Fibers

Fibers as Evidence• Fibers can be collected from crime scenes that

have contact between two people.– Examples are homicide, rape, or assault.

• They can also be collected from scenes where a person comes in contact with an object.– Examples would be a person hit by a car, or

breaking glass in a robbery.• Fibers are not uncommon so they rarely are

effective at giving an individual identification.

Natural Fibers• Natural fibers are taken from plant or animal

sources.• Animal hair – sheep = wool – goats = mohair, cashmere– camel, llama, and alpaca

• Animal fur– mink, rabbit, beaver, muskrat, dalmation (Cruella?)

• Plant - cotton

Manufactured Fibers

• Manufactures fibers are man-made and come from two sources.

• Regenerated fibers – come from cellulose from cotton or wood pulp that has been chemically altered. (rayon, acetate, triacetate)

• Synthetic fibers – created from polymers. (nylon, polyester, acrylics)

Polymers• Polymers are made by chemically joining

monomers which are two usually carbon chains with a double bond.

• Chemically the double bond is broken leaving open bonds that can link with other monomers.

• They link together to form long chains.• The composition of the other bonds on the

monomer will change its properties.• How they are woven together will change the

properties as well.

Identification and Comparison• Certainly if a fabric piece is torn and can be

shown to be an exact fit, the evidence is easy to submit and tie to the crime scene.

• Most times however, a scientist only has a few fibers to use for identification and comparison.

• Use both a microscope for visual comparison, and analytical (determine chemical composition) techniques in the lab.

Microscopes• Can view fibers both from the side, or using a

cross section.• Fibers have characteristic looks and shapes.• Natural fibers look different from synthetic.• Natural fibers look different from each other.• Synthetic fibers look different from each other.• Comparison microscopes allow a side-by-side

comparison.

Natural Fibers

Cotton – has a twist that is easily identifiable.

Wool – looks like hair. (It is!)

Synthetic Fibers

Synthetic fibers have interference patterns that give the different colors. Called birefringence.

Nylon

Very unique cross-section.

Hooks and nylon strands in velcro.

Other Synthetics

Spandex weave Polyester – pitting is due to delustering particles – titanium dioxide

Analytical Techniques• Chemical composition – Chemical tests about

solvency or reactivity are done to determine exact type and dye used for coloring.

• Birefringence – polymers are forced through holes to align fibers. This causes light to split into two parallel waves that interfere. Each fiber has a unique pattern and can be matched by its index or refraction in a fluid.

• Spectroscopy – chemical bonds interact with infrared light. This provides a fingerprint for each fiber.