Forensic Science:
An Introduction
Trace Evidence
Properties of Matter
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Physical properties – describe the behavior
of matter; can change without changing
the composition of the matter (melting,
solubility)
Chemical properties – describes the
behavior of matter in relation to other
substances; if this property changes, then
the substance has changed (gas
production, precipitation, reactivity)
Metric System
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Metric conversion
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English/Standard to Metric Conversions
Nature of Matter
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Matter – anything that has mass and takes
up space
Element – simplest form of matter that
cannot be broken down further by
chemical means. (Periodic Table)
Atom – smallest unit of an element that
still retains the properties of that element
Compound – a pure substance made up of
two or more elements bound together
States of Matter
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Solids – molecules of matter held very
closely together; has definite shape and
volume
Liquid – molecules of matter held more
loosely together by attractive forces; has
definite volume but shape depends on
container
Gas – molecules of matter are allowed to
freely move; has not definite volume or
shape
Changes in the States of Matter
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Freezing
Melting
Condensation
Sublimation
Evaporation
Boiling
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Phase – a separation
of different
substances of matter
due to visible
boundaries between
them
Light as a Wave
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Wavelength – distance from crest to crest
Frequency – number of waves that pass a
given point
Amplitude – how high the waves are
Speed of light – wavelength x frequency
Light as a Wave
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Dispersion – the separating light into its
different component colors
Refraction - the bending of light caused
by a change in its velocity
Reflection – the bouncing of light off a
surface
Transmission – the passing of light
through a substance
Absorption – the
Electromagnetic Spectrum
Light as a Particle
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Photon – a discrete packet or particle of
electromagnetic radiation
Each as a certain amount of energy
related to its frequency such that
E = hf
h is Planck’s constant
f is frequency
Physical Properties of Matter
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Temperature – measure of heat intensity;
average amount of molecular movement in a
substance (Fahrenheit and Celsius)
Weight – the force of gravity exerted on a
substance; differs in the universe w = mg
Mass – amount of matter an object has; a
constant
Density – mass of an object in relation to its
volume; it remains the same regardless of the
size of an object (intensive property)
Physical Properties of Matter
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Refractive Index – the ratio of the speed
of light in a vacuum to its speed of light in
a given medium
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RI = velocity of light in vacuum
velocity of light in medium
Refractive index of water is 1.333 thus it
travels 1.333 times as fast in a vacuum as it
does in water
Crystalline solids may have two refractive
indeces (birefringence) while amorphous
solids do not.
Forensic Analysis of Glass
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Composition – amorphous solid of silicon
mixed with other metal oxides
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Soda-lime glass – windows and bottles
Sodium, calcium, magnesium, aluminum are
common metal oxides added
Borosilicates – boron is added to make heatresistance (headlights, Pyrex)
Tempered – strengthened by heat/cool stress
during manufacturing (car side windows)
Laminated – two sheets of glass with plastic
sheet in between (windshields)
Forensic Glass Analysis
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Comparing fragments
Flotation
Refractive index comparison
Classification of glass samples
Glass fractures
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Concentric fracture
Radial fracture
3R rule: Radial fractures form Right angles on the
Reverse side of the force
Collection and preservation of glass evidence
Trace Evidence
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transferred between individuals and
objects during the commission of a crime
Often can corroborate other evidence
Generally class characteristics and not
individual characteristics
Examples: Hair, Fibers, Metals, Paints, Soil,
Glass, and Impressions
Hair Morphology
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Hair follicle, Shaft, Tip end
Cuticle – outer layer, keratinized scale
structure, animal specific
Cortex – main body of hair shaft, color
Medulla – center; cellular column,
medullary index
Hair Growth
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Root is where the hair grows
Anagen phase – initial growth phase, lasts up to
six yrs, root attached to follicle, provides
follicular tag; grows 1 cm per month
Catagen phase – decreasing growth rate, 2-3
wks,
Telogen phase – growth stops, 2-6 months, hair
is shed
Forensics of Hair
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Identification of animal hair
Comparison of human hairs
Color, length, diameter, medulla structure, cortex
pigments
Microscopic examination is presumptive while
DNA is confirmative
Standard/reference samples
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50 full length head hairs
24 full length public hairs
From hair, can we learn about…
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Body area origin? yes
Racial origin? yes
Age or sex? No
Forcibly removed? Yes
Individualize human hair? Yes, through
DNA
Fibers
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Natural fibers from animal or plant
sources, cotton
Manufactured fibers from natural or
synthetic polymers
Types of Fibers
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Acetate
Acrylic
Aramid
Bicomponent
Lyocell
Melamine
Modacrylic
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Nylon
Olefin
Polyester
PBI
Rayon
Spandex
Forensics of Fibers
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Microscopic examination of fibers
Color, diameter, striation, pitting
Microspectrophotometry
Chemical composition
Birefringerence
Infrared absorption
Metals
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Trace elements of less than 1%
composition may provide invisible markers
to help in comparisons
Composition of Paint
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Electrocoat primer
Primer surfacer
Basecoat
Clearcoat
Forensics of Paint
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Microscopic
Pyrolysis gas chromatography
Infrared spectrophotometry
Emission spectrophotometry
Paint Data Query (PDQ)
Soil
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Examination of soil content include natural and
manufactured objects in it.
