Trace Evidence (PowerPoint) by mikeholy

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									Trace Evidence
(Part I)
Summary

   Microscopic Analysis
   Types of Trace Evidence
       Glass           Paint
       Hair (fur)      Soil
       Fibers          Gunshot Residue
What is Trace Evidence?
Loosely defined, trace evidence is small
  evidence used to link victims, suspects,
  objects, and crime scenes.
Trace analysis uses microscopes and
  other instrumentation.
Other sections (Drugs, Firearms) also use
 microscopes.
Microscopic Analysis
Who analyzes trace evidence?
   Labs sections are arranged differently in
    different labs.
   Different evidence could be assigned to
    different sections (hair:biology, fiber:chemistry)
   Some labs have trace or microscopy
    sections.
Trace > Microscopes


Using the Microscope

   Microscope magnifies sample.
   Also can look at more
    detailed traits of samples.
   Many different types of
    microscopes are used.
Trace > Microscopes


Microscope terminology

3-D object vs. Microscope slide
    3-D object: put any object under the mic
    Slide: requires preparation


Surface analysis vs. Look through object
    Surface: Looking at surface of object (normal)
    Or can shine light through transparent sample
Trace > Microscopes


Microscope terminology

Reflected light vs. Transmitted light
    Refected: bounces off object (normal light)
    Transmitted: passes through object
Trace > Microscopes


Stereo Binocular Microscope

   Look at 3-D objects
   Usually look at surface
   Usually use
    reflected light
Trace > Microscopes


Stereo Binocular Microscope

   Preliminary search of
    objects (clothes) for
    small fibers, hairs, etc.
   Look at paint chip
    layers, measure fibers,
    bullet striations, etc.
Trace > Microscopes


Compound Microscope

   Can be binocular or
    one ocular
   Look at transparent
    object on prepared slide
   Look through object
   Use transmitted light
Trace > Microscopes


Compound Microscope


   Analyze fibers, hair, glass
    for optical traits.
   Biological samples for
    sperm cells
Trace > Microscopes



Key terms:

   Ocular lens
    Lens in the eyepiece

   Objective lens
    Lens above the sample
Trace > Microscopes



Key terms:

   Ocular lens
    Lens in the eyepiece

   Objective lens
    Lens above the sample
Trace > Microscopes


Key terms:

   Micrometer - “ruler” in the eyepiece, allows for
    measurement of sample.
   Magnification - Amount the object is enlarged

   Working Distance - Distance between the
    object and objective lens
Trace > Microscopes


Polarized Light Microscope (PLM)
Trace > Microscopes


Polarized Light Microscope (PLM)

   Takes advantage of the optical
    properties of glass, crystals
    (chemicals), and fibers.

   Sample absorbs light differently
    depending on its orientation in
    polarized light.
Trace > Microscopes


Comparison Microscope
Trace > Microscopes


Comparison Microscope

   An optical bridge allows
    viewing of evidence
    side by side.
   Useful for comparing
    bullets, fibers, hair…
Summary of Trace
   Why is trace evidence useful?
   Collecting trace evidence

Types trace evidence:
     Glass          Paint
     Hair (fur)     Soil
     Fibers         Gunshot Residue
Trace

Why is Trace Evidence Useful?




  Object         Location
                            Victim   Suspect

        Can link objects and people.
Trace

Object

Trace evidence on
hammer may include:

 Blood/Tissue from Victim
 Blood/Fingerprints from Suspect
 Fibers from Rug in van
Trace


Location

Trace evidence on rug
may include:
 Blood/Tissue from Victim
 Blood from Suspect
Trace


Victim

 Trace evidence on
 Victim may include:

 Blood/Semen from Suspect
 Fibers from Rug in van
Trace


Suspect

Trace evidence on
suspect may include:
 Blood/Tissue from Victim
 Fibers from Rug in van
 Trace


 How is Trace Evidence Transferred?

  Locard Exchange Principle: Whenever
    there is contact between two objects,
    they will leave or pick up debris from the
    other object.
  During a crime, there is always be a transfer of evidence.


The difficulty is finding & collecting this evidence.
Collecting Trace Evidence
Who collects the evidence?
   Police Officer
   Crime Scene Investigator
   Forensic Scientist

Depends on the state/community
Often one person to ensure consistency
 of labeling
Trace > Collecting


Collect trace or entire object?

Suppose a glove appears to have
glass, fibers and blood on it.
Should the glass, fibers and
blood be removed and
packaged separately?

Should the entire glove be packaged?
Trace > Collection


Considerations before packaging entire object:

   Object may be too large or difficult to move
   Trace evidence may fall off item during transport.
   Trace Evidence may be transferred to different,
    irrelevant area of object.

If packaging object, package objects separately.

Prevents trace being transferred to other objects.
Trace > Collection

NEVER package known material with evidence.

Example in book:
 Suspect’s clothes had tar on the knees of pants.
    His clothes were collected at the station.
 Tar was collected at the crime scene.

The clothes and the tar were packaged in
 the same bag.
                                         Whoops!
Trace > Collection



These 3 methods can be done at the
 crime scene or in the crime lab.

   1. Visual Inspection
   2. Tape Lift
   3. Vacuum
Trace > Collection


Visible Inspection
 Use naked eye or hand lens.
 Evidence removed and packaged for later
  analysis
 Use bright light and forceps to collect.
Trace > Collection



Visible Inspection (Packaging)

   Small paper envelopes are bad (Holes
    allow small objects to escape).
   Use small plastic bags, glass vial or paper
    using a druggist fold.
   Double package. Label each package.
Trace > Collection


Tape Lift
 Clear tape is used.
 Repeatedly apply tape to small area until
  most of the stickiness is gone.
 Tape is folded back upon itself, taped to a
  glass slide or taped to a piece of plastic.
 Put in separate labeled container.
    Be sure to document specific area covered.
Trace > Collection


Vacuuming

   Nozzle should be
    short and
    transparent.
   Debris is collected
    on a filter or
    membrane
Trace > Collection


Vacuuming
 Small area is vacuumed. (Filters changed
   frequently)
 Filters packaged in separate labeled
  container. (Be sure to document specific area
   covered)
 Most improperly used method because it
  often results in the collection of a lot of
  irrelevant material.
Trace > Analysis


What is the purpose of analysis?
To identify the source of the collected evidence.

Fiber recovered from victim.
Source: Matches fibers from rug in suspect’s van
Soil found on Suspect’s shoe
Source: Matches soil at crime scene
Blood found on suspect’s couch
Source: Matches blood of victim
Trace > Analysis


The Catch:
  With trace evidence, an investigator usually
   cannot say that one piece of evidence
   definitely originated from a specific item.

 The investigator can only tell the jury what
   similarities were found and give them an idea
   of how rare those similarities are.
Trace > Analysis

Classifying Evidence:
Most trace evidence is classified using
 class characteristics (color, shape, refractive
   index, etc.)
 When examining class characteristics,
  absolute identification is not possible.
 The Forensic Scientist’s main objective is
  to give the jury an idea about how rare
  the category is.
Trace > Analysis




Classifying Evidence:

 If physical properties differ, they did not come
    from the same source.

 Exclusion is possible.

								
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