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Forensic Biology by Richard Li

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

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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

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