Blood Stains found at the crime Scene MUST be preserved.
Ways to test blood. 1. To determine the type of unknown blood, test unknown blood with known antiserum, antibodies or with known cells. This technique is useful on liquid blood but not dried blood. Luminol Luminol will cause luminescence( production of light) when added to blood.This is a good option because it can test a large area at once but it needs to be in a darkened area to check for glow or luminescence. Luminol is extemely sesitive. Luminol can detect bloodstains that have been diluted up to 10000 times. Luminol is ONLY used to find blood which is invisible to the naked eye since it DESTROYS IMPORTANT BLOOD FACTORS needed for typing blood. Whose blood is it?
Absorption - elution technique (4 steps) Used for typing dried bloodstains Protocol:
Unreacted antiserum is removed by washing off bloodstain Break antigen-antibody complex apart by heating treated bloodstain to 56o C to denaturize (break bonds) called elution Take eluted antibodies and combine with known RBCs Check for presence or absence of agglutination to determine bloodtype
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Surface texture helpful in interpreting bloodstain patterns Harder, less porous surfaces will cause less splatter When drop of blood hits hard, smooth surface it frequently breaks up on impact casting off smaller droplets These travel in same direction as original drop but their pointed ends always point backwards toward their origin Can determine the impact angle of blood on flat surface Measure degree of circular distortion of stain If blood strikes at right angle, a nearly circular shape is produced As angle decreases, stain becomes elongated in shape |
Preservation and collection of Blood Evidence Preservation of Blood evidence
No Unnecessary exposure of blood to heat, moisture, bacterial contamination This exposure would shorten survival time of antigens and enzymes Collection Record location of bloodstain relative to entire crime scene through notes, sketches and photographs Photograph bloodstains close-up Collect all clothing, from both victim and suspect and send it to lab for examination - even if no apparent bloodstains Collect wet blood by using medicine dropper and then placing blood in glass tube Keep refrigerated If not enough liquid blood is available, let bloodstain dry thoroughly (out of heat and sun) Do not package stain in plastic or airtight container May lead to growth of fungi/bacteria Package each stain separately, in paper bag or well ventilated box Package entire stain, if possible (ex. Cut out section of rug which is stained) May be able to scrape dried blood off surface, onto a clean sheet of paper Use disposable scalpel blade Absorb dried blood onto a clean swab or filter paper dampened with distilled water Compare bloodstains with whole blood (minimum 5cc) from both victim & suspect(s) Blood transfusions can change a person’s blood chemistry Can obscure true blood type Must wait at least 60 days after transfusion to obtain a valid blood sample
Forensic Characterization of blood stains. When examining dried blood the investigator needs to be able to answer the following: Is it Blood? If so from which species? If it is human blood, how closely can it be associated with a particular individual? How to find the answers Is it blood? Use a preliminary color test. Historically the benzidine color test was used, but not now. Benzidine is a known carcinogen. A Kastle - Meyer color test is also available. Its really simple chemistry. Mix phenolphthalein with hydrogen peroxide and bloodstain. If the mixture turns a deep pink color then the mixture has tested positive for the hemoglobin found in blood. This is a presumptive test. the results are not absolute, other substances also test positive such as horseradishand potatoes but these are Not commonly found at crime scenes. Or The Luminol test is also a viable option. Also there is the hemastick it will change color from yellow to green in the presence of blood.
Species? Precipatin test. This test is very sensitive and only needs a very small amount of blood. This test has been used for 10-15 years on dried bloodstains. Even mummies have given positive results to this test. Procedure:(DO NOT TRY THIS!!!!!!!!!!!!!!!!!!!!!) 1.inject an animal (usually a rabbit) with human blood. 2.human antibodies will form. 3.bleed the animal to remove antibodies. Have formed human antiserum. Layer some of the bloodstain on top of the human antiserum (in a capillary tube). If the bloodstain contains human blood it will interact with human antiserum to form a cloudy ring or band where the two substances meet. |
