Benzidine (Forensic Science)
For most of the twentieth century, benzidine was the standard chemical used in presumptive testing for blood at crime scenes. In the presence of heme iron and hydrogen peroxide, benzidine, which is clear in the reduced state, is converted to the oxidized state, which is deep blue. Because heme iron is present in hemoglobin, the protein that carries oxygen in the blood, a positive test can indicate the presence of blood. This test does not distinguish between human blood and animal blood, however; further testing is necessary to make that distinction and, if the blood is human, to determine whose blood it is. In addition, constituents of some plants, such as potatoes and horseradish, as well as oxidizing agents found in some cleansers, can catalyze the reaction. Accordingly, a benzidine test is only presumptive of blood; a positive result must be confirmed by laboratory test.
Developed in 1904, the benzidine test became the most popular presumptive test for blood because of its high sensitivity, specificity, and reliability. Benzidine, however, which was also used for the synthesis of dyes in the textile industry, proved to be highly carcinogenic, and its use and manufacture in the United States was banned by the Environmental Protection Agency in 1974. At that time, 3,3′,5,5′ tetramethylbenzidine (TMB) was developed as a presumptive test for blood. It is not as sensitive as benzidine, but it is much safer to use, although it is a...
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Further Reading (Forensic Science)
Lee, Henry C., Timothy Palmbach, and Marilyn T. Miller. Henry Lee’s Crime Scene Handbook. San Diego, Calif.: Academic Press, 2001.
Nickell, Joe, and John F. Fischer. Crime Science: Methods of Forensic Detection. Lexington: University Press of Kentucky, 1999.
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