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)

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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:

– 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

• 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