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Trace Evidence (Part I) Summary Microscopic Analysis Types of Trace Evidence Glass Hair (fur) Fibers Paint Soil Gunshot Residue What is Trace Evidence? Loosely defined, trace evidence is small evidence used to link victims, suspects, objects, and crime scenes.

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Trace Evidence (Part I)


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summary
Summary
  • Microscopic Analysis
  • Types of Trace Evidence
  • Glass
  • Hair (fur)
  • Fibers
  • Paint
  • Soil
  • Gunshot Residue
what is trace evidence
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
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.
slide5

Trace > Microscopes

Using the Microscope

  • Microscope magnifies sample.
  • Also can look at more detailed traits of samples.
  • Many different types of microscopes are used.
slide6

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
slide7

Trace > Microscopes

Microscope terminology

Reflected light vs. Transmitted light

  • Refected: bounces off object (normal light)
  • Transmitted: passes through object
slide8

Trace > Microscopes

Stereo Binocular Microscope

  • Look at 3-D objects
  • Usually look at surface
  • Usually use reflected light
slide9

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.
slide10

Trace > Microscopes

Compound Microscope

  • Can be binocular or one ocular
  • Look at transparent object on prepared slide
  • Look through object
  • Use transmitted light
slide11

Trace > Microscopes

Compound Microscope

  • Analyze fibers, hair, glass for optical traits.
  • Biological samples for sperm cells
slide12

Trace > Microscopes

Key terms:

  • Ocular lens Lens in the eyepiece
  • Objective lens Lens above the sample
slide13

Trace > Microscopes

Key terms:

  • Ocular lens Lens in the eyepiece
  • Objective lens Lens above the sample
slide14

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
slide15

Trace > Microscopes

Polarized Light Microscope (PLM)

slide16

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.
slide17

Trace > Microscopes

Comparison Microscope

slide18

Trace > Microscopes

Comparison Microscope

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

Types trace evidence:

  • Glass
  • Hair (fur)
  • Fibers
  • Paint
  • Soil
  • Gunshot Residue
slide20

Trace

Why is Trace Evidence Useful?

Can link objects and people.

Object

Location

Victim

Suspect

slide21

Trace

Object

Trace evidence on hammer may include:

Blood/Tissue from Victim

Blood/Fingerprints from Suspect

Fibers from Rug in van

slide22

Trace

Location

Trace evidence on rug may include:

Blood/Tissue from Victim

Blood from Suspect

slide23

Trace

Victim

Trace evidence on Victim may include:

Blood/Semen from Suspect

Fibers from Rug in van

slide24

Trace

Suspect

Trace evidence on suspect may include:

Blood/Tissue from Victim

Fibers from Rug in van

slide25

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
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

slide27

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?

slide28

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.

slide29

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!

slide30

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

slide31

Trace > Collection

Visible Inspection

  • Use naked eye or hand lens.
  • Evidence removed and packaged for later analysis
  • Use bright light and forceps to collect.
slide32

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.
slide33

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.

slide34

Trace > Collection

Vacuuming

  • Nozzle should be short and transparent.
  • Debris is collected on a filter or membrane
slide35

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.
slide36

Trace > Analysis

To identify the source of the collected evidence.

What is the purpose of analysis?

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

slide37

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.

slide38

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.

slide39

Trace > Analysis

Classifying Evidence:

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

Exclusion is possible.