If you are a big fan of the CSI or Forensic Files TV shows, you may have seen the TV scientists analyze soil samples that were found at the scene of a crime. These were soil samples that were perhaps found on the victim’s shoes but no where else to be found in the immediate vicinity of the crime scene. They could be soil samples embedded in the tire tread of a car that is suspected to be used in a crime.
Soil is not just plain dirt. It is a conglomerate of several things. They include minerals, plants, animal matter, and tiny particulates of synthetic products like glass, paint, asphalt, cement, and other things. The contents of soil are not uniform wherever you go. They differ from one place to another. Soil found on the beach in Ocean City, Maryland has different components from soil found on the beach in Myrtle Beach, South Carolina.
The first step involved in analyzing soil samples is visually and microscopically inspecting the soil to make determinations of acidity, consistency, color, and mineral content. Next, the forensic scientist looks for any foreign objects such as plant materials like leaves or animal material like hair, teeth, or nails. For instance, finding evidence of beachfront sandy soil on the carpets of a suspect’s car can contradict his statement that he had never been to the beach on the day the crime was committed. Similarly, finding horse dung in a soil sample could suggest that the soil came from a horse farm and not from the suspect’s backyard.
Further forensic chemical analysis can determine whether two samples share the same chemical properties. Using X-ray diffraction, the criminalist can examine and compare the minerals that are present in soil samples. Gas chromatography/Mass spectroscopy (GC/MS) can be used to identify many individual components in a soil sample. Another laboratory technique called differential thermal analysis is useful. The premise behind differential thermal analysis is that soils release and absorb heat at varying rates. In this analysis, a soil sample is heated, and the point at which the soil breaks down, melts, or boils is recorded. These results are then compared with the same thermal properties of other soils to determine whether or not they are consistent enough to be considered a match.
Forensic laboratory science can be a powerful tool when solving a serious crime such as murder.
The next time you watch your favorite CSI or Forensic Files TV show, you will have a better understanding of why soil samples are important in the investigation of a crime.