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Topic 11 - Gunshot Residue Analysis (Classification of GSR Particles…
Topic 11 - Gunshot Residue Analysis
Any particles or residues discharged from a firearm after the trigger has been pulled. Including chemicals from the primer, propellant, oxidisers, reducing agents, sensitisers and binders.
Sources:
explosive reaction of primer compounds.
bullet and/or jacketing
cartridge case, primer cup
firearm chamber, barrel, residues from previously discharged rounds
foreign material e.g. corrosion, soil, biological material.
How is it formed?
Rapid cooling of the discharge gases and solid matter, originating from partially reacted components of the primer and the propellant. Metallic components of ammo and firearm.
Some gases condense forming spheres, can also interact with solid residue materials to form complex mixtures.
Some residue material can be ejected with little or no physical/chemical modification, most particles show evidence of exposure to v high temps and pressures. (Typically not visible).
Appearance:
Spheroidal particles
range in size
Irregular particles
and
Aggregate particles
(collection/cluster) constitute majority of particles produced, ranging in size
Human Sampling
Use of a sample stub to sample areas on the hands:
back of each hand
thumb and index finger wed and all digits
palm of each had and web and all digits.
Anti-Contamination:
clean equipment and lab
PPE
avoid handling guns beforehand
only work on one sample at a time
seal sample tubes properly
Analysis
Scanning electron microscopy microscope/Energy dispersive X-ray analysis (SEM/EDX)
X-ray fluorescence
Atomic absorbance spectroscopy
Analysis is restricted to the inorganic (metallic) residues, strongly linked to primer related residues.
Primer Compositions
Mercuric:
Mercury fulminate (explosive to drive the ignition process)
Barium nitrate (oxidiser to increase the heat of ignition)
Antimony sulphide (fuel in ignition process and frictionator during ignition)
This is dated now.
Sinoxid:
Lead styphnate (explosive to drive the ignition process)
Barium nitrate (oxidiser to increase the heat of ignition)
Antimony sulphide (fuel in ignition process and frictionator during ignition)
Most common but not only one used.
SEM/EDX
non-destructive
minimal sample prep
individual particles analysed
GSR identified with high level of confidence on the basis of morphology and elemental composition.
highly effective systems for particle detection and calssification
Automated search mode has increased efficiency
can direct the instrument to collect GSR specific information
Classification of GSR Particles
Characteristic of GSR
Compositions rarely found in particles from any other source (more conclusive evidence).
Particles can be classified as characteristic of GSR in SINOXID PRIMERS if the particles contain:
lead
barium
antinomy
Must be in MAJOR components to be characteristic of GSR
Additional elements often found in major/minor amounts: silicon, calcium, aluminium, copper, tin
Found in minor amounts: lead, sulphur, zinc, potassium, chloride, phosphorous and nickel.
Consistent with GSR
Compositions that are found in GSR but also a number of relatively common, non-firearm sources. (Barium, calcium, silicon, or antinomy and barium or lead and barium)
Commonly Associated with GSR
Compositions also commonly found in environmental particles from numerous sources (less conclusive evidence). (lead alone, antimony alone, barium alone)
"unique" is excluded from use in description of GSR particles in court. When reporting two component particles, it must be stated that they originated from firearm
discharge
or
non-firearm related sources
. GSR transfers v easily. A negative result may occur as person washed their hands or wore gloves. Presence of primer residue on an object could be the result of the object being in vicinity of a firearm or coming into contact with primer residue on another item. Primer residue can be found on someones hands due to firing a weapon, handling a weapon, being in proximity of the discharge of a weapon.
False Positives
Non-firearm surfaces of particles having compositions similar to GSR from SINOXID TYPE PRIMERS. Can be distinguished from residues originating from the discharge of a firearm by morphology and/or composition. V small amount can be indistinguishable.
Car Air Bags:
Contain primer device that contains lead, antimony and barium. However many non-primer particles characteristic of air bags but foreign to GSR are also found.
Fireworks and Pyrotechnics:
Lead, antimony and barium RARELY found together. However uncommon fireworks (crackering ball) contains all 3. Majority of particles generated contain elements not typically found in GSR e.g. magnesium. V small proportion of particles generated contain the three-component composition seen in sinoxid primers. GSR particles tend to have
spheroid morphology
, in fireworks its very rare for the particles to have this morphology. Pyrotechnics contain aluminium powder and barium nitrate, can give rise to spheroid particles, occasionally indistinguishable from those generated by ammo.
Car Brake Pads:
Contain compounds of lead, antimony and barium, friction causes shedding of the particles. Can be distinguished from GSR due to presence of element foreign to GSR e.g. elevated levels of iron. However lead is less common in newer cars (2000s onwards).