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Color Tests

By: Evelyn Chavez, Christina Khorn , Liz Masterson, and Monica Valadez. Color Tests. Background information. Color tests are used in the determination of blood . The benzidine color test  was the most common for numerous years  

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Color Tests

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  1. By: Evelyn Chavez, Christina Khorn, Liz Masterson, and Monica Valadez Color Tests

  2. Background information • Color tests are used in the determination of blood. • Thebenzidine color test was the most common for numerous years   • It has been withdrawn because it was acknowledged as a known carcinogen. • The chemical phenolphthalein (Kastle-Meyer color test) is regularly used instead.  •  Both tests involve the examination of perioxidase-like activity in the blood hemoglobin. • Peroxidases – enzymes that speed up the oxidation of a number of classes of organic compounds when peroxides are mixed in.

  3. Kastle-Meyer color test • Mix phenolphthalein (a crystal power compound often used for titration) with hydrogen peroxide and bloodstain • If positive, blood’s hemoglobin will cause the formation of a deep, pink color • Presumptive test: not absolute, other substances (potatoes, horseradish) also test positive although these are not substances usually found at crime scenes so this usually means blood is present

  4. HemastixColor Test • Hemastix strips have been used by field investigators to detect the presence of blood found at the crime scene. • Procedure: • Strip dampened with distilled water • Strip is then placed so it touches the bloodstain in question • If a green color appears then it signifies that the questioned bloodstain is indeed blood

  5. Strengths and Weaknesses

  6. The Trial of Susan May • Susan May was the caretaker of her aunt, Hilda Marchbank, who was found beaten to death in her home.  • Susan May was accused and arrested of the murder of Hilda Marchbank on March 30th 1992. • The main evidence used in the case were stains found on a wall at the crime scene.  • JavaidIqbal, a fingerprint expert examined the stains.  • One of the stains had Susan May's fingerprint on it. • The stains were tested with the Kastle-Meyer test and one stain showed a poor result for blood while another showed a negative result.  • Two other tests were used and they also showed poor results, but still gave a presumptive positive for the presence of blood.  • because the Kastle-Meyer test is PRESUMPTIVE, it only eliminates possibilities.  • The KM test was performed after the investigation, so the results can show false information.  • Susan May was released from prison on march 26th 2005 because the evidence was inly presumptive and was not enough to convict her.   • presumption is “an inference of the truth of a fact from other facts proved”

  7. Review Questions • Which color test did the Kastle-Meyer replace? Why? • What is the weakness of the Kastle-Meyer color test? • What color do Hemastix strips turn to if the sample is positive? • What are two strengths of the Hemastix strips? • Why was the evidence used to convict Susan May not reliable? 

  8. Sources • Helmenstine, A. M. Kastle-Meyer Test To Detect Blood. (n.d.). Retrieved April 1, 2013, from http://chemistry.about.com/od/homeexperiments/a/Kastle-Meyer-Test-To-Detect-Blood.htm • Detection of bloodstains at crime scenes: presumptive tests of color and luminance. (2011). Retrieved April 1, 2013, from http://web2.sbg.org.br/congress/CongressosAnteriores/Pdf_resumos/IIICBGF/CBGF034.pdf • Warrington, R. (2002, May 26). Hemastix. Retrieved April 1, 2013, from http://www.csigizmos.com/products/blooddetection/hemastix.html • May, S. H. (n.d.). Retrieved April 2, 2013, from http://www.susanmay.co.uk/student-report2.htm

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