Physical Evidence Handbook

Trace Evidence Unit

Fire Scene and Trace Evidence The Trace Evidence Unit analyzes a broad spectrum of physical evidence including paint, glass, fibers, fire debris, explosives, plastics, building materials, tapes, ropes and cordage.

Everyday materials become physical evidence when crimes such as hit-and-run, burglary, arson, robbery, homicide, sexual assault and criminal damage to property are committed. During the commission of a crime, minute amounts of material may be transferred from one surface to another. By linking the transferred substance back to its source, a link between the suspect and scene can be established. Almost any object, then, may become evidence at some time and thus require laboratory examination.

Most examinations performed by the Trace Evidence Unit are comparative in nature: The physical and chemical properties of a sample whose origin is known are compared to the same properties of a sample whose origin is unknown. In our laboratory, we use the terms "known" and "questioned" to refer to these materials.

As an example, consider paint transfers in a typical hit-and-run.

During a two vehicle hit-and-run accident, a red suspect vehicle strikes a blue parked car and leaves the scene. Some of the red paint may flake off when the fender of the suspect vehicle is damaged. Blue paint from the parked vehicle may also be dislodged and transfer to the red vehicle. The investigating officer will find a damaged blue car with smears and/or chips of a red paint. These are labeled as "known blue paint" and "questioned red paint from a blue vehicle." Once the suspect vehicle is located, the investigator will recover "known red paint" and "questioned blue paint from a red vehicle."

The lab will conduct comparative analyses of a number of physical and chemical properties that may establish a link between the two red and two blue paints. If the results match, the questioned paints are said to be consistent with the respective known paints.

The term consistent with means that the laboratory has measured the properties of the known and questioned paint chips (see Paint, below) and has found no differences. However, we know that these properties relate to the original batch of paint (thousands of gallons) and are not unique to a single vehicle. Thus the mission of the Trace Evidence Unit is to measure a sufficient number of critical properties such that samples from different origins would be discriminated. Similar results are obtained with other batch made materials such as glass, fibers, tapes, cordage, plastics, metals, drywall, etc.

If a questioned material is found to be inconsistent with the known, then it could not have come from that source. Exclusions are definite; inclusions are general.

An exception to consistency occurs when torn or fractured items can be "jig sawed" back together in a physical match. A physical match is a unique property and conclusively links the two broken/torn items.

Fire Debris
A fire investigation begins when a fire is first discovered and continues even as the fire is being suppressed. Statements of the early witnesses as to what was on fire and of the fire fighters as to how the fire spread assist the investigator in locating the fire's origin. Once the origin has been located, the energy source must be identified. This could be an overheated motor, an overloaded circuit, a cigarette, a flammable liquid or of a chemical composition. The investigator must know both the origin and the cause of a fire before determining if it was an accidental fire or an arson fire. The laboratory is often asked to determine the presence or absence of accelerants or incendiary materials to assist the investigator in making this decision.

In many fires that are purposely started, a ignitable liquid known as an accelerant is used. These materials are aptly named because they promote the rapid combustion of materials and aide in the spread of the fire. Almost anything you can purchase in a hardware store that says "Flammable" in the label can be used to start a fire. Other fires may be started with incendiary devices containing chemicals that give off sufficient heat and energy to ignite the base material.

The investigator must collect the proper sample and seal it in an air-tight container to prevent evaporation. The sample is brought to the laboratory for analysis. Even after being in a fire, traces of an ignitable liquid left may remain. Instruments can detect amounts of sample a small as a few nanograms. (For comparison a typical water droplet weighs about 5 hundredths of a gram [50 milligrams]. A nanogram is one one-billionth of a gram. Therefore the water droplet contains 50 million one nanogram samples. Results may vary. Do not try this at home.)

The major technique for detecting flammable residues is headspace gas chromatography/mass spectrometry. Heating the sample in an oven causes vapors to be liberated. After a several step process the material is injected onto the GC/MS. The resulting chromatogram yields a profile of the volatile components. Each component may be identified from its mass spectrum. In addition, families of compounds found in accelerants can be selectively plotted to assist the analyst in making a determination.

Petroleum-based accelerants can be grouped into several classes. Each class contains a number of commercial products and is defined by a specific molecular weight range. The laboratory cannot identify a particular brand of charcoal lighter, kerosene, etc. or brand/grade of gasoline.

Fibers come in many diameters, lengths, colors and compositions. Fibers may be animal, vegetable, mineral or synthetic in origin. All of these properties are useful when comparing a known fiber to a questioned fiber.

