A presumptive test for tetrahydrocannabinol and other cannabinoids that was formerly widely used. The Duquenois–Levine reagent has a solution containing 2% vanillin and 1% ethanal. Concentrated hydrochloric acid is added and then chloroform. A purple colour in the chloroform layer indicates a positive result. This test was invented by Pierre Duquenois (1904–86) in the 1930s and subsequently modified.