Forensics of Fiber Analysis

Fibers A fiber is the smallest unit of a textile material that has a length many times greater than its diameter. Fibers can occur naturally as plant and animal fibers, but they can also be man-made. A fiber can be spun with other fibers to form a yarn that can be woven or knitted to form a fabric. The type and length of fiber used, the type of spinning method, and the type of fabric construction all affect the transfer of fibers and the significance of fiber associations.

Fiber Evidence Fibers are gathered at a crime scene with tweezers, tape, or a vacuum.  They generally come from clothing, drapery, wigs, carpeting, furniture, and blankets.  For analysis, they are first determined to be natural, manufactured, or a mix of both.

Fiber Number The number of fibers on the clothing of a victim identified as matching the clothing of a suspect is important in determining actual contact. The greater the number of fibers, the more likely that contact actually occurred between these individuals

Forensics of Fiber Analysis Cross transfers of fiber often occur in cases in which there is person-to-person contact Investigators hope that fiber traceable back to the offender can be found at the crime scene, as well as vice versa. Success in solving crimes often hinge on the ability to narrow the sources for the type of fiber found, as the prosecution did with their probability theory on the fibers

Forensic Fiber Analysis Why would this information be valuable to a forensic scientist? The world produced approximately 80 billion pounds of fabric in 1995, about half of which was cotton The other approximately 44 billion pounds of fiber were manufactured or synthetic. Table 1. U.S. Annual Production for Manufactured Fibers: 1995 (millions of pounds) Fiber Product Polyester 3,887 Nylon 270 Olefin 521 Rayon/Acetate/Triacetate 498 Acrylic/Modacrylic 432 (Table 1 [6]). All these fibers were used in a variety of applications including but not limited to clothing, household textiles, carpeting, and industrial textiles. It could be argued that the large volume of fibers produced reduces the significance of a fiber association discovered in a criminal case. It can never be stated with certainty that a fiber originated from a particular textile because other textiles are produced using the same fiber types and color. The inability to positively associate a fiber to a particular textile to the exclusion of all others, however, does not mean that a fiber association is without value. Considering the volume of textiles produced worldwide each year, the number of textiles produced with any one fiber type and color is extremely small. The likelihood of two or more manufacturers exactly duplicating all of the aspects of the textile is extremely remote (see endnote 2). Beyond the comments made previously about color, shade tolerance differs between dyeing companies. Therefore, color may vary demonstrably from batch to batch. Also, the life span of a particular fabric must be considered. Only so much of a given fabric of a particular color and fiber type is produced, and it will eventually end up being destroyed or dumped in a landfill.

Forensic Fiber Analysis It could be argued that the large volume of fibers produced reduces the significance of a fiber association discovered in a criminal case. Considering the volume of textiles produced worldwide each year, the number of textiles produced with any one fiber type and color is extremely small. The likelihood of two or more manufacturers exactly duplicating all of the aspects of the textile is extremely remote

Fiber Evidence The problem with fiber evidence is that fibers are not unique.  Unlike fingerprints or DNA, they cannot pinpoint an offender in any definitive manner.  There must be other factors involved, such as evidence that the fibers can corroborate or something unique to the fibers that set them apart.

Microscopic Examination A compound microscope uses light reflected from the surface of a fiber and magnified through a series of lenses, The comparison microscope (two compound microscopes joined by an optical bridge) is used for more precise identification.  The phase-contrast microscope, reveals some of the structure of a fiber. Electron microscopes either pass beams through samples to provide a highly magnified image, or reflect electrons off the sample's surface.  A scanning electron microscope converts the emitted electrons into a photographic image for display.  This affords high resolution and depth of focus.

Spectrometer The spectrometer, which separates light into component wavelengths.  every organic element has a uniqueness to its constituent parts.  By passing light through something to produce a spectrum, the analyst can read the resulting lines, called "absorption lines."  That is, the specific wavelengths are characteristic of its component molecules of the substance. 

Micro-spectrophotometer This microscope locates minute traces or shows how light interacts with the material under analysis.  Linking this to a computerized spectrophotometer increases the accuracy.   The scientist can get both a magnified visual and an infrared pattern at the same time, which increases the number of identifying characteristics of any given material.

Natural Fibers vs Manufactured Natural fibers come from plants (cotton) or animals (wool).  Manufactured fibers are synthetics like rayon, acetate, and polyester, which are made from long chains of molecules called polymers. 

