboxylates it into THC, losing about 12% of its mass.THC (Δ9-tetrahydrocannabinol) is the neutral, decarboxylated cannabinoid mainly responsible for cannabis's reported intoxicating effects; the living plant makes it almost entirely as THCA.HPLC (high-performance liquid chromatography) separates compounds dissolved in a liquid under pressure; it is the standard cannabis potency method because it measures acid cannabinoids without heating them.A terpene test measures the individual terpenes and total terpene content of a sample, usually by GC-FID or GC-MS, reported in % w/w or mg/g.A residual solvents test measures leftover solvents such as butane, propane, ethanol or hexane in an extract, usually by headspace GC, against safety limits.Acid cannabinoids (THCA, CBDA, CBGA) are the carboxylated forms the plant makes; neutral cannabinoids (THC, CBD, CBG) form when heat or time removes the carboxyl group.
