Trace Evidence

• Trace evidence- small pieces of material

– Debris from mass produced materials– Abundant in nature

Examples of Trace Evidence

• Hair

• Fibers

• Paint

• Glass

• Gasoline

• Soil

• Dust

How is trace evidence collected?

• Vacuum

• Tape lifts

• Picking

• Combing

• Scraping

Types of Evidence

Class evidencecan 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 becausea) Many materials are made through

mass productionb) 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

• 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

• 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

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