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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

How to apply deductive

reasoning to analytical

data.

How to follow qualitative

analytical schemes.

How to gather and use

information to solve

problems.

Students will learn:

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Trace Evidence

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.

Students will be able to:

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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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Unusual Types of

Trace Evidence

A torn piece of

paper

Itching powder

Ashes

A spider

A match

Grease

Linoleum

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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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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 which is using the senses

to describe properties of a substance.

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Qualitative Analysis

1.Microscopic Examination

2. Odor

3. Texture

4. color

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Quantitative Analysis

requires a measurement

1. Solubility

2.pH

3. diameter

4. Length

5. mass

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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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Analysis of Metal Particles

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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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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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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.

http://www.hbo.com/autopsy/episode/episode_6_the_telltale_imprint.html

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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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Lip Prints

Lip prints are different and can be used to identify

suspects. There are several general patterns to

identify under trace evidence.

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Chromatography of

Lipsticks

Thin layer chromatograph

(TLC) or paper chromato-

graphy can be used to

separate the components of

a lipstick. The

chromatograms can then

be compared for a match.