text: The reagent can be prepared by adding 2 grams of vanillin and 2.5 milliliters of acetaldehyde to 100 milliliters of ethanol.
  The test is performed by placing approximately 10 to 20 milligrams of a target substance in a glass test tube, then 10 drops of the Duquenois reagent. After shaking, 10 drops of concentrated hydrochloric acid are added, and the tube is again shaken. Any color that results after the hydrochloric acid step is recorded. Twenty drops of chloroform (or similar solvent) are then added, and the tube is vortexed, then allowed to settle and separate into two layers. Any color that transfers into the organic layer is recorded.
  Marijuana (as well as a variety of other plant substances) becomes purple with the addition of the Duquenois reagent and hydrochloric acid. Upon addition of the organic solvent, the purple color transfers to the organic layer, indicating that cannabinoids may be present.