FORENSIC SCIENCE
Trace Evidence
1
Introduction
Trace Evidence--any small pieces of material,
man-made or naturally occurring
Most common examples:
 Hair
 Fiber
2
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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Hair
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

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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
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:
Mosaic
Pectinate
Imbricate
Petal
Diamond petal
Chevron
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Scale Types
Mosaic
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 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?
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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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Hair Medulla
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
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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. 17
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 Toxicology
Advantages:
– 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 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
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and then a mass spectrometer.
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
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individual characteristics.
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
Fibrous
Material
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
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
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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)
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
insulating properties
Wool 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
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 (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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
Uniqueness
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
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class characteristics.
Man, I was nailed when those forensic guys found fibers
from the kid’s math assignment in my teeth.