Trace Evidence pg 54 Trace evidence – generic term referring to minute physical evidence that may...
-
Upload
june-burke -
Category
Documents
-
view
221 -
download
0
Transcript of Trace Evidence pg 54 Trace evidence – generic term referring to minute physical evidence that may...
Trace Evidencepg 54
• Trace evidence – generic term referring to minute physical evidence that may be transferred (Locard’s Exchange Principle)
• Most common types of trace evidence – hair & fibers, soil & botanicals, glass, paint, and other minute evidence
Microscopespgs 54-59
• Standard Laboratory Microscope – a.k.a. biological microscope; used for serological examination & other specific forensic uses
• Polarized Light Microscope – a.k.a. “polarizing” microscope; most versatile & powerful tool available for analyzing trace evidence. Like standard lab microscope with polarizing light filters added.
Microscopespgs 54-59
• Filters create a dark background to let light pass through trace evidence like crystals.
• Used for examining minerals in soil, synthetic fibers, glass & other birefringent (light passing through them being doubly refracted) materials
Microscopespgs 54-59
• Stereomicroscope – workhorse of crime lab, most versatile & widely used instrument
• Consists of two side-by-side low-power compound microscopes in a common housing, binocular eyepiece (one microscope for each eye) creating “stereoscopic” view with 3-D appearance
Microscopespgs 54-59
• Permits viewing of large objects such as mineral specimens, large documents or other items with a sizable surface area.
• Used in every branch of criminalistics
• Frequently the first instrument used in preliminary evaluation of evidence or for sorting or selecting items for further examination.
Microscopespgs 54-59
• Comparison microscope – essentially a pair of precisely similar compound microscopes arranged so when a viewer looks through the eyepiece, two images are juxtaposed (side-by-side) in a common field plane and can be superimposed onto each other for direct comparison as with two bullets or two shell cases
Microscopespgs 54-59
• Scanning Electron Microscope (SEM) – wide range of magnifications exceeding over 50,000 times. Specimen cannot be alive
• Demonstrates small differences such as in soil or showing common similarity among soil samples
Microscopespgs 54-59
• Microspectrophotometer – two instruments in one: standard lab microscope combined with computerized spectrophotometer (helps identify substance by directing beam of light at it & obtaining its absorption spectrum).
• Suited to examine tiny particles, can better ID & compare types of chemicals from which materials are manufactured.
Microtrace Examining Techniques pgs 54-59
• Microchemical tests – use pipette to add small amount of test chemical (reagent) to questioned material on microscope slide; reaction is observed through microscope.
• Useful for testing very small amounts of material & can confirm visual identification
Microscopespgs 54-59
• Photomicrography – taking photographs through a microscope.
• Permits criminalist to document work such as photographing particle before adding a reagent in case it is altered or destroyed.
• Enlarged photomicrographs can also be used as courtroom exhibits.
Hair & Fiberspgs 59-65
• Hair & fibers are superficially similar but are quite different.
• Hair is an appendage of human or animal skin, growing our of an organ known as a hair follicle.
• Fibers are strands of natural or manufactured material.
Hair & Fiberspgs 59-65
Hair & Fiberspgs 59-65
• Hair represents class or general evidence rather than individual evidence.
• DNA research which involves testing of the root of the hair may make near-individualization possible
• Hair is valuable evidence: it resists chemical decomposition & retains its structural features for a long time
Hair & Fiberspgs 59-65
• Cuticle – scaly exterior portion – sufficiently distinctive to make it helpful for identification by species (human vs dog)
• Cortex – intermediate (middle) layer that gives its color
• Medulla – central canal that may be classified as continuous, interrupted, fragmented or entirely absent.
Hair & Fiberspgs 59-65
Hair & Fiberspgs 59-65
Hair & Fiberspgs 59-65
Hair & Fiberspgs 59-65
Hair & Fiberspgs 59-65
Hair & Fiberspgs 59-65
Hair & Fiberspgs 59-65
Hair & Fiberspgs 59-65
Hair & Fiberspgs 59-65
• An ordinary pencil can be used as a model for the structures of hair (see page 61)
• Remember, the human cuticle is colorless.
• Human hair may reveal donor’s race, part of the body that yielded the hair, whether it was removed forcibly, cut & by what instrument, if it was dyed or bleached, or subjected to force.
