1. Chapter 11 Trace Elements: Metals, Paints and Soil
Objectives:
Describe the usefulness of trace elements for the forensic comparison of various types of physical evidence.
Distinguish between a continuous and line emission spectrum.
Describe the parts of a simple emission spectrograph.
Define protons, neutrons, and electrons, including their mass and charge relationships.
Define atomic number and atomic mass number.
Describe the orbital energy levels that are occupied by electrons.
State what happens when an atom absorbs a definite amount of energy.
Explain the phenomenon of an atom releasing energy in the form of light.
Define an isotope.
Define radioactivity.
Explain how elements can be made radioactive.

2. Chapter 11 Trace Elements: Metals, Paints and Soil
Objectives:
Describe the components of paint.
Classify automobile paints.
List those examinations most useful for performing a forensic comparison of paint.
Describe the proper collection and preservation of paint evidence.
List the important forensic properties of soil.
Describe the density-gradient tube technique.
Describe the proper collection of soil evidence.

3. Forensic Analysis of Metals
Trace Elements
small quantities of elements found as impurities in all natural and manufactured products
useful as “invisible”markers
Kennedy Assassination
bullets and fragments tested
for silver and antimony
match fragments in car
Connally bullets indistinguishable
Q1 and Q1 same Ag Sb ppm
Kennedy bullets indistinguishable
Q2 and Q14
Connally and Kennedy bullets are different

4. Atomic Structure Atoms composed of particles:
Proton
positive charge
mass of 1
Neutron
no charge
Electron
negative charge
mass of 1/1837
Proton number distinguishes elements
Atomic Number = # of protons
Mass Number = sum of protons & neutrons 
nucleus

5. Elements and Isotopes Isotope Radioisotope Atoms of same element
same atomic number (electrons & protons)
Have different atomic mass
different number of neutrons
Radioisotope
unstable isotope
decompose

6. Isotopes atoms of same element with different masses same # of protons
different # of neutrons

7. Radioactivity high-energy particles released during decay Alpha rays
largest/least energy
He nucleus (2 protons & 2 neutrons)
Beta Rays
smaller/more energy
electrons
Gamma Rays
smallest/most energy
electromagnetic radiation

8. Neutron Activation Analysis
atoms in a nuclear reactor gain neutrons
becoming unstable
releasing gamma rays
can be detected to identify trace element
determine if samples have common origin

9. Composition of Paint pigments binder impart color and opacity
organic compounds
binder
polymer and solvent
polymer holds pigments
solvent evaporates after application

10. Automotive Paints Electrocoat Primer Primer Surfacer Basecoat
organic resins electroplated to resist erosion (black or gray)
Primer Surfacer
epoxy modified polyester or urethane
smooth out and hide seams
Basecoat
acrylic polymer
pigments
mica chips aluminum flakes
resist weather, UV, acid rain
Clearcoat
acrylic or polyurethane
improve gloss

11. Microscopic Paint Examination
side by side comparison
layer structure
match # and sequence of colors
“chip” fit

12. Paint Binder Characterization
Pyrolysis Gas Chromatography
decompose paint sample w/heat
pyrolyze
expose to high temperatures ( °C)
sample breaks up into many gaseous parts
run these gases through GC
forms a pyrogram
can distinguish polymers
Infrared Spectrophotometry
each “binder” will have a characteristic absorption spectra

13. Paint Pigment Characterization
Inorganic Pigments
identified by emission spectra, neutron activation analysis, x-ray spectroscopy
detects 15 to 20 elements
most are common to all paints (unimportant)
a few are rarely found (important)
Organic Pigments
UV Absorption Spectra
Carbon Arc Emission Spectrometry
“fingerprint” from “excited” electrons
absorb energy move from ground state to excited state
release “light” energy when they return to ground state
Inductively Coupled Emission Spectrometry (ICP)
argon gas and radio frequency induced coil form “hot plasma”
aerosol of sample forced through the plasma forming an emission spectra

14. Significance of Paint Evidence
Canadian Study
260 vehicle chips in wreck yard
color, layer, and IR spec study
all but one pair were distinguishable
PDQ (Paint Data Query)
Canadian data base started 1975
automotive paints
layers, primer color, binder

15. Collection of Paint Evidence
primary goal
keep chip intact
druggist fold or vial
clothing
package “whole”
hit and runs
get standard sample from undamaged area
scrape to bare metal
¼ inch2 sample sufficient

16. Forensic Examination of Soil
Visual Comparison
color and texture
soils compared must be dried under identical lab conditions
Microscopic Comparison
low power
plant, animal, artificial materials
high power
minerals and rocks (geologist)
Density Tube Gradient
tubes with liquids of various densities
density distribution pattern of soils
useful only in conjunction with other tests

17. Collection of Soil Evidence
must establish variation at scene
standards collected at various intervals within 100 yard radius
only surface soil
each in individual plastic vials
suspect samples
take whole object containing soil
car soil
“lump form” sampling
soil laid down in layers
unique layering may have mre value