Presumptive color tests

Cocaine drug testing kits either manufactured for law enforcement purposes or produced by the underground. These testing kits are simply presumptive color tests. The basic color test used for cocaine is cobalt thiocyanate. The cocaine or any of the other substances from the caine family will form a brilliant blue flaky precipitate in the cobalt thiocyanate. This is an indication that the product is cocaine, procaine, tetracaine, etc. In order to determine whether there is actually any cocaine and not all procaine, stannous chloride is added to the precipitate causing all of the caines except cocaine to dissolve. 

The choice of instrumental technique used in any analysis depends upon the motive behind the analysis and on the nature of the sample. Normal procedure when dealing with bulk samples is to perform presumptive color tests to identify the drug class followed by TLC to identify the specific member of the drug class. When dealing with trace samples, it is often the case that these nonconfirmatory tests are circumvented and confirmatory tests only are employed. The choice of confirmatory test now depends on the motive behind the analysis as indicated below. The choice of technique will also depend upon the nature of the analytes under investigation. 

Out of the many reaction pathways that can lead to dye formation, four are central in presumptive color testing ... 

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. 

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