Trace Evidence

Trace evidence is very important in forensic investigations. This category of evidence encompasses many diverse types of microscopic materials as well as some examples that are easily visible to the naked eye. The subject is broad and diverse because of the number of different types of evidence that are commonly encountered. Trace evidence can be thought of as evidence occuring in sizes so small that it can be transferred or exchanged between two surfaces without being noticed. Varieties of trace evidence can include, but are not limited to: metal filings, glass fragments, feathers, food stains, building materials, lubricants, fingernail scrapings, pollens and spores, cosmetics, plastic fragments, gunshot residue, chemicals, paper fibers and sawdust, human and animal hairs, plant and vegetable fibers, blood and other body fluids, asphalt or tar, vegetable fats and oils, dusts and other airborne particles, insulation, textile fibers, soot, soils and mineral grains, and e!xplosive residues.

Forensic scientists routinely come into contact with a relatively few number of these. They are: hair, glass, paint, fibers, fingerprints, and flamable liquids. These will be covered more in-depth in this paper.

Edmond Locard, a French scientist and one of the early pioneers in forensic science believed strongly that individuals could not enter an area without taking dust particles with them from the scene. This became known as what is now called "Locard's Exchange Principle." This principle states that when two objects come into contact with each other, each of the objects will leave particles of one on the other. It is this principle that is the foundation of the forensic study of trace evidence.

Trace evidence examination is the examination and analysis of small particles in order to help establish a link between a suspect and a crime scene or a suspect and the victim of a crime. These small particles usually include such items as hair, paint, glass, and fibers. Although not considered "trace" items by definition the many Crime Labs also examine and analyze such important evidence as flammables (in arson investigations), fingerprints, footwear (shoeprints), and "fracture matches." Many also perform examinations of automobile headlamps, taillights and speedometers.

The first category of trace evidence I will discuss is hair. Hair is examined grossly (with the naked eye), and with both low power and high power microscopes to determine if questioned hairs, found at the scene or on the clothing of an individual are consistent in characteristics to known hair collected from the suspect and/or victim. Some of these characteristics include more obvious traits such as color, length, and morphological shape and also microscopic aspects of the cuticle, cortex and medulla, which are the three basic components of a hair. A hair cannot be linked specifically to an individual through these methods but vital information developed as to who the suspect may be and significant elimination of other suspects can often be done. It is possible to tell the race, sex, and region of the body that a hair comes from. A relative idea as to the time since the last haircut can also be made.

The second type of trace evidence is glass. When larger samples are available glass can be useful in linking a suspect with the crime scene through "fracture matches". This is when a larger piece of glass, found associated with the suspect, can be physically fitted with one or more pieces from the crime scene. More often when an individual gains access to a business or dwelling by breaking glass the perpetrator will acquire very tiny pieces of glass on his/her clothing. These cannot be physically matched due to their tiny size. However, these pieces, though smaller than a pinhead, can be characterized under the microscope. After proper gross and low power microscopic examinations are performed the Forensic Scientists use microscopic "refractive index" determination to further characterize the samples. Refractive index is a measurement of how light is "refracted" (bent) as it passes through the microscopic glass sample. Glasses having different formulations and used for differe!nt purposes have different RI's. Therefore samples can be compared to determine if the glass from the crime scene could be the source of the glass removed from the suspect's clothes.

The third type of trace evidence is paint. When perpetrators break into businesses or dwellings they have the potential of acquiring small paint samples during