Trace Evidence ii: Metals, paint

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Trace Evidence ii Metals, paint soil

• Forensics

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Forensic Examination of Metal

• Anything present in concentrations of less than
  1 is considered a trace element
• These trace elements can provide invisible
  markers that can establish the source of an item
• See page 389, Table 11-1

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Who Killed JFK?

• Did Lee Harvey Oswald work alone or was he part
  of a conspiracy?
• The Warren Commission - government agency that
  decided Oswald worked alone
• Still a controversy

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The Warren Commissions Reconstruction

• Oswald was hidden on the 6th floor of a nearby
  building. He fired 3 shots from behind the
  president.
• 1 bullet totally missed the president
• 1 bullet hit the president in the back, exited
  his throat and went on to hit Governor Connally
  in the back, wrist and thigh
• 1 bullet lodged in the presidents skull
• Found in the building were 3 cartridge casings

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Questions Criticisms

• Some people believe some shots were also fired
  from the grassy knoll in front of the car
• Some people also argued that the bullet from the
  Governors thigh and wrist couldnt be the same
  bullet that went through the presidents back.
  The recovered bullet did not show enough
  deformation

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The Forensics

• 14 years after the assignation forensics was used
  to determine trace elements in the bullets and
  bullet fragments found
• P 390 Table 11-2
• There is evidence of only 2 bullets
• There were no bullet fragments associated with
  the wound in JFKs back, so they couldnt
  definitively link those wounds to the bullet from
  the Governor.

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Atomic Structure
Particle Symbol Relative Mass Location Charge
proton 1
neutron 1
electron 1/1837
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Atomic Identity

• The atomic number gives an atom its unique
  properties and behaviors
• Atomic number number of protons in an atom

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Isotopes

• Isotopes - atoms with the same number of protons
  but different numbers of neutrons
• This only changes the MASS of the atom, not its
  properties or identity
• Example Hydrogen
• Ordinary hydrogen - mass of 1
• Deuterium - mass of 2
• Tritium - mass of 3
• Most elements have several isotopes. Many
  isotopes are stable, but some decompose over time
  through radioactive decay

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Radioactivity

• Radioactivity - the emission of energy/particles
  when a nuclei breaks down
• Three types
• Alpha rays - helium nuclei (no electrons)
• Beta rays - electrons
• Gamma rays - electromagnetic radiation
• In a nuclear reactor atoms are hit with neutrons.
  This makes new isotopes that are radioactive and
  decompose to produce energy

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Neutron Activation Analysis

• In forensic labs, trace elements can be
  identified using neutron activation analysis
• The specimen is hit with neutrons and the
  resulting gamma rays are measured
• Each element has a unique set of radioactive
  isotopes that can be formed and those isotopes
  release a unique level of energy in the gamma
  rays they release during decomposition.

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Forensic Examination of Paint

• Paint evidence is used in many crimes but is
  typically the most useful in hit-and-runs and
  burglary cases
• Still important to have reference samples to
  compare to the paint evidence
• Thanks to databases like PDQ, a forensic
  scientist can frequently determine the make,
  model and color of a car from paint chips

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Composition of Paint

• Paint is composed of a binder and pigments
• Pigments - give color and some other properties
• Binder - supports the pigments and keeps them on
  the surface
• 4 coatings to automobile paint
• Electrocoat primer - provides corrosion
  resistance
• Primer surfacer - smoothes hides seams/flaws
• Basecoat - provides color and special finishes
• Clearcoat - adds gloss and durability

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Microscopic Examination of Paint

• Compare
• Color, surface texture and color layer sequence
• Color layers are the most important but still are
  considered class evidence in most cases
• Must use chemical composition of paints to
  individualize paint evidence

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Analytical Techniques for Paint

• There are several ways to analyze the composition
  of paint evidence
• Characterization of Paint Binders
• Characterization of Paint Pigments
• Inductively Coupled Plasma Emission Spectrometry

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Characterization of Paint Binders

• Pryolysis Gas Chromatography - (p398 Fig 11-7)
  paint samples are broken down with extreme heat
  and added to a carrier gas. The gas is then sent
  through a column which separates the individual
  elements.
• P 399 Figure 11-8
• Infrared Spectrophotometry - binders selectively
  absorb IR radiation differently

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Characterization of Paint Pigments

• Three ways to identify the chemicals in paint
• Emission spectroscopy, neutron activation
  analysis and x-ray spectroscopy
• Emission spectroscopy is able to detect 15-20
  elements at once. Atoms are excited with
  energy, causing e- to jump energy levels. As they
  fall back down they release energy. The amount of
  energy released corresponds to a color of light
• Emission spectrum - the light a source emits
  broken into its component colors
• Continuous - the rainbow
• Line spectrum - series of lines with black spaces
• P. 400 Fig 11-9

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Inductively Coupled Plasma Emission Spectrometry
(ICP)

• A spark is applied to argon gas in a plasma torch
• This spark takes some e- from the argon atoms and
  these e- are accelerated to collide with other
  argon atoms to take even more e-
• The gas sample is then introduced to the argon
  plasma and generates ions
• The ions produce light that corresponds to the
  elements in the sample

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How Significant is Paint Evidence?

• Many cars are the same color.
• However, the binders and paint composition may be
  different at the chemical level
• Databases allow forensic scientists to identify
  the make and model of cars from very small paint
  samples

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Collection and Preservation of Paint Evidence

• Pick up paint evidence with tweezers or scoop it
  up with a piece of paper
• Package the whole piece of paint
• Collect several reference samples (from undamaged
  areas of questionable cars)
• Make sure reference samples include all 4 layers
  of paint
• Page 407 - Case Study - The Predator

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Forensic Analysis of Soil

• In forensics soil is considered ANY small pieces
  of surface material, natural OR artificial
• Soil Evidence can be significant because it is so
  easily transferred
• Can lead investigators to specific locations

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Forensic Examination of Soil

• Microscopic - reveals plant and animal materials
  as well as artificial debris
• Requires a scientist trained in geology
• Compare the minerals and rocks found
• Compare the building/artificial materials
• Density-Gradient Tube - allows soil to be
  separated by density
• Heaviest particles sink to the bottom

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Variations in Soil

• If the soil around a crime scene is similar to
  other soil it is not very useful
• Soil can only be individualized to a specific
  location when it includes an unusual combination
  of components.

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Collection Preservation of Soil Evidence

• Collect reference samples in a 100-yard radius of
  the crime scene.
• Must be careful to only sample the top layer of
  soil
• Soil evidence should not be removed from the item
  it is found on. The entire item should be wrapped
  up and sent to the lab
• Page 413 - Forensic Brief

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The CBS Murders

• Read the Case Reading on Page 419-421
• Answer the Case Analysis Questions of p 416