Trace Evidence
• Trace evidence- small pieces of material
– Debris from mass produced materials
– Abundant in nature
Examples of Trace Evidence
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Hair
Fibers
Paint
Glass
Gasoline
Soil
Dust
How is trace evidence collected?
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Vacuum
Tape lifts
Picking
Combing
Scraping
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
Problems w/ trace evidence
1. Many samples are VERY similar because
a) Many materials are made through
mass production
b) 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.
3) Trace evidence is least likely to be
individualized.
• Exception: Hair can be individualized if
DNA can be isolated from the root
Locard’s 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
Questions to ask about trace
evidence
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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?
Equipment
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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)
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)
Paint
• Physical Analysis
– Color
– Physical match of broken edges
– # of matching layers
Paint
• Chemical Analysis
– Solubility tests
– Determine type: oil-based, water based,
acrylic, or latex
– analyze chemical make-up
Glass
 Matching pieces
 Surface materials
• Tinting films
• Labels
• Paint
Color
 Refractive index (RI) (ability of glass
to bend light)
 Density
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Gasoline
• Each gas company’s product is different
• Forensic lab can identify the brand and
grade
Soil
Analyze:
- color
- pH
- type of soil
- sizes of particles
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
Dust
– Some dust is unique to an area
• Ex. Bathroom-talcum powder, cosmetics
• Ex. Kitchen- flour, spices
• Ex. Factory- remnants of manufacturing
process