A complete and precise profile of Cannabis’ volatile fraction is not yet available because of terpene and terpenoid content fluctuation during plant growth.  Moreover, the chosen extraction technique can have a big impact on the outcome of the analysis.  In the last decades, an incredible array of terpenes and terpenoids have been identified in Cannabis, but just a few dozen are currently reported for metabolic profiling of different chemovars.  Because of the important contribution of the volatile fraction to the therapeutic effects of Cannabis, it is important to develop and validate methods capable of giving accurate detection and quantitation of terpenes.
Headspace sampling is fundamental if we want to extract volatile molecules from a heavier sample matrix and inject them into a gas chromatograph for analysis. If this extraction technique is applied, it is possible to get a neat chromatogram of volatile compounds, avoiding the chromatographic signals of molecules that could overlap with the peaks of interest and even interact with the stationary phase affecting the quantitation of the terpene fraction. 
The analytical method proposed by Saphira et al  pairs a static headspace sampler (SHS) with gas chromatography (GC) and tandem mass spectrometry (MS/MS), validating for the first time a novel SHS-GC/MS/MS method to simultaneously quantify 93 terpenoids. The analysis workflow starts by putting a sample of ground plant material into an HS vial and heating to 140°C for 40 minutes; the relatively high temperature for a short amount of time avoids the equilibration between the condensed and vapor phases, which could be different between the complex mixture (due to matrix effects) and calibration standards. This is known as the full evaporation technique. Once the volatile compounds are evaporated, they proceed to the inlet of the GC and are injected to an intermediate-polar column through an autosampler, with helium flow rate 0.9 mL/min.
The GC is paired to a triple-quadrupole MS operating in electron impact mode with mass range 35-250 amu. Comparing retention times and protonated masses of the chromatographic peaks with analytical standards, it is possible to identify and quantify terpenes and terpenoids. The elution order from the GC follows the boiling point of the analytes going from the smaller monoterpenes and monoterpenoids to larger and higher boiling point sesquiterpenes and sesquiterpenoids.
A strong point of this analytical method using SHS is the possibility of analysing plant material without prior sample treatment, avoiding the formation of artifacts and the alteration of terpene ratios. Moreover, SHS is preferable to liquid injection, reducing contamination of GC columns and the MS detector. The semipolar column enhanced chromatographic resolution, and MS/MS detection improved sensitivity and identification.
The researchers validated the method against the 93 terpene standards. Looking at 16 distinct chemovars, they then noted “most inflorescences expressed relatively unique terpenoid profiles for the most pronounced terpenoids.”
References: Namdar D, et al. Variation in the compositions of cannabinoid and terpenoids in Cannabis sativa derived from inflorescence position along the stem and extraction methods. Industrial Crops and Products. 2018;113:376–382. doi:10.1016/j.indcrop.2018.01.060 [Times cited = 19 (Semantic Scholar)] [Journal Impact Factor = 4.244]
 Saphira A, et al. Tandem Mass Spectrometric quantification of 93 terpenoids in Cannabis using static headspace injections. Analytical Chemistry. 2019;91(17):11425-11432.
DOI: 10.1021/acs.analchem.9b02844 [Times cited = 7 (Semantic Scholar)] [Journal Impact Factor = 6.785]
 Kolb B, Ettre LS. Static Headspace-Gas Chromatography: Theory and Practice. 2nd ed. Hoboken, NJ: Wiley; 2006.