Natural Fibers Many different natural fibers originating from plants and animals are used in the production of fabric. Cotton fibers are the plant fibers most commonly used in textile materials the type of cotton, fiber length, and degree of twist contributing to the diversity of these fibers. Cotton fibers

Other plant fibers Flax (linen), ramie, sisal, jute, hemp, kapok, and coir. The identification of less common plant fibers at a crime scene or on the clothing of a suspect or victim would have increased significance. Flax fibers viewed with polarized light

Animal Fiber: Wool Wool is the most frequently used in the production of textile materials and the most common wool fibers originate from sheep. Finer woolen fibers are used in the production of clothing coarser fibers are found in carpet. Fiber diameter and degree of scale protrusion of the fibers are other important characteristics. Wool fibers

Other Animal Fibers Although sheep's wool is most common, woolen fibers from other animals may also be found. These include camel, alpaca, cashmere, mohair, and others. The identification of less common animal fibers at a crime scene or on the clothing of a suspect or victim would have increased significance.

Man-Made Fibers More than half of all fibers used in the production of textile materials are man-made. Some man-made fibers originate from natural materials such as cotton or wood; others originate from synthetic materials. Polyester and nylon fibers are the most commonly encountered man-made fibers, followed by acrylics, rayons, and acetates. There are also many other less common man-made fibers. The amount of production of a particular man-made fiber and its end use influence the degree of rarity of a given fiber. Cross section of man-made fibers

Novoloid Olefin Polyester Rayon Saran Spandex Vinal Vinyon Acetate Federal Trade Commission Rules and Regulations Under the Textile Products Identification Act, 1954 Generic names for manufactured fibers Acetate Acrylic Anidex Aramid Glass Lyocel Nylon Metallic Modacrylic Novoloid Olefin Polyester Rayon Saran Spandex Vinal Vinyon

Man-Made Fibers The cross section of a man-made fiber can be manufacturer-specific Some cross sections are more common than others, and some shapes may only be produced for a short period of time. Unusual cross sections encountered through examination can add increased significance to a fiber association. Cross-sectional views of nylon carpet fibers as seen with a scanning electron microscope (SEM)

Fiber Forensics Generally, the analyst gets only a limited number of fibers to work with—sometimes only one. Whatever has been gathered from the crime scene is then compared against fibers from a suspect source, such as a car or home Fibers are laid side by side for visual inspection through a microscope.

Important Considerations An is the length of time between the actual physical contact and the collection of clothing items from the suspect or victim. If the victim is immobile, very little fiber loss will take place, whereas the suspect's clothing will lose transferred fibers quickly. The likelihood of finding transferred fibers on the clothing of the suspect a day after the alleged contact may be remote, depending on the subsequent use or handling of that clothing.

Steps of Fiber Analysis The first step in fiber analysis is to compare color and diameter.  Dyes can also be further analyzed with chromatography, which uses solvents to separate the dye's chemical constituents. 

Fiber Color Color influences the value given to a particular fiber identification. Often several dyes are used to give a fiber a desired color. Individual fibers can be colored prior to being spun into yarns. Yarns can be dyed, and fabrics made from them can be dyed. Color can also be applied to the surface of fabric, as found in printed fabrics. How color is applied and absorbed along the length of the fiber are important comparison characteristics. Color-fading and discoloration can also lend increased value to a fiber association.

Microscopy Fibers should be first examined with a stereomicroscope. Physical features such as crimp, length, color, relative diameter, luster, apparent cross section, damage, and adhering debris should be noted. Fibers are then tentatively classified into broad groups such as synthetic, natural, or inorganic. If the sample contains yarns, threads, or sections of fabric, construction should be recorded

Illumination and Magnification Comparisons should be made under the same illumination conditions at the same magnifications. For comparison microscopes, this requires color balancing the light sources. This is best achieved with two fibers or fiber samples from the same source mounted on two microscope slides, which are then compared. A balanced neutral background color is optimal.

Side-by-Side Comparisons. If all of the characteristics are the same under the stereoscope, the next step is to examine the fibers with a comparison microscope. This side-by-side and point-by-point examination is the best technique to discriminate between fibers, especially those that appear to be similar. The physical characteristics of the must be compared visually with the comparison microscope to determine if they are the same in the known and questioned samples. Photography is recommended to capture the salient features for later demonstration.

Summary of Steps of Fiber Analysis In short, the fiber analyst compares shape, dye content, size, chemical composition, and microscopic appearances, yet all of this is still about "class evidence.“  Even if fibers from two separate places can be matched via comparison, that does not mean they derive from the same source, and there is no fiber database that provides a probability of origin.

Fiber Location Where fibers are found also affects the value placed on a particular fiber association. The location of fibers on different areas of the body or on specific items at the crime scene influences the significance of the fiber association.