1 FORENSIC SCIENCE Trace Evidence

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

3 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?

4 Fiber Evidence NOTE: Fabric is the type of material and fibers are the “threads” that make up the fabric 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. Although this case is unusual, fibers are generally considered of greater value as evidence than that of rootless hairs since they may contain a greater number of variables, thus showing more individual characteristics.

5 Polymers Synthetic fibers are made of polymers which are long string of repeating chemical units. 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.

6 Analysis of Fibrous Material U.S. Department of Justice FBI, April 1999

7 Types of Fibers u Synthetic u Polyester u Rayon u Nylon u Acetate u Acrylic u Spandex u Natural u Silk u Cotton u Wool u Mohair u Cashmere

8 Classification Classified according to their origin: u Vegetable or cellulose u Animal or protein u Mineral

9 Cellulose Fibers u Cotton-- vegetable fiber. Strong, tough, flexible; moisture absorbent; not shape retentive u Ramie-- vegetable fiber. Less flexible than cotton so its often blended with cotton u Rayon-- first man-made fiber; soft, lustrous, versatile fiber Cellulose esters-- cellulose is chemically altered to create an entirely new compound not found in nature. u Acetate-- less expensive, less polluting than rayon

Fiber Comparison Can you tell the difference(s) between the cotton on the left and the rayon on the right?

11 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

12 Protein Fibers u Wool--animal fiber coming most often from sheep but may be goat (mohair), 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 insulating properties Wool Fibers ( 400X )

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

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

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

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

17 Knitted Fabric 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:

18 Identification and Comparison of Fibers u Microscopic examination –Color--compositional differences in the dyes –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

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

20 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 bindle and stored in a paper bag

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