Washington State University staff are working on an equivalent to an alcohol breathalyzer to check whether people are driving while under the influence of cannabis. They say it will help innocent drivers as well as law enforcement officers.

The test would be based on ion-mobility spectrometry, which uses an electric charge to force a material through a “buffer” gas. The different ions of the material are pulled through at differing speeds (because of varying “drift velocities”) and thus can be separated and identified. (Illustrated above, credit Jeff Dahl)

That technique is similar to that already used in some security scanners such as airports where it can identify particles of explosive material almost instantaneously.

In this case the testing would be for the presence of THC (tethrahydrocannabinol) which is the main component of cannabis that gives the effects sought by users.

At the moment law enforcement on the scene must rely on traditional crude sobriety tests (such as asking a suspect to walk in a straight line) or use a blood test, which usually requires taking a suspect to a police station.

Neither is ideal for officers on the scene: Bloomberg cites a 2012 study showing that the sobriety tests only identifies 30 percent of participants who were under the influence of THC. Meanwhile the blood test results take as long as 24 hours to become available.

The WSU test wouldn’t be enough to prove guilt or innocence: it isn’t precise enough to establish a driver had THC levels at or above a legal threshold, such as the 5 nanograms per milliliter of blood required in Washington state.

Instead it would be used merely to establish the presence of THC and justify officers bringing a suspect in for a blood test. Herbert Hill, a WSU chemistry professor, told the Tacoma News Tribune that using the breath test would not only catch some drivers who might have passed crude sobriety tests, but would also reduce false positives from such tests that led to long delays in custody before the blood test cleared a suspect.