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FORENSIC SCIENCEFORENSIC SCIENCE

Trace EvidenceTrace Evidence

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Introduction

Trace Evidence--any small pieces of material, man-made or naturally occurring

Most common examples: Hair Fiber

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Test Questions for Trace Evidence

What is it?

Is it man-made or natural?

What is its source?

How common is it?

Can it be identified to a single source?

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HairHuman hair is one of the most frequently found pieces of

evidence at the scene of a violent crime. It can provide a link between the criminal and the act.

From hair one can determine: Human or animal Race Origin Manner in which hair was removed Treated hair Drugs ingested

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DNA

The hair shaft contains abundant mitochondrial DNA--inherited only from our mothers. It can be typed by comparing relatives if no DNA from the body is available.

Only the root contains nuclear DNA.

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Hair Morphology The Study of Structure and Form

Parts of the hair Shaft--part of the hair

that sticks out of the skin

Root--lies below

the epidermis

Follicle--structure from

which the hair grows

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Hair Growth Terminology

Anagen--hair that is growing

Catagen--hair at rest

Telogen--hair that is dying

Length--00.5 mm per day or 1 centimeter per month; approximately one half

inch per month

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Hair Cuticle

Mosaic

Pectinate

Imbricate

Petal

Diamond petal

Chevron

The cuticle is the outermost layer of hair which is covered with scales. Scales also always point toward the tip of the hair. These scales differ between species of animals and are names based on their appearance. Some of these scales are variations of the same and include:

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Scale TypesMosaic Chevron

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Scale Types (cont)

Pectinate Imbricate

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Scale Types (cont)

Petal Diamond Petal

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HUMAN SCALESIn order to visualize the

scales– paint fingernail polish

on a slide

– place a hair on the polish

– lift off the hair and observe the scale imprints

What pattern is seen in this slide?

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Hair Cortex

The cortex gives the hair its shape.

It has two major characteristics: Melanin--pigment granules that give hair its

color Cortical fusi--air spaces, usually found near the

root but may be found throughout the hair shaft

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The medulla is the hair core that is not always present. The medulla comes in different types and patterns.

Types:

Continuous

Intermittent or interrupted

Fragmented

Absent--not present

Hair Medulla

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Hair Medulla Patterns

Uniserial

Multiserial

Vacuolated

Lattice

Amorphous (without a distinct pattern)

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HUMAN MEDULLA

Human medulla may be continuous, fragmented or absent.

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RABBIT MEDULLA

Rabbit medulla is different depending on the type (location on the rabbit) of hair. The one to the left is multiserial. The one to the right is a uniserial ladder and is found in guard hair.

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Medullary Index

Determined by measuring the diameter of the medulla and dividing it by the diameter of the hair.

Medullary Index for human hair is generally less than 1/3.For animal hair, it is usually greater than 1/2.

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Hair Comparison

Color Length Diameter Distribution, shape and

color intensity of pigments granules

Dyed hair has color in cuticle and cortex

Bleaching removes pigment and gives yellow tint

Scale types Presence or absence

of medulla Medullary type Medullary pattern Medullary index

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Collection of Hair Evidence

Questioned hairs must be accompanied by an adequate number of control samples.– from victim

– possible suspects

– others who may have deposited hair at the scene Control Sample

– 50 full-length hairs from all areas of scalp

– 24 full-length pubic hairs

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Hair ToxicologyAdvantages:

– Easier to collect and store– Is externally available– Can provide information on the individual’s

history of drug use.

Collections must be taken from different locations on the body to get an accurate timeline.

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HAIR TESTINGProcedure

Collect an adequate sample, cut as closely to the scalp as possible.

Wash the hair to remove lipids, oils, cosmetics and any drugs adhering to it

Cut it into one centimeter sections Place hair in a digesting solution Screening test--antibodies are added to the hair that bind

with the drugs. If this shows that drugs are present: A confirmation test is done by gas chromatograph and

then a mass spectrometer.

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Fiber EvidenceNOTE: 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.

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Polymers

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.

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Analysis of

FibrousMaterial

U.S. Department of JusticeFBI, 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

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

Fiber Comparison

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

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Petroleum Plastics(Made from 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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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 insulting propertiesWool Fibers (400X)

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Mineral Fibers

Asbestos--a natural fiber that was used in fire-resistant substances

Metallics (mylar)--a manufactured mineral fiber

Fiberglass--another 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 weaveWarp and weft pass under each other alternatelyCreate even patterns of 1/1 and 2/2Design 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 oneMakes a diagonal weaveDesign resembles a stair stepsDenim is the most obvious example

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Woven Fabric

SATIN

The yarn interlacing is not uniformCreates long floatsInterlacing weave passes over four or more yarnsSatin is the most obvious example

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Knitted FabricKnitted 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 andComparison 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 andComparison of Fibers (cont.)

Polarizing 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. The disappearance of the Becke line is observed under the microscope.

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.

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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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UniquenessEstablishing 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.