FORENSIC SCIENCE
Trace 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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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 Fibers (400X)
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 properties
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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.