Duquenois-Levine color tests

Seized samples of marijuana are analyzed in the laboratory using a color test, thin-layer chromatography, and a microscopic test. The Duquenois-Levine color test, although not specific for marijuana, is often used. Using the microscope one can see on the upper side of the marijuana leaf characteristic bear claw -shaped cystolithic hairs, which contain calcium carbonate. 

A Figure 4 The full Duquenois-Levine color test. Note the purple compound that has extracted into the lower chloroform layer. 

The presumptive color test for marijuana is the Duquenois-Levine test. A portion of the sample is extracted with petroleum ether and transferred to a test tube. The Duquenois reagent is added, followed by hydrochloric acid. A bluish-purple color is indicative of cannabinoids. Several drops of chloroform are added to the test tube, and it is shaken imtil the purple color drops into the chloroform layer. Depending on the concentration of cannabinoids, this transfer can be complete, leaving the top layer in the test tube nearly clear. Potential false positives occur in the presence of some coffees and other oils, but, as with most color-based presumptive tests, an experienced analyst will often recognize a false positive as being subtly different from a true positive. See the color insert for photos of the Duquenois test. 

In some laboratories, the analysis stops with the Duquenois-Levine test, but many add TLC with standards as the final step, using the dye Fast Blue B (which is carcinogenic), or Fast Blue BB or the Duquenois reagent as a developer. Fast Blue B gives the constituents distinctive colors A -THC turns red, CBN purple, and CBD orange. The combination of microscopy, the Duquenois-Levine test, and TLC with standards is considered conclusive identification for marijuana. Quantitation of THC is currently not required in the United States. Derivatization techniques can be used for GC, but HPLC is better suited to such analyses. 

Name the functional group(s) in THC. What presumptive color or crystal tests could be useful, aside from the Duquenois-Levine test Why are crystal tests of limited use with plant extracts ... 

Cannabis usually comes in three forms (1) cannabis (marijuana), (2) cannabis resin (hashish), and (3) extracts of cannabis resin (hashish oil). Most laboratories use a color test (modified Duquenois-Levine), a morphological examination... 

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