Transcript Metal and Paint Analysis

Metal and Paint Analysis
Atomic Structure Review
• An atom is made of
subatomic particles
– Proton – positive (in the
nucleus)
– Neutron – No charge ( in
the nucleus)
– Electron – Negative (orbits
around the nucleus)
• When an atom has no
charge, the number of
protons equals the
number of electrons
Isotopes
• Atoms that are the
same element (because
they have the same
number of protons),
but differ in atomic
mass (because they
have a different number
of neutrons)
Radioactivity
• the emission of energy
from an atom in 1 of 2
ways:
– electromagnetic waves
(energy that moves
through space)
– as moving subatomic
particles (electrons,
protons, or neutrons)
• Three types of
Radiation defined by
what is being emitted:
– Alpha rays – 2 protons
and 2 neutrons are
emitted
– Beta rays – electrons are
emitted
– Gamma rays –
electromagnetic waves
(neutrons)
Metal Analysis
• Neutron activation Analysis – used to identify
trace elements in a sample
– Bombard the sample with neutrons
– Nucleus absorbs neutrons causing instability
– Sample gives off gamma rays
– The energy from the gamma rays is measured
– Different elements have unique gamma ray energy
values.
• This will help identify the element. Then they measure the
concentration of the element in the sample by measuring
the intensity of the gamma ray energy.
Graph showing
what elements
are in a given
sample.
Examples of using neutron activation
• Used to characterize trace elements in metals,
drugs, paint, soil, gunpowder residue, and
hair.
• Compare different types of copper wiring in a
theft.
Examination of Paint
• Common in hit and run and burglary cases
• Usually will compare 2 or more paints to
establish where they came from.
Composition of Paint
• Made of a binder, pigment and additives all dissolved in a
solvent (solvent evaporates once paint dries)
• Paint on a car:
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Electrocoat Primer – 1st layer applied (black  gray)
Primer Surfacer – Corrosion control. Highly pigmented
Basecoat – Provides the color of the car
Clearcoat – Unpigmented to improve gloss, durability, and
appearance.
Microscopic examination of paint
• Crime scene and known specimens are best
compared side by side under a stereoscope for
color, surface texture, and color layer
sequence.
Chemical analysis of Paint Binder
• Pyrolysis gas chromatography
– Heat paint sample to high temperatures so that
they will decompose into numerous gas products.
– Gas products flow through a chamber that
separates them.
– A chromatogram, or pyrogram is created and
analyzed.
– Enables criminalists to distinguish most paint
formulations and the chemical makeup of the
binder
Chemical analysis of Paint Pigments
• Use the following to
identify elements that
make up a pigment:
– Neutron activation
analysis
– Emission spectroscopy
• Sample is heated to
vaporize and excite
sample’s electrons
• Excited electrons emit
light (different
wavelengths based on
energy of electrons)
• Light is recorded and
analyzed
Emission Spectroscopy
The Significance of Paint Evidence
• Paint comparison
• Identifying make and model of a car
Collecting and Preserving Paint Evidence
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Picked up with tweezers or piece of paper
Stored in druggist folds, glass, or plastic vials
If on garments or objects, DO NOT REMOVE
Uncontaimnated standard/reference paint
collected
• Collect all
layers