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Trace Evidence Essay, Research Paper

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 their illegal visit. And usually when cars and/or trucks

collide there is a significant chance that paint from either vehicle will transfer to the other. Many Forensic

Scientists will perform tests on these samples even though they may be pinhead in size. These initial tests

include low power magnification (macroscopic) to determine color, paint layer sequence, thickness, and

overall texture. If the questioned and known paint samples appear the same they then perform analytical

chemical examinations including solubility test, and analytical instrumental examinations. These later tests

give further characterization to the paints in order to determine if they could share a common origin.

The next kind of trace evidence is fibers. Fiber examination usually concerns itself with the potential

transfer of clothing fibers between individuals who have come into contact with each other

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