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Forensic Biology by Richard Li
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  1. Chapter 6: Identification of Blood Forensic Biologyby Richard Li

  2. Identification of Blood • Preliminary Assays • Positive result indicates the possibility of the presence of blood • Not specific only for blood • Quick, easy, sensitive • Not human specific • Screening tool • Confirmatory Assays • Rarely performed…analyzed for DNA Forensic Biology by Richard Li

  3. Biological Properties of Blood • Normal blood volume is 8% of the body weight • Plasma- fluid portion of blood and can clot • Serum- remaining fluid after clot is removed • Cellular Portion • Red blood cells (erythrocytes) (No DNA) • Hemoglobin- transports oxygen • Heme molecule • White blood cells (Leucocytes) (DNA present) • Platelets (Thrombocytes) (No DNA) Forensic Biology by Richard Li

  4. Presumptive Assays • Detect traces of blood • Oxidation-reduction reaction catalyzed by heme • Oxidation- lose electron • Hydrogen peroxide used as an oxident • Reduction- gain electron • Results is a change of color, chemiluminescence, or fluorescence Forensic Biology by Richard Li

  5. Presumptive Assays • Colorimetric Assays • Phenolphthalein • Leucomalachite green (LMG) • Ortho-Tolidine • Tetramethylbenzidine (TMB) Forensic Biology by Richard Li

  6. Presumptive Assays • Chemiluminescent assays • Light is emitted as a product of the chemical reaction • Luminol- emits light blue color • Useful when blood has been cleaned up • Performed in darkness • Can detect small traces of blood • Can detect patterns • May dilute sample Forensic Biology by Richard Li

  7. Presumptive Assays • Fluorescence assays • Fluorescin • Must be exposed to a particular wavelength of light 425-485 nm from ALS • Emits yellowish-green color Forensic Biology by Richard Li

  8. Presumptive Assays • Factors Affecting Presumptive Assay Results • Oxidizing agents • Cause false positives • Metal salts, bleach, household cleaners • Plant peroxidases • False positives • Horseradish • Reductants • False negative Forensic Biology by Richard Li

  9. Confirmatory Assays • Microcrystal assays • Hemochromagen crystal assay • Hematin crystal assay • Other • Chromatographic methods • Electrophoreic method • Spectrophotometric methods • Immunological methods Forensic Biology by Richard Li