Trace Evidence: Fiber

Fiber Evidence

• Fiber evidence can be used to connect a suspect to a victim

  or a crime scene in many of the same ways a hair can.

• Fibers can be one of two types:

  • 1) Natural

  • 2) Artificial

• How do we come to find fibers in a crime scene?

• How do we define a fiber?

  • 1) A material that has a length many times of its width.

• 2) The smallest unit of textile material.

  • Can a hair be considered a fiber?

    • Yes. A very specific type of fiber.

  • Can a fiber be considered a hair?

    • Yes. Natural fibers such as wool are hairs.

• All hairs are considered a fiber but not all fiber considered hair

Natural Fibers

• Obtained from animal, vegetable, or mineral sources.

• Animal fibers

  • Hairs can be considered animal fibers.

  • Wool is the most commonly encountered animal fiber.

  • Silk is considered an animal fiber derived from the silk worm.

• Vegetable fibers

  • Constitute the largest class of natural fibers.

  • The major component of vegetable fibers = cellulose.

    • Cellulose is a naturally occurring polymer.

  • Cotton is the most commonly encountered vegetable fiber.

• Mineral fibers

  • Some minerals occur as a fiber.

  • Asbestos is the most commonly encountered mineral fiber

Artificial Fibers

• Most fibers are polymers.

• Can be divided into two sub-classes.

  • 1) Derived (Regenerated)

  • 2) Synthetic

• Derived Fibers

• Recovered natural polymers (cellulose), which are dissolved and resolidified it. The derived fiber will have properties that are unique.

  • Rayon (Viscose) was the first ever derived fiber (1911).

  • The fibers were woven together to result in sheets of fabric.

  • Acetate is also a derived fiber (from cotton and wool).

  • The derived acetate is soluble in acetone

• Synthetic Fibers

  • Fibers made by human beings (not plant or animal based) where

    the polymers are chemically created.

  • Nylon was the first synthetic fiber (~1938).

    • Nylon is considered a polyamide due to its chemical structure.

  • By rearranging the chemical structure of the polymer, dozens of other synthetic fibers were created.

• The resulting new synthetics all had unique properties.

  • Kevlar

  • Nomex

  • SpandeX

• Polymers: Polymers are long chains of smaller units called monomers

How it’s made

• How are fibers made?

• A process of adding layers of chemical reagents to the

  cellulose to create breakdowns of the monomer bonds and assist them in

  polymerizing.

• The shape of the die inside the Wet Spinning head nozzle will determine the morphology of the fiber. Shape, diameter, and cross-section happen at this step.

• Works like a pasta machine, cuts different sizes and shapes of noodles

  

Fiber Identity

• Methods used to determine fiber identity

  • 1) Microscopic Comparison

• Compound Microscope

  • Can observe the morphology of the fiber under various conditions of light passing through it.

  • Surface form, diameter, and color can be noted easily.

• Visible-Light microspectrophotometer

  • Colors that look identical but are made from different manufacturers will have subtle differences in the chemical composition of the dyes. This technique is much more sensitive to the differences in those chemical compositions.

• Polarized Light Microscope

  • The manufacturing process that makes the fiber also causes the molecules in the fiber to behave in specific ways.

    • Passing polarized white light through the fiber will cause specific colors, called interference colors to be visible to the analyst.

• Methods used to determine fiber identity

  • Physical properties

    • Dissolved in a solvent such as acetone.

    • Dissolved in strong acid.

    • Burn test: Different fibers will have different responses to these tests. Those reactions, plus the Observations under a microscope, will help provide more information about the identity of the fiber.

    • BUT……What if you only have one fiber? How would you proceed?

  • Spectroscopy

    • Instrumental method of determining the identification of material without damaging it.

    • Infrared (IR) spectroscopy passes a beam of IR light through a sample.

    • Each type of fiber material will selectively absorb wavelength of IR

      • Raman spectroscopy passes a laser beam through a sample