1. Forensic Biology by Richard Li
Chapter 6: Identification of Blood
Forensic Biology by 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 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 Other
Hemochromagen crystal assay
Hematin crystal assay
Other
Chromatographic methods
Electrophoreic method
Spectrophotometric methods
Immunological methods
Forensic Biology by Richard Li