TRACE EVIDENCE II: METALS, PAINT & SOIL

Forensics

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

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

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

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

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.

Atomic Structure

Particle SymbolRelative Mass

Location Charge

proton 1

neutron 1

electron 1/1837

Atomic Identity

The atomic number gives an atom its unique properties and behaviors

Atomic number = number of protons in an atom

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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.

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

The CBS Murders

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

416