The trace evidence section of the Forensic Science Laboratory deals with the analysis of both microscopic and macroscopic traces of physical evidence. These may include but are not limited to hairs, fibers, paint, soil, polymers, glass, and impressions. These types of evidence are frequently found at crime scenes such as homicides, rapes, assaults, burglaries and automobile accidents.
During the commission of a crime a suspect may often come into contact with the victim, the environment of the crime scene, or both. During this interaction, exchanges of physical evidence may occur. For example a suspect may leave behind a shoeprint at a crime scene or may take away fibers shed from an item of clothing worn by the victim. The trace evidence laboratory specializes in the examination of this type of evidence with the goal of linking suspects to victims and/or crime scenes. The Trace Evidence section currently employs three full time analysts for the examination of this type of evidence.
Microscopic Trace Evidence
A majority of evidence examined by the trace evidence section is microscopic in nature. Thus this section relies heavily on the use of various types of microscopes to examine physical evidence. An initial examination to detect this microscopic evidence is conducted using a stereomicroscope. The trace evidence examiner may sort through debris removed from an article of evidence or may examine the article directly.
Once the material is sorted and removed, analysis proceeds using a variety of other microscopic techniques. Synthetic fibers are initially identified using the polarized light microscope. Examinations and comparisons can be conducted using the laboratory comparison microscope. More sophisticated chemical analysis on fibers and paints can be conducted using the micro-Fourier Transform Infrared Spectrometer. If the fibers or paints are colored these can be spectrally compared on the Visible Microspectrophotometer. For evidence beyond the capabilities of a normal microscope, the trace evidence section can utilize a state of the art digital Scanning Electron Microscope.
A Pyrolysis Gas Chromatograph-Mass Spectrometer can be used to identify polymers. Solid materials such as paints, fibers and plastics are heated, broken down and volatilized. Based upon the molecular fragments observed, the material can be identified.
Impression Trace Evidence
In addition to microscopic evidence, the trace evidence section also deals with physical evidence such as tire and shoeprint impressions. These can be found in several different types of materials, such as paint, blood, soil and dust. Information can be derived from casts or lifts of these impressions and determinations can be made as to manufacturer and/or type of object that produced them. Other examples of impressions examined by the section may include those produced by fabrics. These can be found in a wide variety of cases. They may occur imbedded in the paint of a car involved in a hit and run case or transferred from an article of bloody clothing at a crime scene. These can be photographically documented, enhanced and subsequently linked back to clothing worn by the victim.
The impression section utilizes both macro and micro casting techniques for comparisons of shoeprints, tireprints and other impressions. In addition to casting techniques, impressions can be lifted from a variety of surfaces. Techniques include gel lifts or utilization of the laboratory's electrostatic lifting device. These lifts can then be compared to impressions of known footwear 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 on other items. This residue is produced when a gun is discharged, and mainly consists of burned and partially burned gunpowder, vaporous lead and other materials. Non-visible residue can be developed using various wet chemical techniques such as the Modified Griess Test and the Sodium Rhodizinate Test. This information, as well as the type of pattern observed, can be used by investigators to determine the approximate distance between the victim and the shooter.
Hit and run examinations
The trace evidence examiner is routinely called upon to link hit and run vehicles to crime scenes. This can be accomplished through physical matches involving scene debris such as broken headlights and comparison to areas of corresponding damage on a suspected vehicle. Microscopic paint samples left on a victim's clothing or in the roadway can be searched against an automobile paint library containing several hundred known specimens. The lab also makes use of the Paint Data Query (PDQ), which is a database of chemical identities of various layers of paint on vehicles. These types of searches can provide investigators with a description of a possible hit and run vehicle even when no eyewitnesses exist. If a suspect vehicle is identified, a paint sample can then be obtained and compared both chemically and microscopically to the sample found at the crime scene.
If a suspect vehicle is identified it can be brought to the laboratorys forensic garage for a detailed examination. Here the trace evidence examiner may examine the speedometer to determine if a speedometer needle slap mark exists. This can be used to determine the speed of the vehicle at the moment of impact. The filaments in a broken headlight can be examined to determine if they were on during the collision. In addition the brake pedal can be examined for shoe print impressions to determine who was driving the car at the moment of impact.