1. Trace Evidence (Part I)

2. Summary Microscopic Analysis Types of Trace Evidence Glass Hair (fur)
Fibers
Paint
Soil
Gunshot Residue

3. 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.

4. 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.

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

6. Microscope terminology
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

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

8. Stereo Binocular Microscope
Trace > Microscopes
Stereo Binocular Microscope
Look at 3-D objects
Usually look at surface
Usually use reflected light

9. Stereo Binocular Microscope
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.

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

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

12. Key terms: Ocular lens Lens in the eyepiece
Trace > Microscopes
Key terms:
Ocular lens Lens in the eyepiece
Objective lens Lens above the sample

13. Key terms: Ocular lens Lens in the eyepiece
Trace > Microscopes
Key terms:
Ocular lens Lens in the eyepiece
Objective lens Lens above the sample

14. 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

15. Polarized Light Microscope (PLM)
Trace > Microscopes
Polarized Light Microscope (PLM)

16. 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.

17. Comparison Microscope
Trace > Microscopes
Comparison Microscope

18. Comparison Microscope
Trace > Microscopes
Comparison Microscope
An optical bridge allows viewing of evidence side by side.
Useful for comparing bullets, fibers, hair…

19. Summary of Trace Why is trace evidence useful?
Collecting trace evidence
Types trace evidence:
Glass
Hair (fur)
Fibers
Paint
Soil
Gunshot Residue

20. Can link objects and people.
Trace
Why is Trace Evidence Useful?
Object
Location
Victim
Suspect
Can link objects and people.

21. Object Trace evidence on hammer may include: Trace
Blood/Tissue from Victim
Blood/Fingerprints from Suspect
Fibers from Rug in van

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

23. Victim Trace evidence on Victim may include: Trace
Blood/Semen from Suspect
Fibers from Rug in van

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

25. 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.

26. 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

27. Collect trace or entire object?
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?

28. Considerations before packaging entire object:
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.

29. Suspect’s clothes had tar on the knees of pants.
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!

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

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

32. Visible Inspection (Packaging)
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.

33. Tape Lift Clear tape is used.
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.

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

35. Vacuuming Small area is vacuumed. (Filters changed frequently)
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.

36. What is the purpose of analysis?
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

37. 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.

38. Classifying Evidence:
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.

39. Classifying Evidence:
Trace > Analysis
Classifying Evidence:
If physical properties differ, they did not come from the same source.
Exclusion is possible.