Forensic soil analysis has grown into a major field of forensic study that includes analysts from many scientific backgrounds, including biologists, toxicologists, entomologists, geologists and botanists. Soil has particular forensic importance at a crime scene and can produce valuable information, such as the presence of vehicles, and type of footwear and clothing.
Samples are collected depending on the nature of the crime scene and the sample that needs to be collected. Indoor samples are usually collected through vacuuming. This method can also be used to collect soil samples from a vehicle. Soils from outdoors might be used to track a suspect's movements. Soils in most gardens are specific to that location because of the gardening techniques and types of plants used. Soil samples at a grave site can be important because they can be linked to soil in the trunk of a car that might have carried tools used to dig the grave. In this case, the tool will be wrapped in plastic and sent to the lab along with the other soil samples. Most tests only require one teaspoonful of soil, which is then placed in a plastic vial. If you are uncovering a body however, soil samples should be taken at regular intervals and a new spoon should be used to collect each sample so as not to contaminate the evidence.
Analysis of Samples
In the laboratory, all items from the suspect and the victim should be handled separately--if possible in different rooms--to avoid contamination. Different people should conduct each examination. To analyze the soil, the first operation is testing the mineral content via microscopic examination. There are other standard tests to help identify the sample's origin such as the density gradient tube. Two differing rations of liquid are added to two glass tubes (each representing a different density). The soil is added to both. When the particles become suspended in the liquid, the bands of separation can be analyzed to reveal the soil's profile. Heat tests can also be used to test soil reaction and electron microscopes can be used to examine the structure of minerals within the soil.
Some soil samples contain evidence that can link the perpetrator to the victim, such as blood, saliva and semen. In these cases, the whole clump of soil should be sent to the laboratory for testing. In the case of grave sites, the soil should be kept moist and any insect samples should be preserved in alcohol and sent to the lab along with the soil samples because these can help to determine how long the body has been discomposing and thus help establish the time of death.
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