Hair, Fiber, and Paints
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Transcript of Hair, Fiber, and Paints
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Hair, Fiber, and Paints
The study of important trace evidence
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Morphology and Structure of Hair
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Hair Morphology Con’t
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Morphology: Cuticle Protective coating made of
overlapping scales, produce a characteristic pattern
Scales always point toward tip of hair Not useful in individualizing human
hair Can be used for species identification
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Morphology: Cortex Made of spindle-shaped cells
aligned in a regular array, parallel to the length of the hair
Embedded with pigment granules that give hair its color
The color, shape and distribution of the granules provide points for forensic comparison
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Morphology: Medulla canal like structure of cells that runs
through the center of the cortex
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Medullary Index Measure of the diameter of the
medulla relative to the diameter of the hair shaft
Usually expressed as a fraction Humans: medullary index < 1/3 Animals: medullary index > 1/2
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Medulla of Different Species
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Forensic Analysis of Medulla Presence of medulla varies quite a bit: even
hair to hair Human head hairs generally have no medulla
or may be fragmented ones; except Mongoloid race whose medulla is usually continuous
Most animals have medulla that is continuous or interrupted
The shape of the medulla can help identify a species
Examples: Most animals and humans: cylindrical Cats: pearl shape Deer: spherical occupying whole hair shaft
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Identification and Comparison of Hair Morphological hair characteristics do not yet allow
individualization of a human hair to any single source Hair when collected with an adequate number of
standards/references can provide strong circumstantial evidence
Scale structure, medullary index, and medullary shape are most often used for hair comparison
Evidential value lies with degree of probability associated with a questioned hair and an particular individual
11 percent of all morphological hair matches are generally found to be non-matches—meaning microscopic hair comparisons are presumptive in nature—must be confirmed by DNA comparisons
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Morphology: Root
Human hair grows in three developmental stages: anagen, catagen, and telogen phases
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Root: Anagen Phase Initial growth phase during which hair follicle
is actively producing hair, phase may last 6 years, root is flame like in appearance
When pulled this root may contain a follicular tag (rich source of DNA)
Anagen hair rootRoot w/ follicular tag
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Root: Catagen Phase A transition phase—hair grows at a
decreasing rate for two to three weeks—elongated appearance as root bulb shrinks and is being pushed out of hair follicle
Catagen hair root
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Root: Telogen phase Hair growth has ended—root takes
on a club-like appearance—during two-six month period, the hair will be pushed out of the follicle causing the hair to shed naturally
Telogen hair root
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Important Forensic Questions Can the body area from which a hair
originated be determined? Can the racial origin of hair be
determined? Can the age and sex of an individual be
determined from a hair sample? Is it possible to determine if a hair was
forcibly removed from the body? Are efforts being made to individualize
human hair?
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Fibers: Natural Derived entirely from animal or plant
sources Most prevalent plant fiber is cotton. Its widespread use has made its evidential
value almost meaningless Cotton has a ribbon-like shape with twists
at regular intervals Animal sources include sheep (wool), goats
(mohair, cashmere) and others
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Man-Made Fibers Fibers derived from either natural
or synthetic polymers The fibers are made by forcing
polymeric material through the holes of a spinneret
Rayon and then nylon were the first two man-made fibers (year 1911)
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Man-Made Fibers Con’t Regenerated Fibers
Made from regenerated cellulose (wood or cotton pulp)
Include such fibers as rayon, acetate, and triacetate
Synthetic Fibers Currently manufactured Made from synthetic chemicals called
polymers Include such fibers as nylons, polyesters, and
acrylics
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Polymers Basic chemical substance of all
synthetic fibers Consist of long chains of repeating
molecules. The repeating molecular units in the polymer are called monomers.
Often referred as macromolecules or “big” molecules
Countless varieties exist
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ID and Comparison of Man-Made Fibers Fabrics that can be fitted together at their torn
edge are easy to match Microscopic comparison of color and diameter Comparison of lengthwise striations and pitting
on the surface of a fiber The shape of the fiber—ex. Wayne Williams case Note: Combined factors of color, size, shape,
microscopic appearance, chemical composition, and dye content make it very unlikely to find two different people wearing identical fabrics
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Tools and Techniques to Aid in Comparing Fibers Light infrared spectrophotometer—
compares colors and chemical composition through spectral patterns
Chromatography—compares dye composition
Refraction—ID’s fiber by refractive index Comparison microscope—reveals shape,
coloring, pitting and striations
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Forensic Examination of Paint Paint evidence is frequently
encountered in hit-and-run and burglary cases
Most examinations consist in comparing two or more paints to establish their origin
Often color, make and model of a vehicle can be determined
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Paint Characteristics Paint spread on a surface will dry into a hard
film consisting of pigments and additives suspended in a binder
The binder provides the support medium for the pigments and additives.
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Paint CharacteristicsModern automotive finishing consists of at least four
coatings: Electrocoat Primer: first layer, electroplated to the car—
provides corrosion resistance—color from black to grey Primer Surface: second layer, smoothes out and hides
any seams on the car—color pigments are used to minimize contrast between primer and topcoats
Basecoat: third layer, provides the basic color and appearance of the car
Clearcoat: final coat, provides great appearance (glossiness) and protection for the car
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Techniques to Aid in Paint Examination Questioned (L) and known (R) specimens are
compared side by side under a stereomicroscope for color, surface texture, and color layer sequence
*Note: Layer sequence is very important evidence: forensic scientists will try to match layers with respect to number and sequence of color
*Note: Layer structure alone will not provide enough information to be individualized to a single source
Chemical analysis of the paint’s pigments and binder composition provides further points of comparison.
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Tools to Aid in Paint Examination Gas chromatography is used to determine
the chemical make-up of the binder material.
Infrared spectrophotometry is also used to determine the binder composition of paint.
Elements of the paint pigments can be identified with a number of techniques, including spectroscopy, neutron activation analysis, and x-ray diffraction
Using these techniques the odds against crime-scene paint originating from another randomly chosen vehicle is approximately 33,000 to one.