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Presumptive Tests For Blood


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Presumptive Tests For Blood

  1. 1. Forensic Tests Vide
  2. 2. Presumptive Tests 1. Determines if a sample COULD BE evidence. They are quick, cheap, and easy tests 2. Usually a simple color change test. Can be done in the field or at crime scene 3. Some preliminary tests: – Blood, semen, bodily fluids – Illegal drugs • Examples: – Luminol (blood) – Acid phosphatase (semen) – Cobalt thiocyanate (cocaine)
  3. 3. Confirmatory Tests 1. CONFIRMS that the sample IS that specific evidence. They are expensive, need special equipment, and can take a long time to complete 2. Must be done in the laboratory 3. Tests for: – Blood, semen, urine, bodily fluids – Illegal drugs – DNA – Poisons 1. Example tests: – Takayama Test (human blood) – Micrsocope- Christmas Tree stain (semen) – GC/MS- Gas Chromatography/Mass Spectrometry (cocaine)
  4. 4. Light Source Investigators will first examine the crime scene to look for areas that may contain blood. They may use a high-intensity light or UV lights to help them find traces of blood as well as other bodily fluids that are not visible under normal lighting conditions. How is blood evidence detected at a crime scene? Blood Reagent Tests These presumptive tests are used to detect blood at crime scenes based upon the properties of hemoglobin in the blood. Further tests at the crime lab can determine if it is human blood or not. Examples: • Phenolphthalein is a chemical that is still utilized today and is usually referred to as the Kastle-Meyer test and produces a pink color when it reacts with hemoglobin. We will use! •HemaStix is a strip that has been coated with tetramethylbenzidine (TMB) and will produce a green or blue- green color with the presence of hemoglobin. We will use! Kastle-Meyer Test HemaStix
  5. 5. • Luminol We will use This chemical is used by crime scene investigators to locate traces of blood, even if it has been cleaned or removed. Investigators spray a luminol solution is throughout the area under investigation and look for reactions with the iron present in blood, which causes a blue luminescence. One problem is that other substances also react, such as some metals, rust, paints, cleaning products, and plant materials (such as horseradish). Another problem is that the chemical reaction can destroy other evidence in the crime scene. Luminol Reaction LCV or Leuco Crystal Violet, is one type of chemical process that is used for blood enhancement. Using this test helps to make the blood evidence more visible so it can be photographed and analyzed. • Fluorescein This chemical is also capable of detecting latent or old blood, similar to luminol. It is ideal for fine stains or smears found throughout a crime scene. After the solution has been sprayed onto the substance or area suspected to contain blood, a UV light and goggles are used to detect any illuminated areas, which appear greenish-white if blood is present. It may also react to many of the same things as luminol (copper and bleach). Fluorescein Reaction in UV Light
  6. 6. POSITIVE A blood test is POSITIVE when the test response is POSITIVEand the sample IS BLOOD. 4. POSITIVE AND NEGATIVE TESTS NEGATIVE A blood test is NEGATIVE when the test response is NEGATIVE and the sample IS NOT BLOOD. FALSE NEGATIVE A blood test is NEGATIVE when the test response is NEGATIVE and the sample IS BLOOD. FALSE POSITIVE A blood test is POSITIVE when the test response is POSITIVE and the sample is NOT BLOOD.