Trace Evidence (Part I)

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Trace Evidence(Part I)
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Summary

• Microscopic Analysis
• Types of Trace Evidence

• Glass
• Hair (fur)
• Fibers

• Paint
• Soil
• Gunshot Residue

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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.
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Microscopic Analysis

• Who analyzes trace evidence?

• Labs sections are arranged differently in
  different labs.

• Different evidence could be assigned to different
  sections (hairbiology, fiberchemistry)

• Some labs have trace or microscopy sections.

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Trace gt Microscopes

• Using the Microscope

• Microscope magnifies sample.

• Also can look at more detailed traits of samples.

• Many different types of microscopes are used.

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Trace gt 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

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Trace gt Microscopes

• Microscope terminology

• Reflected light vs. Transmitted light
• Refected bounces off object (normal light)
• Transmitted passes through object

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Trace gt Microscopes

• Stereo Binocular Microscope

• Look at 3-D objects
• Usually look at surface
• Usually use reflected light

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Trace gt Microscopes

• Stereo Binocular Microscope

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

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Trace gt Microscopes

• Compound Microscope

• Can be binocular or one ocular
• Look at transparent object on prepared slide
• Look through object
• Use transmitted light

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Trace gt Microscopes

• Compound Microscope

• Analyze fibers, hair, glass for optical traits.
• Biological samples for sperm cells

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Trace gt Microscopes

• Key terms

• Ocular lens Lens in the eyepiece

• Objective lens Lens above the sample

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Trace gt Microscopes

• Key terms

• Ocular lens Lens in the eyepiece

• Objective lens Lens above the sample

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Trace gt 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

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Trace gt Microscopes

• Polarized Light Microscope (PLM)

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Trace gt 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.

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Trace gt Microscopes

• Comparison Microscope

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Trace gt Microscopes

• Comparison Microscope

• An optical bridge allows viewing of evidence side
  by side.

• Useful for comparing bullets, fibers, hair

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Summary of Trace

• Why is trace evidence useful?

• Collecting trace evidence

Types trace evidence

• Glass
• Hair (fur)
• Fibers

• Paint
• Soil
• Gunshot Residue

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Trace
Why is Trace Evidence Useful?
Location
Object
Suspect
Victim

• Can link objects and people.

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Trace
Object
Trace evidence on hammer may include
Blood/Tissue from Victim
Blood/Fingerprints from Suspect
Fibers from Rug in van
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Trace
Location
Trace evidence on rug may include
Blood/Tissue from Victim
Blood from Suspect
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Trace
Victim
Trace evidence on Victim may include
Blood/Semen from Suspect
Fibers from Rug in van
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Trace
Suspect
Trace evidence on suspect may include
Blood/Tissue from Victim
Fibers from Rug in van
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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.
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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
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Trace gt 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?
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Trace gt 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.
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Trace gt Collection
NEVER package known material with evidence.
Example in book
Suspects 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!
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Trace gt Collection

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

1. Visual Inspection 2. Tape Lift 3. Vacuum
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Trace gt Collection
Visible Inspection

• Use naked eye or hand lens.
• Evidence removed and packaged for later analysis
• Use bright light and forceps to collect.

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Trace gt 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.

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Trace gt 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.

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Trace gt Collection

• Vacuuming

• Nozzle should be short and transparent.
• Debris is collected on a filter or membrane

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Trace gt 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.

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Trace gt 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 suspects van
Soil found on Suspects shoe Source Matches soil
at crime scene
Blood found on suspects couch Source Matches
blood of victim
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Trace gt 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.
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Trace gt 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 Scientists main objective is to
give the jury an idea about how rare the category
is.
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Trace gt Analysis

• Classifying Evidence

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