As states across the country legalize marijuana use, either for medicinal or recreational purposes, a predictable outcome is that over time more drivers will get behind the wheel under its influence. However, as compared to drivers pulled over for suspicion of drinking and drunk driving, determining impairment from smoking is significantly harder to do.
That's because testing for the active ingredient in cannabis is much more difficult, mainly due to the fact the molecule in that ingredient is much larger and more complex than its counterpart found in ethyl alcohol. That difference has proven to be an obstacle for would-be manufacturers of roadside "breathalyzer-type" devices, since their accuracy and reliability are essential in order to market them to law enforcement officials.
But researchers at the National Institute of Standards and Technology (NIST), a part of the U.S. Department of Commerce, have figured out a way to accurately measure "a fundamental physical property" in pot's primary active ingredient, delta-9 tetrahydrocannabinol, or THC.
This scientific achievement, never achieved before say the researchers in a statement, lays the "technical groundwork for manufacturers to develop accurate devices."
Scientists say they have perfected the measurement of "vapor pressure," which according to Tara Lovestead, a NIST chemical engineer and the lead author of the study, "describes how a compound behaves when it transitions from a liquid to a gas. That's what happens in your lungs when a molecule leaves the blood to be exhaled in your breath. So if you want to accurately measure blood levels based on breath, you need to know the vapor pressure."
The study, titled "Determination of Cannabinoid Vapor Pressures to Aid in Vapor Phase Detection of Intoxication," was recently published in the journal Forensic Chemistry.
Since ethyl alcohol molecules are small, they are released into the air easily, which is why they're usually – there are some exceptions – easy to smell and detect when a bottle is opened, or when they're on someone's breath. THC molecules, on the other hand, are heavier and they stick together, resulting in a low vapor pressure. Here's an image of the two molecules, compared side by side.
The NIST researchers developed a new technique, called PLOT-cryo, described in more detail here, "for capturing and analyzing things in the vapor phase."
There are at least two manufacturers actively competing to roll out an effective, accurate and reliable roadside device for police to use. They are Cannabix Technologies, a Canadian company, and the Oakland-based Hound Labs, which has been testing a prototype with volunteer drivers since late last year.
Up until now, some law enforcement agencies have been using saliva testing, since it can more accurately determine recent pot usage. But it hasn't proven reliable enough to use to charge impaired motorists, casting doubt on the roadside process and opening the doors for eventual legal challenges in court. But this latest vapor pressure technique, if incorporated by device manufacturers, has the promise to more accurately identify recent marijuana usage, and impairment.