Fibers

Locard’s Exchange Principle When a criminal comes in contact with an object or person, a cross-transfer of evidence occurs. Suspect may transfer evidence to scene or pick up evidence from the scene

Cases involving fibers Physical Contact (hugging, etc.) Any physical struggle between two people Carpet fibers adhering to shoes Hit and run - pedestrian fibers on suspect vehicle Primary and Secondary Transfer Primary – direct transfer Secondary – indirect…home: you: crime scene

Types of Fibers Natural Fibers Man-made Fibers (synthetic) derived from animal, plant or mineral sources Man-made Fibers (synthetic) manufactured from raw materials or synthetic chemicals

Natural Fibers Broken into 3 classifications Animal fibers Silk, sheep (wool), goats (mohair, cashmere), camels, horse, rabbits Vegetable Fibers Cotton, hemp, jute, ramie, flax, linen Mineral Fibers asbestos

Cotton Most prevalent natural fiber!! White cotton fibers have no evidentiary value Cotton jean fibers have little to no evidentiary value Microscopic appearance: Ribbon-like shape with irregular twists

White Cotton Fibers

Polymers Basic chemical substance of all synthetic fibers Assumes different chemical and physical properties Macromolecules of thousands to millions of atoms Huge chains of repeating monomers Natural polymers also exist

Man-made Fibers Regenerated Fibers – produced from the naturally occurring polymer, cellulose Cellulose = repeating units of glucose rayon Acetate Triacetate

More Man-made Fibers Synthetic Fibers - produced from synthetic chemicals (polymers) Nylon Polyesters Acrylics

Identification and Comparison Microscopic Comparison Color, diameter, lengthwise striations, delusterant particles, inclusions, fluorescence, refractive indices (parallel and perpendicular), birefringence Polarized Light Microscopy (PLM) Cross section shape Microspectrophotometer (color) Chemical composition (FT-IR)

Natural vs. Synthetic Thru the Microscope Natural fibers Usually have a “twisty” appearance Variable diameter NO delustrant particles Scales and medulla (animal hairs) Cross hatching Synthetic fibers Smooth edges, lengthwise striations Consistent diameter (usually) Very clear or has delustrant particles

Collection of Fiber Evidence Post-It notes and paper packets (individual or small numbers of fibers Tape Lifts cover large areas (i.e.. Care seats, victim’s clothing, etc.) ALWAYS collect known samples (if several different carpets in the scene, collect a known from each one) Package items separately to avoid cross-contamination

Single Fibers Collected onto a Post-It Note

Cross Sectioning Fibers – determine shape Synthetic fibers are forced out of a nozzle when they are hot, and then they are woven. The holes of the nozzle are not necessarily round; therefore, the fiber filament may have a unique shape in cross-section.

Cut square piece from edge of polypropylene heat seal bag

Polypropylene Piece Fibers

Once the fibers are sandwiched between in the polypropylene, place another slide on top so fibers are trapped between

Place slides with fibers/polypropylene on hot plate and push down with micro spatula or other object until the polypropylene melts (will turn clear) PLAY VIDEO

Use scalpel or razor blade to slice across to an area where good cross sections can be made (perpendicular to the fibers)

Hold the polypropylene with a forceps and carefully shave off thin layers from the edge (this is the most difficult step, you want to shave off layers like a deli would slice sandwich meat)

Good cross sections thru the microscope 20X Good cross sections thru the microscope 40X

Burn Tests Different fiber types will react differently when exposed to a flame Some fibers fuse and shrink, some continue to burn after removal from the flame, so give off smoke, others have distinct smells when burned The resulting ashes will also vary based on fiber type

Burn Tests Silk and wool produce burning hair smell with a black crushable ash Nylon burns slowly with melting and usually self-extinguishes to form a hard gray bead (supposedly has a celery-like odor) Polyester burns/melts with black smoke, usually self-extinguishes to form a hard black bead (supposedly has a sweet odor) Cotton and flax burns with an afterglow and produces a gray feathery smooth edged ash (produces a burning paper odor)

Solubility Tests Acetate fibers are soluble in acetone (other fibers are not) and glacial acetic acid Nylon fibers are soluble in hydrochloric acid

Class Exercises on Fibers Mount different types/colors of fibers permanently and sketch them thru the microscope Perform Burn tests on various fiber samples Perform chemical tests on various chemical fibers Make tape lifts and then compare to known fibers