Terpenes have stormed the cannabis and wellness industries. The ability to measure terpenes precisely and accurately in samples has become imperative for product designers and analytical laboratories. Although there are several options, some techniques stand out as more commercially viable than others.
- Gas Chromatography (GC) with Flame Ionization Detection (FID)
- GC with Mass Spectrometry (GC-MS)
- Liquid Chromatography with MS (LC-MS)
Additionally, they recommend pairing these methods with two auto-sampling techniques:
- Liquid Autoinjector
- Headspace (HS) Autosampler
In GC, the sample is heated and vaporized molecules (terpenes in this case) are carried by a gas (usually nitrogen or helium) into a column where the different components separate at unique rates. Terpenes are volatile and vaporize relatively easily, making GC the top choice.
Gas chromatograph. By Offnfopt; Public Domain.
A detector then produces electrical signals based on the composition of each separated molecule. At the basic level, a flame ionization detector (FID) can be used. FID is suitable for detecting organic compounds at 0.1 ppm. It works by torching the sample molecules with a hydrogen flame which produces ions that are electrically recorded.
MS is more expensive than FID and can detect other compounds such as pesticides. The sensitivity and selectivity of this instrument are superior. The MS device ionizes the sample and subsequent fragments are analyzed by the ratio of mass to charge. The ion peaks of sample compounds are matched to those in established libraries. When multiple mass analyzers are paired together, the technique is known as tandem mass spectrometry (MS/MS). This improves the ability of the analyst to precisely quantify certain compounds. A recent study used GC-MS/MS to identify 93 distinct terpenes in cannabis samples. 
LC-MS may be appropriate for heavier terpenes (e.g., sesquiterpenes); it also brings the advantage of not decarboxylating cannabinoids (for cannabis samples) prior to quantification. However, separation from the sample matrix is considered more challenging and requires multiple solvents. Equipment manufacturer ThermoFisher Scientific quantified about 20 terpenes in hops samples using LC-MS/MS. Limits of detection ranged from 2-10 ppb for terpenoids and sesquiterpenes.
Liquid auto-sampling may be required for heavier terpenes that vaporize less readily (e.g., sesquiterpenes or triterpenes). That said, headspace (HS) sampling is more commonly used for volatile terpenes. HS essentially involves applying the minimum heat needed (perhaps even ambient conditions) to lift terpenes from the liquid base sample, and into a gas phase. The vaporized, separated terpenes become the sample. This leaves behind many molecules that would complicate the analysis.
- Shapira A, Berman P, Futoran K, Guberman O, Meiri D. Tandem mass spectrometric quantification of 93 terpenoids in cannabis using static headspace injections. Anal Chem. 2019;91(17):11425-11432. doi:10.1021/acs.analchem.9b02844. [Impact Factor: 6.785; Times Cited: 6 (Semantic Scholar)]