Trace Evidence is responsible for the examination of small (trace) particles of evidence left behind during a crime such as hair, fiber, paint, glass, tape, fire debris, and gunshot residue. This kind of evidence is often submitted in response to violent crimes including rape, homicide, robbery, arson, and hit-and-run incidents.
Trace Evidence examinations are based on the Locard Exchange Principle which states that any time two objects come into contact, there is an exchange of information. That exchange of information could be hairs in a sexual assault, paint in a hit and run, or glass in a breaking and entering.
Hair examinations include the microscopic comparison of evidentiary hairs to known origin samples and a microscopic examination of a hair to determine if there is material present for DNA analysis. Hair comparisons are not a form of positive identification.
Fiber and Tape Examinations
Fiber and tape examinations compare evidentiary fibers or tape to known origin sources by microscopy, infrared spectroscopy, microspectrophotometry, and pyrolysis-gas chromatography-mass spectrometry. These comparisons are very powerful in demonstrating associations; however since many fibers and tapes are manmade and produced in bulk, sole source attribution is not possible.
Glass examinations compare the physical properties (color, thickness, and type), elemental composition, and refractive index of evidentiary samples to known origin samples. Elemental composition is determined with x-ray fluorescence. Refractive index is how light bends from one medium into another, in this case air and glass. The Glass Refractive Index Measurement System (GRIM) measures the refractive index of glass fragments using the oil-immersion method, a process that exploits the variation of refractive index with temperature. Sole source attribution is not possible with glass examinations.
Paint is a mixture of pigments and additives that are intended to color and/or protect a surface. Typical paints include automotive and architectural paints. Scientists compare evidentiary samples to known origin samples using microscopy including a scanning electron microscope with an energy dispersive r-ray system, infrared spectroscopy, microspectrophotometry, and pyrolysis-gas chromatography-mass spectrometry. In cases of a hit and run where the potential source is unknown, scientists can examine paint chips and enter its characteristics into the Paint Data Query (PDQ) database to determine a possible make and model of car to provide investigators with a lead
Fire Debris Examinations
Fire debris examinations extract volatile compounds from fire debris collected from potential arsons. These extracts are examined using gas chromatography and mass spectrometry to identify ignitable liquids such as gasoline, charcoal starters, paint thinners, and diesel fuel.
Gunshot Residue Examinations
Gunshot residue examinations look at samples that have been collected for the presence of primer residue. Primer residue typically contains the elements barium, antimony, and lead. Inductively coupled plasma-mass spectrometry and scanning electron microscopy are used to identify characteristic gunshot residue particles. The presence of gunshot residue particles could have originated from the discharge of a firearm, the handling of a discharged firearm, being in close proximity to a firearm when it was discharged, or from some other source which produces similar particles.
For more information, please contact the Crime Lab.