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Trace Evidence

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Trace Evidence Trace evidence- small pieces of material Debris from mass produced materials Abundant in nature Examples of Trace Evidence Hair Fibers Paint Glass ... – PowerPoint PPT presentation

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Title: Trace Evidence


1
Trace Evidence
2
  • Trace evidence- small pieces of material
  • Debris from mass produced materials
  • Abundant in nature

3
Examples of Trace Evidence
  • Hair
  • Fibers
  • Paint
  • Glass
  • Gasoline
  • Soil
  • Dust

4
How is trace evidence collected?
  • Vacuum
  • Tape lifts
  • Picking
  • Combing
  • Scraping

5
Types of Evidence
  • Class evidence
  • can be associated with a general group
  • Ex. Blood type
  • Ex. Print from a Size 7 Nike sneaker
  • Individual evidence
  • can be associated with a single source
  • Ex. DNA typing
  • Ex. Size 7 Nike sneaker w/ specific wear
    pattern

6
Problems w/ trace evidence
  • 1. Many samples are VERY similar because
  • Many materials are made through mass production
  • Tight quality control in the factories means
    there is little variation.
  • 2. Mobility of criminals
  • - Makes it difficult to trace the evidence back
    to a specific source.

7
  • 3) Trace evidence is least likely to be
    individualized.
  • Exception Hair can be individualized if DNA can
    be isolated from the root

8
Locards principle
  • When two objects come into contact, material is
    exchanged.
  • How much material is transferred is affected by
  • Intensity of contact
  • Duration of contact
  • Type of surfaces

9
Questions to ask about trace evidence
  • 1. What is it?
  • 2. Man-made or natural?
  • 3. What is its source?
  • 4. How common is it?
  • 5. Can it be identified to a single source?

10
Equipment
  • Stereomicroscope 10-60X
  • Compound microscope 40-1000X
  • Comparison microscopes
  • Phase contrast microscope 100-1000X
  • Better resolution
  • Scanning electron microscope (100,000X)
  • Ex. Layered paint
  • FTIR- Fourier Transform infra Red
    Spectrophotometer
  • GC- Gas chromatograph (ex. Gasoline)

11
Identification process
  • Compare physical and chemical properties
  • Look at the probabilities that each piece of
    evidence could be present
  • Case Study Wayne Williams (Atlanta serial
    killler)

12
Paint
  • Physical Analysis
  • Color
  • Physical match of broken edges
  • of matching layers

13
Paint
  • Chemical Analysis
  • Solubility tests
  • Determine type oil-based, water based, acrylic,
    or latex
  • analyze chemical make-up

14
Glass
  • Matching pieces
  • Surface materials
  • Tinting films
  • Labels
  • Paint
  • Color
  • Refractive index (RI) (ability of glass
  • to bend light)
  • Density

15
Gasoline
  • Each gas companys product is different
  • Forensic lab can identify the brand and grade

16
Soil
  • Analyze
  • - color
  • - pH
  • - type of soil
  • - sizes of particles

17
Information from soil
  • Where was the crime committed?
  • Soil type may be unique to a given area
  • Clay
  • Sand
  • Coal dust
  • Seeds or pollen in the soil may be from specific
    plants

18
Dust
  • Some dust is unique to an area
  • Ex. Bathroom-talcum powder, cosmetics
  • Ex. Kitchen- flour, spices
  • Ex. Factory- remnants of manufacturing process
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