Microscopy plays an important role in comparing fibers. Stereomicroscopy is used to observe the gross characteristics of fibers and the solubility of small segments (1mm) of fiber. Several classes of synthetic fibers may be differentiated based upon their solubility in a series of organic solvents.

Optical Microscopy is used to look at surface details and cross sections of fibers. It is very useful for identifying animal hairs and vegetable fibers and for sorting out fibers of the same type that were spun through different shaped spinnerets.

Polarized Light Microscopy is used to examine many types of fibers. Most fibers, especially synthetic ones, exhibit birefringence. Light whose direction of polarization is parallel to the fiber axis travels through the fiber differently than does light polarized perpendicular to the fiber axis. Thus the apparent index of refraction of light depends on the orientation of the fiber. These values can be measured and compared to known values for different families of fibers (nylons, acrylics, polyesters etc.). A fiber's birefringence can also be seen directly when it is viewed through crossed polars. The difference in the indices of refraction yields a color that is dependent on the fiber's thickness.

A known and a questioned fiber can be compared side-by-side using a comparison polarizing microscope. Fibers that have different properties can be excluded as having a common origin very rapidly. Fibers with similar properties need to undergo further analyses before they can be considered as consistent.

Other techniques used for fiber analysis include Fourier Transform Infrared Spectrometry (FTIR), a specialized type of infrared spectrometry. This technique can readily differentiate the various classes of fiber and can separate members of the same class.

Physical Matches
Physical matches occur when a solid material is fractured or torn. These actions are random in nature and cannot be reproduced. Even when perforated paper or plastic bags are torn, the area between the cuts must tear. Even torn fabric can be reconstructed.

Comparison of physical matches is more than just seeing the fit of the irregular edges. The analyst must check that any manufacturer markings, design characteristics, random scratches or surface coatings (paints, labels) that start on one side continue smoothly across the fracture. For glass, plastic or any thick material, the analyst must examine the vertical edge as well as the top and bottom surfaces.

Paint samples are most commonly submitted to the laboratory in crimes of hit-and-run and burglary. In the commission of these crimes, one or more painted surfaces may be damaged. Paint from the damaged surface may be transferred to another surface. The example of a hit-and-run was given above. Another example is the use of a pry bar used to break into a residence. Most pry bars (and other tools) are painted to retard rusting and to identify a brand name. When used to force open a storm window of a home, some of this paint will be left as smears on the painted wood or aluminum framing of the window. Paint from the framing may also be transferred to the pry bar.

Paint consists of a plastic film containing solids that give it body and color. The analysis of paint relies on examining the organic film and the inorganic solids (pigments, fillers, extenders). Vehicular and building paints may have multiple layers. If possible each layer is analyzed separately.

Initially a small chip (2-5 mm square) of the known paint is examined side-by-side with a chip of the questioned paint. The number of layers and the color and texture of each layer is noted. There are no instruments that can differentiate shades of color as well as the human eye.

Glass samples, like paint, are most commonly submitted to the laboratory in crimes of hit-and-run and burglary. In the commission of these crimes, window/ windshield glass may be damaged.

Glass is a super-cooled liquid that at room temperatures has the characteristics of an amorphous solid. There are different types of glass such as float, tempered float, or bottle glass to name a few.

Glass from a broken window may be transferred to a burglar's clothing or tools. Windshield or vehicle side window glass may be transferred to a victim's clothing or person. Known glass recovered from the window frame can be compared to the questioned glass using color, thickness, refractive index, density and elemental composition.

When the known and questioned glass are compared in all these areas and found to be consistent we can say the questioned glass is consistent with the known glass and could share a common origin.

NOTE: Intact devices must be rendered safe by police officers or military personnel who are trained to take part in this dangerous undertaking. The Crime Laboratory will not accept unexploded devices that have not been rendered safe. Because the laboratory is not certified to ship explosive materials, it will be necessary to hand carry suspected low explosives to the Milwaukee lab as well as pick up evidence when analysis has been completed. This does not include fragments from explosive devices that have already exploded.

There are five classes of low explosive mixtures that may be encountered in forensic cases involving explosions or bomb making. Black powder, black powder substitutes, flash powder, smokeless powder and other low explosive mixtures. Another type of explosive commonly encountered is an acid/foil type bomb where common household chemicals can be combined with aluminum foil to create a chemical reaction pressure type bomb.

Through spot tests, extractions, microscopic examinations and instrumental analyses the trace unit can usually identify what type of explosive material was used in the manufacture of a suspected improvised explosive device.

Other Areas of Analysis
Other types of evidence encountered in the trace unit include duct tape, bank security dye packs, plastics, building materials, ropes and cordage.