Trace EvidenceThe Trace Evidence Unit is one of the most diverse and unique of all disciplines in the Forensic Science Division (FSD) of the Michigan State Police (MSP). An examiner in this unit will see a wide range of evidence and must be proficient in several areas. The following types of evidence are analyzed in Trace Evidence:
- Hairs and Fibers
- Paints (automotive and architectural)
- Headlamp Filaments
- Footwear and Tire Impressions
- Physical Matches
- Arson Debris
- Miscellaneous Unknowns
A Trace Evidence Analyst must possess a bachelor's degree in chemistry or other natural science with a strong background in microscopy, analytical instrumentation, and photography. An incoming analyst has a very extensive training period under a senior examiner. The incoming analyst may also be required to travel to other Michigan State Police Laboratories for training in certain disciplines. Each of the above disciplines must be studied, practiced, and mastered before the analyst begins performing their own casework. A Trace Analyst may use a variety of instrumentation and visualization tools in their analysis of evidence: Stereoscopic Microscopy, Polarized Light Microscopy (PLM), Fluorescence Microscopy, Scanning Electron Microscope/Energy Dispersive X-Ray (SEM/EDX), X-Ray Fluorescence (XRF), Fourier Transform Infrared Spectroscopy (FT-IR), Gas Chromatography / Mass Spectrometry (GCMS), Pyrolysis Gas Chromatography (PGC), Ion Chromatography (IC), and Microspectrophotometry.
Fiber evidence is found in a variety of cases. When a struggle occurs between two people, fibers may be transferred from the suspect's clothes to the victim's clothes and vice versa. Carpet fibers from a home may adhere to a breaking and entering (B&E) suspect's shoes. A pedestrian struck by a vehicle may leave fibers from their clothing on the suspect's vehicle bumper or windshield. These are just a few of the types of crimes that may be solved by fiber analyses.
A questioned (Q) and a known (K) fiber can be compared using PLM. The fiber type (i.e. Nylon, rayon, cotton, polyester, etc.), cross sectional shape, sign of elongation, and refractive index can be determined with PLM. Analytical instrumentation like FT-IR and Microspectrophotometry can be used to further identify and compare the Q and K fiber's chemical composition and color to determine if the two fibers could have originated from the same source.
Paint can be transferred from one vehicle to another in an accident. An automotive paint chip left at the crime scene or on a hit-and-run victim's clothing can be used to determine the make and model of the vehicle it came from. Paint from a B&E that was transferred from a window to a suspect's tool, can place that tool at the crime scene. Paint is examined with microscopy and several analytical instruments to determine its layer sequence, binder type, and pigment content. If the Q and K paints are found to be similar in all these analyses, then they could have originated from the same source.
MSP Laboratories examine powders and exploded/unexploded devices to determine what type of explosive may have been used. After the MSP Bomb Squad renders a device safe, they submit a sample of the explosive (when the device is unexploded) or the debris (after an explosion occurs) to the Trace Unit. These items are then analyzed with chemical spot tests and analytical instrumentation (FTIR, SEM/EDX, and IC) to determine their chemical make-up. This information will then be used to inform the submitting agency what type of explosive was used. These results can then be compared to any evidence found in the suspect's possession.
Fragments of glass can be compared to determine if they originated from the same source. When a pedestrian is struck by a vehicle, fragments of glass from the headlamp area or windshield may transfer onto the victim's hair and clothing. A criminal unlawfully entering a building or vehicle by breaking through glass will likely get fragments of glass on their clothing and on the tool used to break the glass. In both of the above situations, glass particles (questioned glass) found by examining the clothing can be compared to particles collected from the crime scene (known glass) to determine if they have a common origin. Properties of the glass like tint, thickness, UV fluorescence, density, and refractive index, must all be similar for the questioned and known samples to have originated from the same source.
Imagine you are carefully pulling out of your driveway late at night and your car is struck by a vehicle with its lights off. The police arrive and the person who hit you claims you pulled out in front of him. You explain to the police officer that the person who hit you did not have their lights on, so you did not see them. The other party replies that his lights were "definitely on". How is the officer to know who is right and who is wrong? The headlamp(s) from the vehicle in question can be submitted to the Trace Evidence Unit for examination. The filaments in the light bulbs are examined for oxidation, hot stretch, cold breaks, rainbowing, and fused glass particles. The analyst can then make the determination whether the headlamp(s) was on or off when the collision occurred.
Footware and Tire Impressions
Footwear impressions and tire impressions are examined in the same fashion. Impressions can be three-dimensional when left in snow or soft soil, or they can be two-dimensional when a dirty, bloody, or other impression is left on a hard surface. Footwear impressions can be as individual as a fingerprint. There may be thousands of other people that have the exact same shoe as you, yet your shoe impression may be unique. As you wear your shoe, the tread wears down uniquely to your walking style. Also, accidental scratches, nicks, and cuts are left on the bottom of your shoe, which may be unique. Tires undergo the exact same changes making them unique as well.
Questioned impressions from crime scenes can be photographed, lifted, or cast with dental stone to compare to suspect shoes or tires. A comparison of tread designs or wear patterns can lead to a positive identification.
What is a physical match? It is a comparison between two cut, broken, or torn objects to determine if they were at one time part of the same object. For example, if a car strikes a pedestrian and part of the grill breaks off and is left at the scene, it can be collected as evidence. Once the suspect car is found, the questioned piece of grill from the scene can be compared to the car's grill to see if they were at one time one piece. When an object breaks, tape is torn, or something is cut, two unique edges can be formed. These edges can be compared by the naked eye, and under high magnification to see if they fit together like puzzle pieces. If the edges fit together like a lock and key, they are said to physically match one another. It can then be said that the two objects were at one time a single object.
When a suspicious fire occurs, an investigator from the Michigan State Police or local agency investigates the scene. If they suspect an accelerant may have been used, the investigator can collect areas where they believe the accelerant was used. These samples are placed in nylon bags or sterile metal cans, sealed air tight, and submitted to the Trace Evidence Unit for examination.
The samples are then documented and prepared for extraction. The samples are heated, causing any accelerant to go from the liquid phase to the gaseous phase and be adsorbed onto a strip of activated charcoal. The charcoal will absorb any accelerant molecules that may be present. The charcoal is then used for analysis. A solvent is used to extract the accelerant from the charcoal. The resulting solution is injected into a GC/MS to identify the type of accelerant used.
Sometimes the Trace Evidence Unit is referred to as the "Catch-all" Unit. There are many different types of evidence or unknowns found at crime scenes which are submitted to the Trace Unit to make a determination as to what it is or to make a comparison. Acid/foil bottle bombs are submitted to determine what type of chemicals were used. Bank dyes from a suspect's clothing can be compared to bank dye from exploding dye packs to determine if they are similar. Lubricants and cosmetics (lipstick, mascara, nail polish, etc.) can be examined and compared. When a piece of evidence is brought to the laboratory and no other units are able to analyze it, the Trace Evidence Unit will often receive the evidence and attempt to identify it through the use of numerous microscopic and chemical analyses.