Although several companies are working to develop marijuana breathalyzers, testing a person’s breath for marijuana-derived compounds is far more complicated than testing for alcohol.
But scientists at the National Institute of Standards and Technology (NIST) have taken an important step toward that goal by measuring a fundamental physical property of the main psychoactive compound in marijuana, delta-9 tetrahydrocannabinol (THC). Specifically, they measured the vapor pressure of this compound—a measurement that, due to the compound’s chemical structure, is very difficult and has not been accomplished before.
One reason it is difficult to design a reliable marijuana breathalyzer is that delta-9 tetrahydrocannabinol (THC), the primary psychoactive compound in marijuana, is a large molecule with a complex structure. Ethyl alcohol is more easily measured with a breathalyzer. In these images, carbon atoms are dark gray, hydrogen atoms are light gray, and oxygen atoms are red. 3D model based on computer rendering, not experimental data.
Credit: Kelly Irvine/NIST
• PLOT-cryo was used to measure the vapor pressures of CBD and Δ9-THC.
• PLOT-cryo stabilizes labile solutes because collection is done at low temperature.
• Vapor pressures were much lower than for compounds with similar molecular weights.
The quest for a reliable means to detect cannabis intoxication with a breathalyzer is ongoing. To design such a device, it is important to understand the fundamental thermodynamics of the compounds of interest. The vapor pressures of two important cannabinoids, cannabidiol (CBD) and Δ9-tetrahydrocannabinol (Δ9-THC), are presented, as well as the predicted normal boiling temperature (NBT) and the predicted critical constants (these predictions are dependent on the vapor pressure data). The critical constants are typically necessary to develop an equation of state (EOS). EOS-based models can provide estimations of thermophysical properties for compounds to aid in designing processes and devices. An ultra-sensitive, quantitative, trace dynamic headspace analysis sampling called porous layered open tubular-cryoadsorption (PLOT-cryo) was used to measure vapor pressures of these compounds. PLOT-cryo affords short experiment durations compared to more traditional techniques 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 magnitude lower than those measured for n-eicosane, which has a similar molecular mass. Thus, the difference in polarity of these molecules must be impacting the vapor pressure dramatically. The vapor pressure measurements are presented in the form of Clausius-Clapeyron (or van’t Hoff) equation plots. The predicted vapor pressures that would be expected at near ambient conditions (25 °C) are also presented.