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