Microscopic examination can identify minerals by
color, shape, refractive index, or birefringence
Density-gradient tubes determine the density
distribution of soil
Spectrophotometry determines soil settling
Bullet and Cartridge Comparisons
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Gum barrel has grooves (rifling) to help
spin the bullet
The diameter of the bore of the barrel is
called the caliber
Various processes of creating rifling can be
used to identify maker of gun
Various processes of creating rifling all
create striations that are unique to gun
Bullet and Cartridge Comparisons
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The striation markings on the bullet can provide
individual characteristics for gun comparison.
FBI maintains a record known as the General
Rifling Characteristics File which lists class
characteristics for known weapons.
Shotguns do not have rifling thus no striations.
The size of the shotgun is referred to as its
gauge.
Cartridge casings also will have markings on it
from the loading, firing or ejection mechanisms.
Automated Firearms Search
Systems
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DRUGFIRE developed by the FBI focused
on the casings
IBIS developed by the ATF focused more
on the bullets
NIBIN incorporates both systems
Ballistic Fingerprinting of guns prior to sale
is being discussed.
Gunpowder Residues
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Incomplete combusted gun powder residue deposited on
target allows for distance determination
Without the weapon, generalizations can be made about
the distance
 Melted clothing fibers and star shape hole – less than
1 inch away
 black smoke around hole – 12-18 inches away
 scattered specks around hole – 25 inches away
 only bullet wipe – greater 3 feet away
Shotgun distances can be determined by the scatter of
the pellets (1 inch scatter for
every yard of distance)
Gunpowder residues can be developed
on clothing using chemical tests
Gun Shot Residue
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Primer residue is propelled backward and
deposited on the hand of the shooter
GSR can be detected chemically or
microscopically (SEM)
Ballistics
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A bullet encounters four ballistic regimens
Internal ballistics – primer and propellents
Internal ballistics – movement throught
the barrel; grooves and lands
External ballistics – gravity, Coriolis effect,
aerodynamic, Magnus force, yaw, ballistic
coefficient
Terminal ballistics – hitting the target;
yaw, lacerations, crushing, cavitation
Terminal ballistics
Toolmarks
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Any impression, cut, gouge or abrasion caused
by a tool coming into contact with another
object.
Typically, impression comes from prying action
of tool
Damage, wear patterns as well as manufactured
striations may be left in impression
Silicone casting material is used to create a cast
of the impression for comparison with the
suspected tool.
The impression may also contain trace evidence
such as paint from the tool.
Other Impressions
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Shoe print
Tire prints
Fabric impressions
Impression must be photographed, lifted or
casted, and then compared
Shoe and tire marks in soft earth can be casted
Bitemarks can be casted
Dust left on a floor can be lifted electrostatically
Bloody footwear impression can be enhanced
with chemical treatment.
Document Examination
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Questioned document – any document
which some issue has been raised or that
is the subject of an investigation.
Requires knowledge in microscopy,
photography, and chromatography
Handwriting Comparisons
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General style
Variations in handwriting
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Angle, slope, speed, pressure, spacing, dimension,
connections, pen movement, writing skill, finger
dexterity, margins, spacing, alignment, phraseology,
spelling, grammar
Exemplars – an authentic sample for comparison
purposes, such as handwriting
Natural variations – normal deviations found
between repeated specimens of an individual’s
handwriting
Typescript Comparisons
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Photocopier – random debris on the glass
cover or machinery can be reproduced
Fax machines – transmitting terminal
identifier (TTI) font can be altered
Computer printers – letter shape and
toners may vary
Typewriters – type face alignment will vary
Alterations in Documents
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Erasure – removal of writing or printing from a
document normally accomplished by either
chemical or abrasive means
Obliterations – blotting out or smearing over
writing to make it unreadable
Charred document – darkened and brittle by
exposure to fire or heat
Indented Writings – impressions left on paper
positioned under a piece of paper that has been
written on
Ink and paper Comparisons
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Chromatography and ink tags allow
comparison of ink
Cellulose content, weight, color,
watermarks, and additives vary in paper