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Cannabinoid Vapor Pressures to Aid in Vapor Phase Detection of Intoxication



Tara Lovestead, Thomas J. Bruno


The quest for a reliable means to detect cannabis intoxication with a br eathalyzer is ongoing. In order to design such a device, it is important to understand the basic fundamental thermodynamics of the compounds of interest. Herein, the vapor pressures of two important cannabinoids, cannabidiol (CBD) and Δ9-tetrahydrocannabinol (Δ9-THC), are presented. These compounds are difficult to analyze in the vapor phase because they have high molecular masses (and low vapor pressures) and are reactive in the presence of oxygen (unstable). Accurate and therefore useful vapor pressure data depends on a measurement technique that can collect the data prior to decomposition. An ultra-sensitive, quantitative, trace dynamic headspace analysis technique called porous layered open tubular-cryoadsorption (PLOT-cryo) was used to measure these compounds. PLOT-cryo affords short experiment durations compared to more traditional technique for vapor pressure determination (minutes versus days). Additionally, PLOT-cryo has the inherent ability to stabilize labile solutes because collection is done at reduced temperature. The measured vapor pressures are approximately 2 orders of magnitudes lower than those measured for n-eicosane, which has a similar molecular mass. Thus, these molecules must be investigated further to understand the intermolecular interactions that impacting the vapor pressure so dramatically. The measurements are presented in the form of van't Hoff equation plots. The linear relationship of the mass collected (normalized by the volume of sweep gas used to collect the mass) as a function of inverse collection temperature is an indication of the thermodynamic consistency and predictive capabilities of the methodology employed here. The predicted vapor pressures that would be expected at near ambient conditions (25 C) are also presented, experimentation at this temperature would be nearly impossible.
Forensic Chemistry


cannabinoids, CBD, headspace, PLOT-cryo, THC, vapor pressure


Lovestead, T. and Bruno, T. (2017), Cannabinoid Vapor Pressures to Aid in Vapor Phase Detection of Intoxication, Forensic Chemistry, [online], (Accessed April 15, 2024)
Created June 26, 2017, Updated November 10, 2018