Principles of Trace Evidence

In the early 20th century, Dr. Edmond Locard, a forensic science pioneer in France, formulated the theory which states, “Every contact leaves a trace”. This became known as Locard’s exchange principle and is the basis for all forensic science as we know it today.

Linking People, Places and Things

Trace evidence can be used to link people or objects to places, other people or other objects, and often serves as a starting point, or lead, for a particular line of investigation. Trace evidence helps to put together pieces of the investigative puzzle—from which direction did the perpetrator arrive? How close was the victim to the window when the bullet shattered the glass? Were stolen goods transported in a particular vehicle? The answers to these questions can significantly impact the outcome of a trial and these answers may be found via careful examination of tiny bits of evidence.

Important developments in trace evidence came alongside advances in microscopy, chemical analysis, and for evidence comparison purposes, database technology.

As the capabilities, availability and networking of comparison databases from scientists and manufacturers became more robust, samples of items such as paint, glass and even soil could be compared against known standards to provide solid and consistent classifications. For example, the National Automotive Paint File is a Federal Bureau of Investigation (FBI) database containing more than 45,000 samples of automotive paint from manufacturers dating back to the 1930s1. Sherwin-Williams® Automotive Finishes also maintains a large database, Formula Express®, which can be very helpful in identifying year, make and model based on color availability. The National Institute of Justice maintains a list of some available databases.

Trace investigators must stay abreast of advances in manufacturing techniques, materials, coatings and processes. Every item that can be touched or transported has the potential to become trace evidence, therefore, investigators and analysts must consider the potential that a product may have a new or updated version available.

1FBI Laboratory Services, Chemistry

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