Fibers

Fibers are one of the several pieces of forensic evidence known as trace evidence. Even though fibers are small and can be difficult to detect, their importance can be considerable. For example, textile fibers from an article of clothing can be influential in linking a suspect to the scene of a crime.

A fiber is the smallest portion of a textile material. Whether synthetic (i.e., rayon) or natural in origin (i.e., cotton), all fibers share the trait of being very much longer than their diameter. A short length of sewing thread is a good visual analogy of a fiber. The different origins of the materials that make up a fiber, and the differing ways that a fiber can be formed together to create the finished fabric, are all important in identifying the fiber.

Analysis of fibers that are found on a victim will involve determining the types of fibers present at the scene. For example, a fiber can be transferred from a carpet to a body. This fiber will not be as significant as a fiber found on a victim that is not present anywhere else at the scene. If a similar fiber is found on a suspect, this can be a powerful piece of evidence linking the suspect to the scene.

Fibers tend not to cling to other fabric tenaciously. Thus, forensic examiners must handle a victim with care, to minimize fiber loss. Retrieving the victim's clothing as soon as possible is a prudent step to preserving as many fibers as possible.

Fibers are typically collected using adhesive tape. The strips of tape are examined for fibers that match the fibers that are thought to be a normal part of the crime scene. This collection and analysis of fibers are tedious tasks.

Among the natural fibers, cotton and wool are the most popular. Other examples include flax, jute, hemp, and kapok. Each type can present a different appearance under an examination technique such as polarized light microscopy. Different fibers will refract light differently. Depending on a fiber's shape, the fiber can appear brighter along the edges or in the middle. Natural fibers tend to be circular when viewed in cross-section. In contrast, synthetic fibers can have a variety of shapes.

The number and location of fibers on a victim and a suspect are important in connecting the individuals together, especially if the fibers match. Matching fibers involves comparing the fiber type, color and type of dye used. The latter can be especially significant, given the nearly unlimited number of dye combinations that are possible.

Color is determined using a visible light microspectrophotometer or by thin-layer chromatography, which separates the various dye components. Synthetic fibers can also be examined by infra-red spectroscopy, which can also yield information on the chemical makeup of the fiber.