Trace Evidence. Trace evidence- small pieces of material –Debris from mass produced materials –Abundant in nature.

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Presentation transcript:

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 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 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 X Comparison microscopes Phase contrast microscope X –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