FORENSIC SCIENCE. Trace Evidence

Transcription

1 FORENSIC SCIENCE Trace Evidence 1

2 Analysis of Fibrous Material Adapted from U.S. Department of Justice FBI, April

3 Types of Fibers Synthetic Polyester Rayon Nylon Acetate Acrylic Spandex Natural Silk Cotton Wool Mohair Cashmere 3

4 Classification All fibers are polymers and are classified according to their origin: Vegetable or cellulose Animal or protein Mineral 4

5 Cellulose Fibers Cotton--vegetable fiber. Strong, tough, flexible; moisture absorbent; not shape retentive Ramie--vegetable fiber. Less flexible than cotton so its often blended with cotton 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. Acetate--less expensive, less polluting than rayon 5

6 Cellulose (plant) Fiber Remember cellulose from Biology? a. Names for sugars end in ose. a single sugar monomer is called a monosaccharide and one name for a particular monosaccharide is glucose. (C6H12O6) b. Cellulose is made of glucose sugars in a polymer chain: polysaccharide Fig. 1 Whole polymer of cellulose Fig. 2 Basic hexagon shape of one single glucose

7 Cellulose (plant) Fiber 7

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

9 Protein Fibers Wool--animal fiber coming most often from sheep but may be goat (mohair), rabbit (angora), camel, mink, beaver Wool Fibers (400X) Silk--animal fiber that is spun by a silk worm to make its cocoon; fiber reflects light and has insulating properties 9

10 Protein Fibers Remember animal protein structure from Bio? Proteins are polypeptide polymers of more than 50 amino acids- (up to about 50 amino acid combinations from the 20 available).

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

12 Mineral Fibers Asbestos--a natural fiber that was used in fire-resistant substances Metallics (mylar)--a synthetic manufactured mineral fiber Fiberglass--another synthetic manufactured mineral fiber 12

13 Fabric Production Fabrics are composed of individual threads or yarns, made of fibers, that are knitted, woven, bonded, crocheted, felted, knotted or laminated. Most are either woven or knitted. The degree of stretch, absorbency, water repellence, softness and durability are all individual qualities of the different fabrics. 13

14 Woven Fabric Woven fabric are made by interlacing warp (lengthwise) and weft (filling) yarns. Warp run the length of the fabric and parallel to the selvage which is the edge of the fabric. Weft cross over and under the warp threads. Types include: Plain Twill Satin 14

15 Woven Fabric PLAIN 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 15

16 Woven Fabric TWILL Create by passing the warp yearn over one to three weft yearns before going under one Makes a diagonal weave Design resembles a stair steps Denim is the most obvious example 16

17 Woven Fabric SATIN The yarn interlacing is not uniform Creates long floats Interlacing weave passes over four or more yarns Satin is the most obvious example 17

18 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

19 Identification and Comparison of Fibers Microscopic examination Color--compositional differences in the dyes Fibers surface--delustering particles that may be added by manufacturers Microspectrophotometer--compares fiber colors through spectral patterns Chromatography--gives a more detailed analysis of the dye composition 19

20 Identification and Comparison of Fibers Microscopic examination Color--compositional differences in the dyes Fibers surface--delustering particles that may be added by manufacturers Microspectrophotometer--compares fiber colors through spectral patterns Chromatography--gives a more detailed analysis of the dye composition 20

21 Identification and Comparison of Fibers (cont.) Density-sink float method -one liquid that is less dense (fiber will sink) -one liquid is more dense (fiber will float) 21

22 Identification and Comparison of Fibers (cont.) Refractive Index -RI is the measure of bending of light as it passes from air into a solid or liquid. -Light is slowed down when it passes through dense fluid mediums -Speed of light in air is 300 million meters/s, but in water it is only 225 million m/s -Water s refractive index=300 mil/225mil=

23 Identification and Comparison of Fibers (cont.) Refractive index differences cause Becke lines -Becke line: a halolike glow or shadow appearing around an object immersed in a liquid of a different RI -When object or specimen has same RI as medium it is in, the specimen edges disappear (become transparent)! 23

24 Identification and Comparison of Fibers (cont.) Refractive index differences cause Becke lines When the liquid medium has lower RI, the object appears to glow from inside. When the liquid medium has a higher RI, the object appears to have an outer halo of light/glow. 24

25 A grain or fiber that has greater refractive index than its surroundings will refract and reflect light inward like a crude lens (middle image) A grain or fiber that has lower refractive index than its surroundings will refract and reflect light outward like a crude diverging lens. (last image to right) 25

26 Practice Glycerine has an RI of Look at the table to determine which fiber edges should disappear when mounted in glycerine? Which fibers should glow with a Becke line inside the edges? Are there any fibers that will appear darker than glycerine with an outer halo glow around outside edge? 26

27 Collection of Fiber Evidence Bag clothing items individually in paper bags. Make sure that different items are not placed on the same surface before being bagged. Make tape lifts of exposed skin areas of bodies and any inanimate objects Removed fibers should be folded into a small sheet of paper and stored in a paper bag 27

28 Uniqueness and Discrimination Establishing Individual Characteristics If there is only one source for the transfer material with a controlled environment where the contact took place If there is contamination of several different materials from surface onto surface two If there is a method available to characterize the material, such as applying DNA Otherwise, trace evidence would have only class characteristics. 28

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