Hair, Fiber, and Paints

The study of important trace evidence

Morphology and Structure of Hair

Hair Morphology Con’t

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

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

Morphology: Medulla canal like structure of cells that runs

through the center of the cortex

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

Medulla of Different Species

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

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

Morphology: Root

Human hair grows in three developmental stages: anagen, catagen, and telogen phases

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

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

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

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?

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

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)

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

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

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

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

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

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.

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

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.

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.