Forensic Examination of Fibers

Important evidence in incidents involving personal contact- homicide, assault, sexual

Cross-transfers may occur between the clothing of a suspect and victim

Hit-and-run victims can leave fibers, threads, or whole pieces of clothing on a vehicle

Fibers can also become fixed in screens or glass broken during a breaking-and-entering attempt

Natural Fibers Are derived from animal or plant

sources

Animal fibers comprise most natural fibers encountered in crime lab exams

Can include hair coverings: sheep (wool), goats (cashmere), camels, llamas

Or fur fibers: obtained from mink, rabbit, beaver, etc.

Most common plant fiber is cotton

Manufactured Fibers Are derived from natural or synthetic polymers They have increasingly replaced natural fibers in

clothing & fabric

They are made by:

1) Processing raw material from cotton/wood pulp and extracting cellulose

2) The cellulose may be chemically treated and dissolved in a solvent

3) It is then forced through small holes of a spinning jet (spinneret) to produce the fiber

Fibers made from natural raw materials (regenerated cellulose) are called regenerated fibers: rayon, acetate, triacetate

Fibers made from synthetic chemicals are synthetic fibers: nylon, polyester, acrylic

Microscopic Exam of Fibers

The first and most important step is a comparison for color & diameter by a comparison microscope

Other features that may aid in the comparison are lengthwise striations (lined markings) on the surface of some fibers and pitting of the fiber’s surface with delustering particles (titanium dioxide particles added to reduce shine)

Cross-sectional shape of a fiber may also help

Analytical Techniques Used

Two fibers may seem to be the same color, differences may exist in the dyes applied to them when made

Most fibers are dyed with a mixture of colors to obtain a desired shade

Can use a visible light microspectrophotometer to compare the colors of fibers

A fiber as small as 1 mm long or less can be examined

Analytical Techniques Used

A more detailed analysis of the fiber’s dye composition can be obtained by a chromatographic separation of dye

Small strands of fibers are compared for dye content by extracting the dye off each fiber with a solvent and then spotting the dye solution onto a thin-layer chromatography plate

The dye of the questioned and standard fibers are separated on the plate and compared for similarity

Others When fibers are compared, they must be

shown to have the same chemical composition (belong to the same class)

For example, the standard and questioned fabric both being nylon instead of one being nylon while the other is cotton

Many manufactured fibers exhibit double refraction or birefringence which will make it look crystalline

Polarized white light will split into two rays that are perpendicular to each other and produce interference colors, polarization

Significance of Fiber Evidence

Once a match has been determined, the significance is bound to be raised

No technique can associate a fiber definitively to any single garment

No database is available for determining the probability of a fiber’s origin

Despite this, one should not discount the significance of a fiber match

By observing what people wear, it is unlikely to find two different people wearing identically colored fabric ( exception: jeans and cotton tees)

There are thousands of different colored fibers Combine this with the fact that scientists not only

compare color, but also size, shape, microscopic appearance, chemical composition and dye

There will still be a good chance of linking a questioned fiber to a standard

Collection and Preservation

Clothing should be packaged in paper bags Each article must be placed in separate bags to

prevent contamination Must keep clothing from different people from

coming into contact Carpets, rugs, and bedding should be folded to

protect areas suspected of containing fibers Knife blades should be covered to protect

adhering fiber If a body was wrapped in a carpet or blanket,

tape lifts must be done on the body If individual fibers are found, they must be

removed with clean forceps and placed in a small sheet of paper, must be folded and labeled and placed in another container

Types of FiberCotton (ribbon-like) Nylon Polyester

WoolRayon fibers Silk (fractures)