Intro to Forensic Science-chapter 13 Hairs, fibers, and paint

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cuticle

the scale structure covering the exterior of the hair; the scales always point towards the tip of the hair; the scale pattern is useful in species identfication

hair

is an appendage of the skin that grows out of an organ known as the hair follicle; it extends from its root (or bulb) embedded in the follicle, continues into a shaft, and terminates at a tip end; it is not possible to individualize a human hair to any single head or body but it is still physical evidence; can be used for placing a person at a crime scene;

hair shaft

composed of three layers: the cuticle, cortex, and medulla; mDNA can be extracted from the hair shaft

cortex

main body of the hair shaft; its major forensic importance is the fact that it is embedded with pigment granules that impart hair with color; the color, shape and distribution of those granules proved the criminalist with important points of comparison among hairs of different individuals

medulla

is a cellular column running through the center of the hair; human hair generally has a medullary index of less than 1/3 ; the hair of most animals has an index of 1/2 or greater; may be continuous, interrupted, fragmented, or absent; the presence of the medulla varies from individual to individual and even among hairs of a given individual; it also has different shapes, depending the species.

medullary index

measures the diameter of the medulla relative to the diameter of the hair shaft

root

provide the tools to produce hair and continue its growth

follicular tag

translucent tissue surrounding the hair's shaft near the root that may stay on hair after it's been pulled from the head; using DNA on this the hair may be individualized

Comparison microscope

used for comparing the morphological characteristics of hair; these tend to be subjective and highly dependent on the skills and integrity of the analyst; as a rule all positive microscopical hair comparisons must be confirmed by DNA analysis

animal hair identification

scale structure, medullary index, and medullary shape; if a medulla exhibits a patterned shape

Factors for comparing human hair

the presence or absence of a medulla; the distribution, shape, and color intensity of the pigment granules present in the cortex; matching the color, length and diameter; only head hair or public hair; 50 full length head hairs from all areas of the scalp for reference sample; minimum of 2 dozen full length public hairs; hair sample are also collected from victims during an autopsy

anagen phase

initial growth phase during which the hair follicle actively produces hair; may last up to six years; probability of detecting DNA is at this stage

catagen phase

a transition stage between the anagen and telogen phases of hair growth; hair continues to grow but at a decreasing rate; can last anywhere from two to three weeks; roots typically take on an elongated appearance

telogen phase

the final growth phase in which hair naturally falls out of the skin; the root takes on a club-shaped appearance; over two to six months the hair is pushed out of the follicle causing the hair to be naturally shed

hair length

3 inches of grown hair can tell if a person has done drugs in the last ten months; hair grows at an avg of 1 cm per month

natural fibers

derived in whole from animal or plant sources; examples: wool, mohair, cashmere, furs, and cotton; animal fibers constitute most of the natural fibers encountered in crime labs; most prevalent plant fiber is cotton; the microscopic view of cotton fiber shows a ribbon like shape with twists at irregular intervals

manufactured fibers

Rayon 1911, nylon 1939; regenerated and synthetic fibers; the fibers are typically made by forcing the polymeric material thought the holes of a spinneret

polymers

aka macromolecules are synthetic fibers composed of large numbers of atoms arranged in repeating units known as monomers

regenerated fibers

made from natural raw materials and include rayon, acetate, and triacetate; produced from regenerated cellulose

synthetic fibers

made solely from synthetic chemical and include nylons, polyesters, and acrylics; made from polymers; possess the physical property of birefringence because they are crystalline

Comparison microscope

used on fibers; morphological features that could be important: lengthwise striations on the surface of the fiber, the presence of delustering particles that reduce shine, the cross sectional shape of the fiber

methods for fiber comparison

(1) the visible light micro-spectrophotometer is a convenient way to compare the colors of fibers through spectral patterns;(2) a more detailed analysis of the fiber's dye composition can be obtained through a chromatographic separation;(3) Infrared spectrophotometry is a rapid and reliable method for identifying the generic class of fibers, as does the polarizing microscope.Depending on the class of fiber, each polarized plane of light will have a characteristic index of refraction.

Collection of fibers

place it in a small sheet of paper, fold and label the paper and place the paper packet inside another container

Paint

dries into a hard film that is described as consisting of pigments and additives suspended in the binder

automobile paint

one of the most common types of paint examined in the lab; auto makers normally apply a variety of coatings to the body of an automobile; these coatings may include: electrocoat primer (provides corrosion resistance), primer surfacer, basecoat, and clearcoat

Paint comparison

two most important components of dried paint are the color and the layer structure; side by side using a stereoscopic microscope for color, surface texture, and color layer sequence; pyrolysis gas chromatography and infrared spectrophotometry are used to distinguish most paint binder formulations; paints can be individualized to a single source only when they have a sufficiently detailed layer structure

collection of paint

paper druggist folds and glass or plastic vials; paint smeared or embedded in garments or objects required the whole item to be packaged and sent to the lab; standard/reference paint must always be collected; tools can have paint samples on them and must be collected also

molecule

two or more atoms held together by chemical bonds

marcromolecule

a molecule with a high molecular mass

monomer

the basic unit of structure from which a polymer is constructed

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