FORENSIC SCIENCE Trace Evidence Fiber 1 Introduction Trace

FORENSIC SCIENCE Trace Evidence Fiber 1

Introduction Trace Evidence: u any small pieces of material, man-made or naturally occurring Most common examples: u Hair u Fiber 2

Test Questions for Trace Evidence u What is it? u Is it man-made or natural? u What is its source? u How common is it? u Can it be identified to a single source? 3

Fiber Evidence u NOTE – Fabric is the type of material and fibers are the “threads” that make up the fabric u The use of fiber evidence in court cases is used many times to connect the suspect to the victim or to the crime scene. – In the case of Wayne Williams, fibers were the entire case. – Williams was convicted in 1982 based on carpet fibers that were found in his home, car and on several murder victims (product rule). 4

Classification Fibers are classified according to their origin: u Natural Vegetable u Animal or protein u Mineral u u Man-Made Regenerated u Pure Synthetic u 5

Vegetable or Plant Fibers u u u Cotton Hemp Linen (Flax) Ramie Sisal 6

Protein Fibers Wool Fibers (400 X) u Wool--animal fiber coming most often from sheep but may be goat (mohair, cashmere), rabbit (angora), camel, mink, beaver u Silk--animal fiber that is spun by a silk worm to make its cocoon; fiber reflects light and has insulting properties 7

Mineral Fibers u u u Asbestos--a natural fiber that was used in fire-resistant substances Metallics (mylar)--a manufactured mineral fiber Fiberglass--another manufactured mineral fiber 8

Cellulose Fibers u Rayon--first man-made fiber; soft, lustrous, versatile fiber u Cellulose esters-cellulose is chemically altered to create an entirely new compound not found in nature. u Acetate--less expensive, less polluting than rayon 9

Polymers u u u Synthetic fibers are made of polymers which are long string of repeating chemical units. The word polymer means many (poly) units (mer). The repeating units of a polymer are called monomers. By varying the chemical structure of the monomers or by varying the way they are weaved together, polymers are created that have different properties. As a result of these differences, forensically they can be distinguished from one another. 10

Petroleum Plastics (Made from derivatives of petroleum, coal and natural gas) u Nylon--most durable man-made fabric; extremely light weight u Polyester--most widely used man-made fiber u Acrylic--provides warmth from a lightweight, soft and resilient fabric u Spandex--extreme elastic properties 11

Fabric Production u Fabrics are composed of individual threads or yarns, made of fibers, that are knitted, woven, bonded, crocheted, felted, knotted or laminated. u Most are either woven or knitted. u The degree of stretch, absorbency, water repellence, softness and durability are all individual qualities of the different fabrics. 12

Woven Fabric u Woven fabric are made by interlacing warp (lengthwise) and weft (filling) yarns. – Warp run the length of the fabric and parallel to the selvage (the edge of the fabric). – Weft cross over and under the warp threads. Types include: u Plain u Twill u Satin 13

Woven Fabric PLAIN v v Simplest and most common weave Warp and weft pass under each other alternately Create even patterns of 1/1 and 2/2 Design resembles a checkerboard 14

Woven Fabric TWILL v v Create by passing the warp yarn over one to three weft yarns before going under one Makes a diagonal weave Design resembles a stair steps Denim is the most obvious example 15

Woven Fabric SATIN v v The yarn interlacing is not uniform Creates long floats Interlacing weave passes over four or more yarns Satin is the most obvious example 16

Knitted Fabric u u Knitted fabrics are made by interlocking loops into a specific arrangement. It may be one continuous thread or a combination. Either way, the yarn is formed into successive rows of loops and then drawn through another series of loops to make the fabric. . Diagram: 17

Identification and Comparison of Fibers u Microscopic examination – Color--compositional differences in the dyes – Diameter – Fibers surface--delustering particles that may be added by manufacturers u Microspectrophotometer--compares fiber colors through spectral patterns u Chromatography--gives a more detailed analysis of the dye composition 18

Identification and Comparison of Fibers (cont. ) u Polarizing u microscope Can be used to determine the refractive indices of various fibers. The fiber is immersed in a fluid that has a comparable refractive index. u The disappearance of the Becke line is observed under the microscope. u u In addition, fibers will absorb infrared light in a characteristic pattern. u This can be observed through the use of an infrared microspectrophotometer and a microscope. 19

Collection of Fiber Evidence u Bag clothing items individually in paper bags. Make sure that different items are not placed on the same surface before being bagged. u Make tape lifts of exposed skin areas of bodies and any inanimate objects u Removed fibers should be folded into a small sheet of paper and stored in a paper bag 20

Uniqueness u Establishing Individual Characteristics If there is only one source for the transfer material with a controlled environment where the contact took place u If there is a method available to characterize the material, such as DNA u u Otherwise, trace evidence would have only class characteristics. 21

22 Figure F-1. Cross sections of several natural and synthetic fibers. Reproduced from Melone, et. al. , Lab Manual for Crimminalistics, 6 th ed, Prentice Hall.

Man, I was nailed when those forensic guys found fibers from the kid’s math assignment in my teeth.