1. Chapter 9 Trace Evidence
“Breadth of view is one of the essentials of our profession. The interplay of ideas and the oblique use of knowledge are often of extraordinary interest.”
—Arthur Conan Doyle’s Sherlock Holmes,
in The Valley of Fear
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Trace Evidence
Students will learn:
How to apply deductive reasoning to analytical data.
How to follow qualitative analytical schemes.
How to gather and use information to solve problems.
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Trace Evidence
Students will be able to:
Identify metals.
Identify traces of white powder.
Classify lip prints.
Use chromatography to compare lipsticks.
Design and conduct scientific investigations.
Identify questions and concepts that guide scientific investigations.
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Trace Evidence
Trace evidence is physical evidence found in small amounts at a crime scene. Common examples would be hair, fiber, paint chips, body fluids, stains, powders, explosive residue, glass particles, vegetative matter, metal particles, and soil. It may also include more unusual types of evidence.
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5. Unusual Types of Trace Evidence
A torn piece of paper
Itching powder
Ashes
A spider
A match
Grease
Linoleum
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6. Physical and Chemical Properties
Physical property: A characteristic that does not involve a change in the identity of a substance, such as odor, color, boiling point, density, refractive index.
Chemical property: A characteristic that determines how a substance will change into another substance with different physical properties.
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Metal Analysis
Bits of metal can be identified from their physical and chemical properties.
Solid particles—microscopic examination, magnetism, malleability, density, color, solubility, reactivity
Dissolved metals—separation by chromatography with comparison of Rf values to known metals, specific reactions and color tests
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Product Liability
Solid metal particles found in a loaf of bread.
Were they in the flour from the manufacturer?
Or were they introduced by the plaintiff for personal gain?
Their identity answers these questions.
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9. Analysis of Metal Particles
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10. Environmental Contamination
Seasonal flooding caused crop failure and livestock sickness
The water was tested by chromatography
The separated metals had to be located with uv light
The spots were developed chemically and compared to standards
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11. Environmental Pollution
The metals were identified, but where did they come from?
A magnet manufacturer?
A maker of brass hardware?
A rechargeable battery company?
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12. Trace Evidence: Qualitative Analysis
When investigators find substances at the scene of a crime and send them to the laboratory for identification, the forensic chemist uses several techniques or lab tests to identify them. One of these techniques is qualitative analysis.
A number of white powders that appear the same can be identified by their physical and chemical properties.
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Qualitative Analysis
Microscopic Examination
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Qualitative Analysis
Solubility pH
Chemical reactions
Color
Precipitate formation
Evolution of gas
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A Historical Crime
In 1912, Emile Gourbin was a bank clerk in Lyons, France. He came under suspicion of strangling his girl friend, Marie Latelle. Gourbin was arrested but had what appeared to be an air-tight alibi. Edmond Locard went to Gourbin’s cell and removed scrapings from under his fingernails. The scrapings contained tissue possibly from Marie’s neck, but this was not provable. Locard noticed that the tissue was coated with a pink dust, which he identified as rice starch. On the particles he found bismuth, magnesium stearate, zinc oxide, and a reddish iron oxide pigment called Venetian red. Examination of the face powder used by Marie revealed that a powder prepared for her by a Lyons druggist was similar in composition. In these days of mass-produced face powder, this evidence would have far less significance. However, in 1912, because of the special preparation, it led to the confession of Gourbin.
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A More Recent Crime
A bank robber was startled by an alarm just as the teller handed her the money. She grabbed it and in her haste to get away, ran smack dab into a glass door. Nevertheless, she recovered and got away. Subsequent examination of the door revealed a red lipstick imprint of the perpetrator’s mouth. Later police picked up a suspect, but needed evidence to link her to the robbery.
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Lip Prints
Lip prints are different and can be used to identify suspects. There are several general patterns:
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Lip Prints
What happened?
Of several suspects, one was identified by matching his lip print to that on the bank door.
The lipstick used by the suspect could also have been compared to the residue on the door. How?
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19. Chromatography of Lipsticks
Thin layer chromatograph (TLC) can be used to separate the components of a lipstick. The chromatograms can then be compared for a match.
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