FORENSIC SCIENCE Trace Evidence. Introduction. Trace Evidence--any small pieces of material, man-made or naturally occurring Most common examples: Hair Fiber. Test Questions for Trace Evidence. What is it? Is it man-made or natural? What is its source? How common is it?
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Trace Evidence--any small pieces of material, man-made or naturally occurring
Most common examples:
Human hair is one of the most frequently found pieces of evidence at the scene of a violent crime. It can provide a link between the criminal and the act.
From hair one can determine:
The hair shaft contains abundant mitochondrial DNA--inherited only from our mothers. It can be typed by comparing relatives if no DNA from the body is available.
Only the root contains nuclear DNA.
Parts of the hair
The cuticle is the outermost layer of hair which is covered with scales. Scales also always point toward the tip of the hair. These scales differ between species of animals and are names based on their appearance. Some of these scales are variations of the same and include:
In order to visualize the scales
What pattern is seen in this slide?
The cortex gives the hair its shape.
It has two major characteristics:
The medulla is the hair core that is not always present. The medulla comes in different types and patterns.
Amorphous (without a distinct pattern)
Human medulla may be continuous, fragmented or absent.
Rabbit medulla is different depending on the type (location on the rabbit) of hair. The one to the left is multiserial. The one to the right is a uniserial ladder and is found in guard hair.
Medullary Index for human hair is generally less than 1/3.
For animal hair, it is usually greater than 1/2.
Collections must be taken from different locations on the body to get an accurate timeline.
NOTE: Fabric is the type of material and fibers are the “threads” that make up the fabric
The use of fiber evidence in court cases is used many times to connect the suspect to the victim or to the crime scene. In the case of Wayne Williams, fibers were the entire case. Williams was convicted in 1982 based on carpet fibers that were found in his home, car and on several murder victims. Although this case is unusual, fibers are generally considered of greater value as evidence than that of rootless hairs since they may contain a greater number of variables, thus showing more individual characteristics.
Synthetic fibers are made of polymers which are long string of repeating chemical units.
The word polymer means many (poly) units (mer).
The repeating units of a polymer are called monomers.
By varying the chemical structure of the monomers or by varying the way they are weaved together, polymers are created that have different properties.
As a result of these differences, forensically they can be distinguished from one another.
U.S. Department of Justice
FBI, April 1999
Classified according to their origin:
Cellulose esters--cellulose is chemically altered to create an entirely new compound not found in nature.
Can you tell the difference(s) between the cotton on
the left and the rayon on the right?
Wool Fibers (400X)
Fabrics are composed of individual threads or yarns, made of fibers, that are knitted, woven, bonded, crocheted, felted, knotted or laminated. Most are either woven or knitted. The degree of stretch, absorbency, water repellence, softness and durability are all individual qualities of the different fabrics.
Woven fabric are made by interlacing warp (lengthwise) and weft (filling) yarns. Warp run the length of the fabric and parallel to the selvage which is the edge of the fabric. Weft cross over and under the warp threads.
Knitted fabrics are made by interlocking loops into a specific arrangement. It may be one continuous thread or a combination. Either way, the yarn is formed into successive rows of loops and then drawn through another series of loops to make the fabric..
Establishing Individual Characteristics
Otherwise, trace evidence would have only class characteristics.
from the kid’s math assignment in my teeth.