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Hair, Fiber, and Paints. Morphology and Structure of Hair. 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: 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
Preserving scale pattern • Since examination of internal structure of hair requires loss of scale pattern, a scale case is made. • Clear nail polish on microscope slide • Hair embedded and allowed to dry before removed.
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: Cortex (cont’d) • In order to examine the cortex of the hair, it is suspended in a liquid with a refractive index similar to that of the hair.
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
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 (Asian) 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 Characteristics do not allow individualization of a human hair to any single head or body • 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 a 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
Hair type • Scalp hair has consistent diameter and uniform distribution of pigment • Pubic hairs have continuous medullae • Beard hairs have triangular cross-sections • Eyebrow hair has decreasing diameter from root to tip
Can the racial origin of hair be determined? • Anthropological terms: Caucasian, Negroid, Mongoloid • Mongoloid has continuous medullae • Caucasian has even distribution of pigment in cortex • Negroid has unevenly distributed pigment.
Root: Human hair grows in three developmental stages: anagen, catagen, and telogen phases • 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, which is a rich source of DNA Anagen hair root Root w/ follicular tag
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 (see text for picture) • Animal sources include sheep (wool), goats (mohair, cashmere) and many other sources
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. (see pic p210) • 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 • Cross sections are generally helpful • 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 • UV-Vis or IR 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. • Modern 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
Tools and Techniques to Aid in Paint Examination • Questioned and known 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. • Typically, 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 Visible spectroscopy • Using these techniques the odds against crime-scene paint originating from another randomly chosen vehicle is approximately 33,000 to one.