Trace Evidence: Proof of Our Everyday Interactions

Trace evidence is the term applied to evidence left behind when an interaction between the perpetrator of a crime and the crime scene. Often this type of evidence is not seen with the naked eye or is not the first thing noticed about the crime scene. Types of trace evidence include:

• Fingerprints on a door frame
• Saliva on the rim of a drinking glass
• Strands of hair on a victims' clothing
• Gun residue on the criminals' hands
• Skin cells on lipstick
• A strand of thread on a broken piece of glass.

Collecting trace evidence requires the investigator to examine the evidence with close scrutiny and often involves the use of a microscope. Depending on the type of evidence the investigator suspects to be present, they may also spray the surface with chemicals or use other extraction processes to reveal the evidence.

One extraction process that you commonly see on popular crime shows is the use of superglue fumes to reveal latent fingerprints. The fumes react to oils that are commonly on our skin and particles from the fumes bond to the oil and form a fingerprint. Another common method of collecting fingerprints is dusting with powder. Once the fingerprint is revealed, it is collected using special adhesive tape that allows the print to be preserved rather than damaged or destroyed during the collection process.

One of the most popular methods of revealing trace evidence that isn't seen by the naked eye is to spray an area with a Chemiluminescent substance, such as Luminol. The Chemiluminescent substance reacts with the hemoglobin in the blood and produces a distinct glow when viewed in the correct lighting conditions. This requires complete darkness in the room and the blood will not glow for long. The hemoglobin in the blood cannot be cleaned off with ordinary household cleaning products.

Trace evidence collection is achieved through the use of special vacuum equipment, cotton swabs and tweezers among other methods. In the lab evidence is examined through the use of special microscope equipment. This equipment reveals finer particles of evidence as well as specifics about the evidence. Through the use of microscopes, thin layer chromatography and infrared spectroscopy procedures examiners can determine the thread count of fabric, the size and color of a grain of powder and reveal clues as to the type of animal a strand of hair came from. There are kits available on the market to let you try your hand at finding trace evidence. A CSI Crime Scene Investigation Kit would be wonderful for your next crime solving party or just to test your skills as an aspiring forensic examiner.