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FORENSIC SCIENCE Trace Evidence - PowerPoint PPT Presentation

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FORENSIC SCIENCE Trace Evidence. Introduction. Trace Evidence--any small pieces of material, man-made or naturally occurring Most common examples: Hair Fiber. Test Questions for Trace Evidence. What is it? Is it man-made or natural? What is its source? How common is it?

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

Most common examples:

  • Hair
  • Fiber
test questions for trace evidence
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?

Human 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

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.

hair morphology the study of structure and form
Hair MorphologyThe 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
hair growth
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
hair cuticle




Diamond petal


Hair Cuticle

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:

scale types
Scale Types



scale types cont
Scale Types (cont)



scale types cont11
Scale Types (cont)


Diamond Petal

human scales

In 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?

hair cortex
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
hair medulla
Hair Medulla

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


  • Continuous
  • Intermittent or interrupted
  • Fragmented
  • Absent--not present
hair medulla patterns
Hair Medulla Patterns





Amorphous (without a distinct pattern)

human medulla

Human medulla may be continuous, fragmented or absent.

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.

medullary index
Determined by measuring the diameter of the medulla and dividing it by the diameter of the hair.Medullary Index

Medullary Index for human hair is generally less than 1/3.

For animal hair, it is usually greater than 1/2.

hair comparison



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

Hair Comparison
collection of hair evidence
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
hair toxicology
Hair Toxicology


  • 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.

hair testing procedure
  • 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.
fiber evidence
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.


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.

analysis of fibrous material
Analysis of FibrousMaterial

U.S. Department of Justice

FBI, April 1999

types of fibers













Types of Fibers

Classified according to their origin:

  • Vegetable or cellulose
  • Animal or protein
  • Mineral
cellulose fibers
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
Fiber Comparison

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

the left and the rayon on the right?

petroleum plastics made from derivatives of petroleum coal and natural gas
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
protein fibers
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)

mineral fibers
Mineral Fibers
  • Asbestos--a natural fiber that was used in fire-resistant substances
  • Metallics (mylar)--a manufactured mineral fiber
  • Fiberglass--another manufactured mineral fiber
fabric production
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.

woven fabric
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
woven fabric35
Woven Fabric
  • 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
woven fabric36
Woven Fabric
  • 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
woven fabric37
Woven Fabric
  • The yarn interlacing is not uniform
  • Creates long floats
  • Interlacing weave passes over four or more yarns
  • Satin is the most obvious example
knitted fabric
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..


identification and comparison of fibers
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
identification and comparison of fibers cont
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.
collection of fiber evidence
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

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.