Color presumptive tests based

In forensic laboratories, chemical spot tests based on color reactions, have been replaced over the years by modem, more accurate instrumental methods. However, analytical techniques based on color formation are stiU commonly employed in field tests for explosives. Being inexpensive, simple, easy-to-operate and often quite sensitive (see above), they are most suitable for use outside the laboratory as presumptive field tests for the presence of explosives. 

Analysis starts with qualitative presumptive tests that narrow down the list of potential analytes and directs subsequent analysis. P resumptive tests belong to a family of analytical techniques referred to as wet chemical methods. Most are based on observing results when specific reagents are added to small portions of the samples. Color and crystal tests, as they are commonly called, are used in analyzing drugs, gunshot residues, and explosives. These tests will be discussed in detail in subsequent chapters. 

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. 

An older color-based presumptive test for saliva is based on the presence of the enzyme amylase. This test employs starch gel and iodine in an interesting way. Explain how the test is performed, and discuss its chemical basis and its limitations. How is tills test related to material presented in the chapter ... 

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