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Summary
Microscopic Analysis Types of Trace Evidence
Glass Hair (fur) Fibers
Paint Soil Gunshot Residue
What is Trace Evidence?
Trace analysis uses microscopes and other instrumentation.
Other sections (Drugs, Firearms) also use microscopes.
Loosely defined, trace evidence is small evidence used to link victims, suspects, objects, and crime scenes.
Microscopic Analysis
Who analyzes trace evidence?
Different evidence could be assigned to different sections (hair:biology, fiber:chemistry)
Some labs have trace or microscopy sections.
Labs sections are arranged differently in different labs.
Using the Microscope
Microscope magnifies sample.
Many different types of microscopes are used.
Also can look at more detailed traits of samples.
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:
Magnification - Amount the object is enlarged
Working Distance - Distance between the object and objective lens
Micrometer - “ruler” in the eyepiece, allows for measurement of sample.
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
An optical bridge allows viewing of evidence side by side.
Useful for comparing bullets, fibers, hair…
Trace > Microscopes
Summary of Trace
Why is trace evidence useful?
Collecting trace evidence
Glass Hair (fur) Fibers
Paint Soil Gunshot Residue
Types trace evidence:
Object
Trace evidence on hammer may include:
Blood/Tissue from Victim
Blood/Fingerprints 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.
Trace
Collecting Trace Evidence
Who collects the evidence?
Depends on the state/community
Police Officer Crime Scene Investigator Forensic Scientist
Often one person to ensure consistency of labeling
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 > Collecting
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
Use naked eye or hand lens. Evidence removed and packaged for later
analysis Use bright light and forceps to collect.
Trace > Collection
Visible Inspection
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
Visible Inspection (Packaging)
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
Tape Lift
Vacuuming
Nozzle should be short and transparent.
Debris is collected on a filter or membrane
Trace > Collection
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.
Vacuuming
Trace > Collection
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 shoeSource: Matches soil at crime scene
Blood found on suspect’s couchSource: 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