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The study of important trace evidence

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Presentation on theme: "The study of important trace evidence"— Presentation transcript:

1 The study of important trace evidence
Hair, Fiber, and Paints The study of important trace evidence

2 Morphology and Structure of Hair

3 Hair Morphology Con’t

4 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


6 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

7 Morphology: Medulla canal like structure of cells that runs through the center of the cortex

8 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

9 Medulla of Different Species

10 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

11 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

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

13 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 root Root w/ follicular tag

14 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

15 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

16 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?

17 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

18 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)

19 Man-Made Fibers Con’t Regenerated Fibers Synthetic 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

20 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

21 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

22 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

23 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

24 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.

25 Paint Characteristics
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

26 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.

27 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.

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