When a dark substance is found at a crime scene, analysts must determine whether or not it is blood.
If the tests determine there is blood at the scene, the blood analyst can examine the blood spatter. The spatter can determine where the victim was killed, at what angle and with what weapon.
But, the spatter must be determined to be be blood first.
There are several tests that can differentiate between blood and other substances, and each test must be performed carefully as not to destroy any evidence.
- A powerful light is moved across the surfaces to check for traces.
- If blood is suspected, luminol is sprayed across the scene. The luminol reacts to blood by making it luminescent. When the lights are off, blood will show as a faint blue. Luminol works with old blood and diluted stains, but it can destroy the blood’s properties.
- The Kastle-Meyer Color Test uses phenolphthalein and hydrogen peroxide on a piece of filtered paper. The paper turns pink when blood is present.
- Sometimes microcrystalline tests are used- chemicals are added to the blood to make it form crystals with hemoglobin derivatives.
- A precipitin test will distinguish animal blood from human blood.
- After these tests have been performed, a blood analyst can determine the blood type with an ABO test and work on determining the gender of the source.
If chemicals are present, the tests may be vulnerable to corruption.
A blood analyst will also analyze the splatter and stains to determine how the blood projected from the body and potentially what weapon was used. The splatter can be traced back to their converging point by considering the surface where the blood fell, the angle it hit, the shape and the distance it traveled.
The shape of the blood drops reveals a lot about the scene.
Taking several splatter stains, a basic trigonometric functions can determine a 3D re-creation of the scene.
Before any blood is analyzed, photos are taken to prevent potential contamination. Blood that is removed from the scene is protected from heat, moisture and bacterial contamination.