Forensic Analysis of Fibers

Types of Fibers

Natural – fibers that come from plants or animals

Man-made – fibers that come from natural or synthetic polymers

animal – wool, cashmere, cotton

Examples – rayon, polyester, nylon

Animal Fibers

Of all the natural fibers, these are examined the most in a crime lab

Examples: wool, mohair, cashmere, fur (rabbits, mink)

Analysis of animal fibers is the same as analyzing hair from animals:

Microscopic evaluation of color and morphology

Make sure to obtain a good number of control samples to get a range of the fiber’s properties

Varying thicknesses of individual hairs are evident in the photomicrograph, which also shows the different textures characteristic of this fur.

Mink Furmicro.magnet.fsu.edu/.../ mink.html

Woolwww.fbi.gov/.../backissu/ july2000/deedric3.htm

Plant Fibers

Most abundant plant fiber is cotton (also linen)

- Use as evidence can be meaningless since it is so widely used

CottonDistinguishing feature: ribbon

shape, twisting at different spots

Man-made FibersFirst man-made fiber created was rayon (1911) and then nylon (1939) – now are more common than natural fibers.

Man-made fibers have generic names for consumers but there are several manufactures for each fiber type.

Polymers-

Basic chemical component of all synthetic fibers (also used to make plastics, paints, adhesives)

Man-made Fibers

Polymers-

Resemble a chain of repeating units (monomers)

It is by changing chemical structure of one monomer that scientists have been able to create such a wide range of polymers and types of fibers

Polymers are also produced naturally

Ex: Proteins are made of repeating amino acids

Starch and cellulose are made from repeating carbohydrate units

DNA is made from repeating nucleic acids

Types of Man-made Fibers

Regenerated fibers –

• fibers made from cellulose (component of plants and a natural polymer), which is chemically treated and placed through a machine to make the fiber.

Examples: rayon and acetate

Synthetic fibers –

• fibers that are made only from synthetic chemicals.

Examples: nylon, polyester, acrylic

Identification and Comparison of Man-Made FibersFibers are class evidence (unless actual piece of ripped fabric can be fit back into it’s original place)

Steps for Examination:

-Microscopic comparison – look at color, diameter and surface characteristics of individual fibers

-Additional morphological features examined are weave patterns and cross sectional shape.

Identification and Comparison of Man-Made Fibers

Steps for Examination:

-Look at dye composition by chromatography – separates dye molecules by size (just like electrophoresis)

-Physical and chemical composition must be determined

a. IR spectrophotometry – different fibers absorb radiation at specific wavelengths- provides a unique chemical “fingerprint” of fiber.

b. Also perform chemical/acid tests, burn tests, strength tests, water absorbency tests.

Try to narrow this down to certain class of fibers and ultimately to the manufacturer.