Duquenois–Levine reagent

In the 1960s and 1970s, various studies showed that the D–L test was not specific to cannabis, although some flawed studies claimed to show the opposite. A 1969 UK government scientist reported twenty-five plant substances giving a D–L test result very similar to that of cannabis and warned that the D–L test "should never be relied upon as the only positive evidence." Another 1969 study found false positives from "a variety of vegetable extracts". Still another study, in 1974, showed that 12 of 40 plant oils and extracts studied gave positive D–L test results. In 1975, Dr. Marc Kurzman at the University of Minnesota, in collaboration with fourteen other scientists, published a study in The Journal of Criminal Defense that concluded: "The microscopic and chemical screening tests presently used in marijuana analysis are not specific even in combination for 'marijuana' defined in any way." In the 35 years since that study was published, no one has ever refuted this finding. The study most cited in favour of the specificity of the D–L test is Thornton and Nakamura (1972). The authors themselves reported that the D–L test gave false positives, but declared the D–L test confirmatory when combined with the presence of cystolithic hairs, which marijuana plants possess. However, many plant species have such hairs, and the study only confirmed that 82 of them did not give D–L test false positives. A 2000 study by the US NIST A 2012 Brazilian study tested 40 vegetal drugs with the Duquenois–Levine test and obtained false positive results from Chilean boldo (Peumus boldus Molina), pot marigold (Calendula officinalis L.), leather hat (Echinodorus grandiflorus (Cham. & Schltdl.) Micheli.), cecropia (Cecropia hololeuca Miq.), lemon balm (Melissa officinalis), anise (Pimpinella anisum L.), guaraná (Paulinia cupana Kunth.), jaborandi (Pilocarpus jaborandi Holmes.) and laurel (Laurus nobilis L.). ==Process==

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. Image:Duquenois_levine_step1.jpg|Step 1 – addition of Duquenois reagent to dried petroleum ether extract Image:Duquenois_levine_step2.jpg|Step 2 – addition of hydrochloric acid Image:Duquenois_levine_step3.jpg|Step 3 – addition of chloroform ==References==