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.


• 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


• 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


• 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.


• 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


• 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


• 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


• 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 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


• 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.


• 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.


• 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


• 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


• 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…


• 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


• 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.


• 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


• 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


• 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


• 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).


• 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


• 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.


• 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.


• 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.


• 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.


• 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


• 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.


• 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

trace evidence pg 54 trace evidence – generic term referring to minute physical evidence that may...

Documents

polarizing microscope

comparison microscope

biological microscope

standard lab microscope

hair fiberspgs

standard laboratory

microscope slide reaction

hair follicle