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Trace Evidence

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.

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Trace Evidence

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  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. 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

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

  15. Gasoline • Each gas company’s 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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