Reagents (Forensic Science)
Reagents are used to test questioned substances at crime scenes and in forensic laboratories. In the presence of the substances with which they are designed to react, reagents produce chemical reactions that usually involve color changes or other changes that can be seen, such as causing a substance to glow under a black light. If the substance with which a reagent is known to react is not present, the chemical reaction does not occur. For example, if a reagent known to react with blood does not react on a stain, that stain is not blood. If that reagent does react on a stain, the stain may be blood, but because other substances could cause the same reaction, the scientist cannot conclude that the stain is blood until further testing is done.
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Chemical Reactions (Forensic Science)
Reagents react chemically with substances usually either through synthesis or through replacement. In synthesis, the reagent and the substance form a new substance. In replacement, the reagent acts on the substance by replacing some of the substance’s elements with its own elements.
Forensic scientists must take care when preparing and using reagents. A reagent must be pure enough to react properly with the substance it is intended to detect, so it must be prepared according to strict procedures. In addition, because reagents may be carcinogenic, caustic, or explosive, scientists must handle them carefully; the use of protective clothing and equipment is recommended.
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Uses in Biological and Chemical Labs (Forensic Science)
Many different reagents can be used to identify blood or other bodily fluids on different surfaces. The choice of reagent depends on the surface on which the bodily fluid is thought to be and whether the reagent is being used at the crime site or in the laboratory. For example, some reagents work well on porous materials but are poor choices for nonporous materials. Other reagents may cause violent chemical reactions and therefore must be used only in laboratory settings.
Some of the reagents that identify blood are amido black, leuco crystal violet, luminol, and Takayama reagent. Amido black reacts with the protein in blood to produce a blue-black color, whereas leuco crystal violet, along with hydrogen peroxide, reacts with the hemoglobin or its derivatives in blood to produce a deep violet color.
Luminol reacts on the oxidizing activity of hemoglobin derivatives in blood to produce a blue-white chemiluminescent reaction (that is, it glows in the dark), so luminol tests must be conducted in darkness. The reaction produced by luminol does not last long (perhaps one to two minutes), and photographs of the tested area need to be taken quickly to preserve the results. Takayama reagent (a mixture of glucose, sodium hydroxide, pyridine, and deionized water) reacts with the hemoglobin in blood when heated to form pink needle- or rhomboid-shaped crystals that can be seen under a microscope.
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Uses with Impression Evidence (Forensic Science)
Powder or chemical reagents can be used to bring out fingerprints where none can be seen. For example, ninhydrin is a reagent that reacts with some of the protein left behind by fingerprints. When ninhydrin is placed on a surface that has fingerprints, a deep blue/purple color highlights the prints. Gentian violet is another substance used to develop fingerprints. These types of reagents must be used with caution, however, because they destroy any DNA (deoxyribonucleic acid) evidence that may have been left behind with fingerprints.
Reagents can also be used with shoe prints to determine the type of soil in which the person wearing the shoe walked. For example, if the soil contains iron, ammonium/potassium thiocyanate will react with the iron to produce a reddish-brown color. Hydroxyquinoline reacts with calcium, magnesium, iron, aluminum, and other metals that may be present in small amounts in shoe or tire impressions to produce fluorescence that can be seen under an ultraviolet light. Iodine reacts with small amounts of waxy or oily substances on footprints or tire tracks and may be used in enhancing wet impressions.
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Further Reading (Forensic Science)
American Chemical Society Committee on Analytical Reagents. Reagent Chemicals: Specifications and Procedures. Washington, D.C.: American Chemical Society, 2005. Discusses the requirements and methods for determining the purity of analytical reagents.
Bell, Suzanne. Forensic Chemistry. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006. Describes the methods that forensic scientists use involving chemistry and chemical reactions.
Cobb, Cathy, Monty L. Fetterolf, and Jack G. Goldsmith. Crime Scene Chemistry for the Armchair Sleuth. Amherst, N.Y.: Prometheus Books, 2007. Presents basic information on chemistry-related forensic techniques, including the use of reagents in identifying body fluids.
Johll, Matthew. Investigating Chemistry: A Forensic Science Perspective. New York: W. H. Freeman, 2007. Textbook designed for nonscience majors includes introductory material on basic chemical concepts, including the use of reagents, in crime solving.
Lundblad, Roger L. Chemical Reagents for Protein Modification. Boca Raton, Fla.: CRC, 2004. Discusses how protein modification relates to the analysis of individual amino acids.
Wang, Zerong. Comprehensive Organic Name Reactions and Reagents. New York: Wiley-Interscience, 2008. Describes the reactions, applications, and related reactions of reagents and provides experimental examples.
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