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Articles » Crime Scene And
Physical Evidence
Forensic Spot Testing
Matthew Senison
One of the jobs of a Forensic Biologist is to perform spot test analysis of physical evidence. These tests, which are in the most part a type of analytical chemistry, are supposed to be quick, inexpensive, and precise. The physical evidence collected at a crime scene, if not tested on the spot, are sent to the laboratory. If the chain of custody is followed and the evidence is not tampered with, the results can greatly aid an investigation. Firearm discharge residues (nitrates, nitrites, barium, antimony, and lead) are to be tested on suspect clothing in a homicide, or on the web between thumb and forefinger for a suicide. The composition of extremely small amounts of explosive residue is very important for court use. Luckily, the presence or absence of certain groups of chemicals allow exact judgements (aluminum is extremely common in homemade explosives) to be made. Drug testing has become such a huge part of a forensic biologist's job that many centers have been devoted to the task. While most spot tests are quick, they often reveal false- positive and negative results, so extreme care must be taken in testing. They are, however, useful in revealing the class of the drug. A positive Duquenois test will show Tetrahydrocannabinol, or THC, the most active ingredient in Marijuana, and a positive Marquis test will reveal the Opiates. Finally, testing for blood is extremely necessary in the detection of blood where visual inspecting is not conclusive. Blood testing actually detects the heme in the blood, and although false positive results will occur in the case of crushed insects, the distinction between that and actual human blood is easily achieved due to the wide array of tests that have been created.
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heme
- The deep red, nonprotein, ferrous component of hemoglobin, C34H32FeN4O4.
Marquis test - (2 percent formaldehyde in sulfuric acid.) One of the five primary color tests used to test for reagents. The reagent turns purple in the presence of heroin and morphine and most opium derivatives.
Duquenois-Levine Test - (Solution A is a mixture of 2 percent vanillin and 1 percent acetaldehyde in ethyl alcohol; solution B is concentrated hydrochloric acid: solution C is chloroform.) This is a valuable color test for marijuana, preformed by adding solutions A, B, and C, respectively, to the suspect vegetation. A positive result is shown by a purple color in the chloroform layer.
Crime
Scene to Court: The Essentials of Forensic Science Ed. White, Peter.
Cambridge, UK:
Royal Society of Chemistry, Information Services, 1998.
Jungreis,
Ervin Spot test analysis: clinical, environmental, forensic, and geochemical
applications 2nd ed. New York: J Wiley, 1997.
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