is it possible to measure the vapor pressure of THC (Delta-9)- according to NIST it is on the way to success despite the complexity of its molecular structure. Unlike ethyl alcohol a very small molecule- it escapes due to its a simple shape. But THC molecules are large and complex, with loops and spurs that cause them to stick together. This results in a very low vapor pressure -- so low that you can't measure it the usual way, which would involve putting THC in a closed container and waiting for the pressure to equalize..which would take forever no exactly ideal for roadside test. However researchers at NIST have overcame that obstacle by using a technology called PLOT-cryo -- short for porous layer open tubular cryogenic adsorption. "PLOT-cryo is an extremely sensitive technique for capturing and analyzing things in the vapor phase." The researchers used pure THC, and first swept an inert gas across the sample to capture escaping molecules, then chilled the gas to collect them (that's where the "cryo" part of the name comes from). By measuring the mass of the recovered molecules in a known volume and temperature of sweep gas, the researchers calculated the vapor pressure. I think now it is up to someone or some company to convert this into reality and make a field device, could be sometime before that happens.

For more information:

Tara M. Lovestead, Thomas J. Bruno. Determination of Cannabinoid Vapor Pressures to Aid in Vapor Phase Detection of Intoxication. Forensic Chemistry, 2017; DOI: 10.1016/j.forc.2017.06.003