Hair & Fiberspgs 59-65
• Tentative judgment as to sex & age of donor may sometimes be determined
• Certain drugs or chemicals can be determined by analyzing different segments of a single hair donor since it grows about 1 mm/day
• Other trace evidence may be found adhering to hairs – blood, various types of dust, minute (small) fibers & paint
Hair & Fiberspgs 59-65
• A comparison microscope is an essential instrument for examining known & questioned hair samples in the lab
• Characteristics of human hair are:– Color -Reflectivity -Length– Diameter -Spacial configuration– Tip -Base -Root– Cross section -Pigment
Hair & Fiberspgs 59-65
• Characteristics of human hair cont.:– Medulla -Cortical Fusi– Cortical Cells -Birefringence– Cosmetic Treatment -Cuticle– Scales
• After human hair analysis, usually one of three findings can be expected…
Hair & Fiberspgs 59-65
• 1-the questioned sample is consistent with a known one in terms of microscopic characteristics & either originated from the same individual or from another whose hair has the same qualities
• 2-the samples are NOT similar in their characteristics & therefore did not come from the same person
Hair & Fiberspgs 59-65
• 3-the examiner is unable to reach a conclusion. “A negative finding may serve to disprove erroneous theories. A positive finding is of importance when it is correlated with other newly discovered facts.”
• Hair discovered at the crime scene should be collected carefully by using tweezers or forceps & should be placed in snap-on top bottles.
Hair & Fiberspgs 59-65
• In rape cases, medical personnel should collect samples using a clean comb to remove foreign hair from the pubic area & taking samples from the victim for controls
• Hair differs in different parts of the body. It’s necessary to remove samples from each area relevant to a case especially head & pubic areas with which most forensic comparisons of hair are involved
Hair & Fiberspgs 59-65
• Forensic authorities disagree over how many strands should be collected, but somewhere between 30 & 50 from each location should be sufficient.
• Strands from different body areas – obtained by combing or cutting if necessary – should be packaged separately and properly labeled
Hair & Fiberspgs 59-65
• Fibers & cloth fragments offer much greater evidential value because they incorporate numerous variables:– Number of fibers in each strand, diameter of
strands & fibers, direction & number of twists, type of weave, dye content, presence of any foreign material that may be adhered to fibers or embedded among them
Hair & Fiberspgs 59-65
• Fibers may be classified into two broad categories – natural and manufactured.
• Natural fibers – animal (wool, silk, animal hair fibers such as mohair), vegetable (seed fibers such as cotton; bast fibers such as flax, hemp, & jute; leaf fibers including manila), & mineral (asbestos).
Hair & Fiberspgs 59-65
• Man-made (manufactured) fibers include natural-polymer fibers (rubber, rayon & cellulose ester), synthetic-polymer fibers (polyvinyl derivatives, polyurethane, & polyester), & other fibers (carbon, glass, metal & ceramic).
• Fibers from hair coverings are identified by their color & structural characteristics under the microscope
Hair & Fiberspgs 59-65
• Plant fibers – distinct morphology/structure under microscope makes them easy to ID
• Cotton fibers have a twisted, ribbon-like form. Linen fibers resemble knobby tubes.
• Cotton is the most prevalent of plant fibers. Alone its evidential value is weak, but presence of dye adds a significant measure of distinctiveness.
Hair & Fiberspgs 59-65
• For all but the most commonly found natural fibers specialized techniques may be required.
• Maceration is necessary for examining bast & leaf fibers that occur in bundles.
• Man-made fibers generally show fewer identifying features than natural fibers. So several methods of identification are used.
Hair & Fiberspgs 59-65
• Infrared spectroscopy – differentiating several types of nylon fibers
• Solubility tests – differentiate between certain other fiber types
• Cross sectioning – cutting in half (perpendicular to long axis)
• Standard microscopic examination used to determine optical properties such as birefringence & refractive indices.
Hair & Fiberspgs 59-65
• A fiber’s dye may be analyzed by thin-layer chromatography where dye components soak through an absorbing medium to separate them.
• Criminalists seek to learn type of fiber, natural or man-made; generic type & subtype, color & shade, expected use, manufacturer, period of manufacture, relative commonness or rarity of fiber.
Hair & Fiberspgs 59-65
• That transfer of fibers occurs is not just a theory (Locard’s Exchange Principle) but has been established by scientific tests.
• 80% of fibers can be expected to be lost in 4 hours with just 5% – 10% remaining at the end of 24 hours.
• Important to collect evidence from both complainants & suspects ASAP following alleged offense
Hair & Fiberspgs 59-65
• Fibers may be retrieved by tape lifts using clear adhesive tape pressed to the surface of the item or by vacuuming.
• Brushing & shaking aren’t recommended.
• Fibers can be collected on a sheet of clean, pre-folded paper that is then placed in an evidence bag or envelope & properly labeled.
Hair & Fiberspgs 59-65
• Value of fiber evidence demonstrated in Atlanta Child Murders case study.
• Proper collection & identification of distinctive series of fibers o the clothing of victims provided considerable information on the killer’s environment long before his arrest provided confirmation of the accuracy of the fiber evidence