Trace evidence

The Trace Evidence section of the Forensic Science Laboratory examines microscopic and macroscopic materials that can help establish connections between individuals, objects, and locations. Common evidence types include, but are not limited to hairs, fibers, paint, polymers, glass, gunshot residues, and impressions.

Trace evidence is frequently recovered from scenes involving homicides, sexual assaults, physical assaults, burglaries, and motor vehicle incidents. During a crime, a suspect may come into contact with the victim or the surrounding environment, creating opportunities for the transfer of physical material. A shoe impression left at the scene or fibers transferred from the victim’s clothing are typical examples of these exchanges.

The Trace Evidence section specializes in the analysis of these materials to support the identification, comparison, and association of evidence, helping to link suspects to victims and crime scenes.

Microscopic Trace Evidence
Much of the evidence examined by the Trace Evidence section is microscopic in nature, making advanced microscopy essential. Initial examinations are performed with a stereomicroscope, which allows examiners to detect and sort microscopic materials either directly on an item or within debris collected from it. The stereoscopic examination is depicted above.

Once the materials are sorted, additional analytical techniques are applied. Synthetic fibers are initially identified using a polarized light microscope, while further examinations and side-by-side comparisons are conducted on a comparison microscope. More complex chemical analysis on fibers and paints is conducted using a micro-Fourier Transform Infrared Spectrometer. When fibers or paints are colored, they can be spectrally compared on the Visible Microspectrophotometer.

For evidence requiring higher magnification or elemental analysis, the section can utilize a state-of-the-art digital Scanning Electron Microscope with an Energy Dispersive Spectrometer.

Polymer materials, including paints, fibers, and plastics, can be further analyzed using a Pyrolysis Gas Chromatograph-Mass Spectrometer. With this technique, solid materials are heated, volatilized, separated, and identified based on the molecular fragments produced.

Impression Trace Evidence
In addition to microscopic evidence, the Trace Evidence section also examines physical impression evidence, including tire and footwear impressions. These may be present in materials such as paint, blood, soil, and dust. Casts, lifts, and photographs of these impressions preserve detail that provides valuable information, such as the manufacturer or type of object that produced them.

The section also analyzes fabric impressions. As examples, these impressions may be embedded in the paint of a car involved in a hit-and-run case or transferred from blood-stained clothing at a crime scene. Fabric impressions can be photographed, enhanced, and compared to clothing worn by anyone connected to an incident.

To compare impression evidence, the section utilizes both macro and micro casting techniques for footwear, tire, and patterned impressions. In addition to casting, impressions can be lifted from many surfaces using gel lifts or an electrostatic lifting device. These lifts are then compared to known impressions prepared in the laboratory.

Gunshot Residue on Clothing and Other Items
Trace evidence examiners use microscopy to identify propellant residue particles deposited on clothing and other items. These residues are produced when a gun is discharged and typically consist of burned and partially burned gunpowder, vaporous lead, and other materials. When residue is not visible to the naked eye, it can be developed using wet chemical techniques such as the Modified Griess Test and the Sodium Rhodizonate Test. The presence, distribution, and pattern of this residue provide investigators with valuable information, including an approximate distance between the firearm and the victim at the time of discharge.

Hit-and-run examinations
Trace evidence examiners routinely assist in linking hit-and-run vehicles to crime scenes. This may involve physically matching scene debris, such as broken headlight fragments, to corresponding areas of damage on a suspected vehicle. Microscopic paint chips recovered from a victim's clothing or from the roadway can also be searched against an automobile paint color library, which contains several hundred known specimens.

The section additionally uses the Paint Data Query (PDQ), a database containing the chemical profiles of paint layers used in vehicle manufacturing. PDQ searches can provide investigators with a description of a possible hit-and-run vehicle even when no eyewitnesses exist. When a suspect vehicle is located, its paint can be sampled and compared both chemically and microscopically to the material recovered from the crime scene.

Forensic Garage
Probes can be used to document the path of bullets and locate them if present.When a suspect vehicle is identified, it may be brought to the laboratory’s forensic garage for a detailed examination. Trace evidence examiners assess multiple vehicle components to help reconstruct the events surrounding a collision. The speedometer may be inspected for a needle slap mark, which can indicate the speed of the vehicle at the moment of impact. The brake pedal can be examined for footwear impressions to identify who was driving the vehicle at the time of impact.