Vapor pressure measurements on Δ 9 -tetrahydrocannabinol, cannabidiol, and cannabinol to inform cannabis breathalyzer development

Abstract Δ 9 -tetrahydrocannabinol (THC), the main psychoactive compound in cannabis, and other drug molecules that have large molar masses, are often described as ‘nonvolatile’ and are presumed to be carried in exhaled breath aerosols. Large variabilities in THC concentrations in breath have been measured with devices that only collect aerosols; it is possible that neglecting the vapor phase could be responsible. Partitioning of compounds between vapor and aerosol phases is directly dependent on vapor pressure ( p sat ), which itself is strongly dependent on temperature. We describe p sat measurements for THC, cannabidiol (CBD), and cannabinol (CBN) using a gas-saturation apparatus. The measured values of p sat for 364 K to 424 K are 0.0459 Pa to 7.833 Pa for THC, 0.0826 Pa to 13.44 Pa for CBD, and 0.0199 Pa to 5.678 Pa for CBN. The combined standard ( k = 1, 68% confidence) measurement uncertainty in p sat ranges from 2.9% to 5.3% for CBD and CBN, and from 5.2% to 9.5% for THC. To obtain the p sat at human body and exhaled breath temperatures, we extrapolated the measurements for each cannabinoid with a thermodynamic correlation. Then a vapor-aerosol partitioning model was used to predict mole fractions of each cannabinoid in each phase of exhaled breath. All three cannabinoids were predicted to reside primarily in the vapor phase of exhaled breath. However, relatively small changes in temperature or aerosol concentration can significantly impact the predicted partitioning. This work illustrates the utility of low-uncertainty p sat measurements for any drug, including those thought to be too low in volatility for vapor-phase sampling, and may extend the market for forensic drug tests and clinical diagnostic tests via breath analysis.