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Crime Labs

Forensic Analysis Units

Trace Evidence Unit

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 "standard" and "questioned" to refer to these materials.

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

The term consistent with means that the laboratory has measured the properties of the standard 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, wood, metals, drywall, etc.

If a questioned material is found to be inconsistent with the standard, 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.

Click on a topic below to see how that type of evidence is analyzed.

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 over heated 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 flammable 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 lable 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 a flammable liquid left may remain. Instruments can detect amounts of sample a small as a few nanograms. (For comparison a typical water dropplet weighs about 5 hundredths of a gram [50 milligrams]. A nanogram is one one-billionth of a gram. Therefore the water dropplet 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 can not identify a particular brand or grade gasoline nor brand of charcoal lighter, kersoene, etc.

Vehicle Light Filaments

It is often possible to determine whether a vehicle's light filament(s) was/were energized (i.e., on) at the time of an accident. A cold filament that receives a strong shock will break without stretching. This is referred to as a brittle break. If the outer glass envelop was also broken, the cold metal will not oxidize.

When a hot filament receives the same shock, it will stretch and deform. The spacing of the coils will become irregular (some coil spacing increased, others compressed) and the filament will be bent out of shape. If sufficient force is applied, the filament will also break. This is referred to as a ductile break. As the hot metal is drawn out to a narrow point, its temperature increases and the tungsten may melt, leaving a sphere of metal on one end after the break. In addition, exposure to air will cause extensive oxidation of the tungsten. Small chips of glass from the outer envelope may even become frozen onto the hot metal.

Fibers

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 standard 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 spinnerts.

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 standard 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 can not 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

Paint sample 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 transfered to the pry bar.

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

Initially a small chip (2-5 mm square) of the standard 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.

Then

Glass
Explosives
Household Chemicals
Other Areas of Analysis

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