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Trace Evidence

Few people outside law enforcement had heard of trace evidence identification before the recent popularity of crime scene investigation shows like CSI: Miami. Now, everybody’s talking about it. Nowadays, many people can describe an example or two of how trace evidence was used on TV to figure out who committed a crime. But that doesn’t mean they really understand trace evidence identification. Let’s examine this interesting and often overlooked field.

Before we learn about trace evidence identification, we need to know more about trace evidence – what exactly is it? Actually, trace evidence refers to a wide variety of materials left behind at a crime scene in small quantities when a crime is committed. (In fact, in the course of your everyday life, you leave trace amounts of material wherever you go!) These materials have unique physical and chemical properties that can be used to prove that a suspect was at the scene of a crime.

For example, if a robber snags a sweater on a nail at the crime scene, detectives can connect the material left at the scene to the torn sweater in the suspect’s apartment.
Materials that are typically useful for identifying suspects include everything from hair to clothing fibers to makeup. Even paint can be used to tie a suspect to the crime scene.

Trace evidence identification is the process of comparing the physical and chemical properties of samples from a crime scene to those of known origins. This is no easy task – trace evidence identification uses a wide range of analytical methods to identify the wide range of materials involved!

Before trace evidence can be identified, it must be collected. When a crime is committed, crime scene investigators typically use special lights to find trace evidence that is not obvious to the naked eye.

After locating a fiber or other trace material, crime scene investigators photograph it and collect it using techniques such as ‘lifting’ it with special lifting tape or collecting the sample with forceps. They also document the location of the sample and begin a meticulous record of the individuals who handle or transport the evidence as it makes its way to the forensic laboratory, where trace evidence identification occurs. Protection of trace evidence from loss or contamination is essential to ensure that it can be used in a court of law if need be.
Forensic scientists who analyze trace evidence in the lab are typically called trace evidence examiners.

The trace evidence examiner’s job begins when sample arrives at the lab from the scene of the crime. The procedure for identifying the evidence depends on the type of evidence. For example, examiners look at fibers under the microscope to determine the fiber type. It may also be helpful to use chemical analysis techniques to learn more about the fiber. However, trace evidence examiners don’t have the luxury of this option when there is a very small amount of fiber available – chemical approaches destroy the evidence! Ultimately, examiners are comparing the properties of the sample against the properties of known materials to determine if they are from the same source.

The examiner must thoroughly document every step of the process and write a full report about the identification process and its conclusions. If the trace evidence is used in a case that goes to trial, the examiner may be required to testify in court about the work. Trace evidence isn’t usually enough to convict a suspect, but it can be critical in placing the suspect at the scene of the crime!

Trace evidence identification is a continually evolving field that plays an important role in crime solving.

About the author: Emily Nelson earned an M.S. in Electrical Engineering from the Massachusetts Institute of Technology before beginning her career as a science writer.