1. Trace Evidence ii: Metals, paint & soil
Forensics

2. 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

3. 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

4. The Warren Commission’s 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 president’s skull
Found in the building were 3 cartridge casings
The head shot killed the president. The 2nd bullet was later found on the governor’s stretcher in the hospital after it fell out of his thigh.

5. 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 Governor’s thigh and wrist couldn’t be the same bullet that went through the president’s back. The recovered bullet did not show enough deformation

6. 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 JFK’s back, so they couldn’t definitively link those wounds to the bullet from the Governor.
Q 1& Q9 are the same. Q2, 4, 5 and 14 are also the same.
These tests required the use of neutron activation analysis, which we’ll discuss in this chapter.

7. Atomic Structure Particle Symbol Relative Mass Location Charge proton1
neutron
electron
1/1837
Symbols: p+, n, e-
Locations: nucleus, nucleus, electron cloud
Charges: +. 0, -

8. Atomic Identity
The atomic number gives an atom its unique properties and behaviors
Atomic number = number of protons in an atom

9. 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

10. 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
Many isotopes occur naturally, but not so much or so many that the ‘natural’ radiation is dangerous

11. 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.
This is similar to different colors produced in something like our flame test lab we do (chemistry students will understand this).
One benefit is that this kind of testing is non-destructive and can be done on most types of specimens (metals, drugs, paint, soil, gunpowder residues and hair)

12. 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

13. 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
Special finishes = metallic shine provided by aluminum flakes in the basecoat.

14. 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

15. 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

16. 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
Figure 11-8 shows how samples from two different cars produce different GC results

17. 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 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
These line spectrum are the ‘fingerprint’ of an element.

18. 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

19. 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

20. 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 Case Study - The Predator

21. 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

22. 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

23. 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.

24. 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 Forensic Brief

25. The CBS Murders Read the Case Reading on Page 419-421
Answer the Case Analysis Questions of p 416