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FORENSIC SCIENCE

Trace Evidence

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Analysis

of

Fibrous

Material

Adapted from U.S.

Department of Justice

FBI, April 1999

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Types of Fibers

Synthetic

Polyester

Rayon

Nylon

Acetate

Acrylic

Spandex

Natural

Silk

Cotton

Wool

Mohair

Cashmere

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Classification

All fibers are polymers and are classified

according to their origin:

Vegetable or cellulose

Animal or protein

Mineral

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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

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

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Cellulose (plant) Fiber

Fiber Comparison

Can you tell the difference(s) between the cotton on

the left and the rayon on the right?

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Protein Fibers

Wool--animal fiber coming most often from sheep but may be goat (mohair), rabbit (angora), camel, mink, beaver

Silk--animal fiber that is spun by a silk worm to make its cocoon; fiber reflects light and has insulating properties

Wool Fibers (400X)

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).

Protein Fibers

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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

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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

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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.

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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

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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

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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

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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

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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:

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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

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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

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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)

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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=1.333

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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)!

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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.

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

Practice

Glycerine has an RI of 1.473

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?

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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

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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.

Man, I was nailed when those forensic guys found fibers

from the kid’s math assignment in my teeth.