Gunshot Residue Particle Analysis Technique

S.S. Baisoya

Central Forensic Science Laboratory Chandigarh

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Introduction

Gunshot residue particle analysis pertains to: Identification of bullet holes Estimation of range of fire

Linking the suspect with the firing of a gun-in-question

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Gunshot Residues When a firearm is discharged –

discharge products are expelled from its muzzle: -

Gases and vapours Particulate materials (inorganic &

organic) Burnt and unburnt gunpowder

particles Metallic chips

(collectively known as Gunshot Residues)

Gunshot residues disperse down the range (in and around the bullet hole)

Gunshot residues disperse in backward and lateral directions (falls on the hand, face or clothing of a person)

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Formation of GSR particles

When firing pin strikes percussion cap, it ignites the primer mixture

Primer mixture vaporizes due to high Temperature, condense to droplets due to super-saturation,

Passes through various meta-stable stages of Temperature and Pressure, grow into bigger sizes by coalescence,

With the expansion and cooling, rapid condensation of droplets, freeze in their existing form of tiny particles

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Sources of Gunshot Residues

Primer consisting of: Initiating explosive, Oxidizing agent, Fuel, & Sensitizer

(Initiating explosive: Lead stephnate, lead azide, mercury fulminate Oxidizing agent: Potassium chlorate, barium nitrate, lead nitrate, lead peroxide and other components such as calcium silicide, lead thiocynate, aluminum powder, zirconium powder, magnesium and titanium)

Propellant Single base powders and double base powders

(Nitro-cellulose, nitro-cellulose along with nitro-glycerin) Additives

Stabilizer, plasticiser, coolants, flash inhibitors, lubricants, moderants etc.

Metals of bullet, cartridge case and barrel

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GSR Analysis Techniques Bulk analysis techniques: - Neutron activation analysis

and atomic absorption spectroscopy Total quantity of elements (Pb, Ba, Sb) estimated The measured amount should be more than the threshold limits Very low success rate of about 10% Lack of specificity, time consuming and costly

Particle analysis technique: - Technique using SEM-EDXA

Morphological analysis and elemental analysis simultaneously Morphology in combination with elemental composition makes

particles unique, so no threshold limit Superior success rates Automated GSR software reduced the problem of time and

manpower consumption

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GSR Technique in Casework

Equipment: SEM coupled with EDX/WDX, Automated GSR analysis software

for particle detection/ classification

Software for EDX/WDX automation Sputter Coater

Sampling procedure: Using GSR kit

Method of experiment: Sample pre-treatment Autorun of sampling stub

Protocol: The protocol is based on the

Aerospace recommendations

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Sampling Each GSR kit contains

Instructions A pair of rubber gloves Labels Two sample vials

Each vial contains adhesive coated aluminum stub mounted on the base of a rubber stopper

Stub pressed and lifted on the surface being sampled

Hand Face Clothing

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Identification of GSR Particles

Surface Morphology Varying size (0.1m - 10m,

occasionally up to 55m or even more)

Predominately spheroid (60-70%), also irregular, cluster and flake

Elemental Composition Varying elemental composition (Pb,

Ba, Sb, & elements such as Cu, Fe, Zn, Al, Si, S, K & Ca)

Distributions of elements (Pb-Ba-Sb) corresponds to various meta-stable stages of T & P

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Particle Classification Recommendations of Aerospace Corporation (1977)

Unique Particles (with no known source other than GSR) Pb-Ba-Sb, Ba-Sb (at least one component is a major peak)

Typical Particles (considered to be typical of GSR but not unique to it) Ba-Ca-Si (S is absent or trace), Pb-Sb, Pb-Ba (at least one

component is a major peak) Compatible Particles (commonly found in association with GSR, but

for which a wide variety of potential sources can be identified.) Pb, Ba, Sb (S is absent or trace), any combination of allowed

elements (Si, Ca, Al, Cu, Fe, S, P, K, Cl) Not GSR – Inconsistent with GSR and another source is known,

and Others – Inconsistent with GSR and source is unknown

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Detection of GSR Particles

Each element present in residues emits characteristic X-rays when bombarded with electrons in vacuum chamber of SEM

X-ray analyzer coupled with SEM detects X-ray quanta, records and prints X-ray spectral peaks automatically

It also classifies particles according to their elemental composition

The X-ray spectral printout shows all the elements present at the major and minor levels and their relative intensities

X-ray peak height and comparison criteria Major peak: 1/3 highest peak or higher Minor peak: > 1/10 but <1/3 highest peak Trace peak: 1/10 highest peak or less, but still clearly

identifiable

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Collecting X-ray Spectra EDX spectrum printout of a given unique particle will show all

elements present at major & minor levels, and their relative intensities. (if necessary, more than one spectrum may be taken)

Presence of Pb, Ba and Sb in a given unique particle should be documented by EDX spectrum showing the following peaks

Lead M1 L1 L1 L2

Barium L1 L1 L2

Antimony L1 L1

(If necessary, more than one spectrum may be taken) Caution is needed in case when the following peaks overlap.

Antimony-Tin-CalciumLead-Sulphur-MolybdenumBarium-titanium

(It may require to resolve the peaks further)

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Interpretation and Documentation

Interpretation of data The primary goal of the analytical analysis of a sample

stub is to determine the presence of a minimum of one unique particle required to state that GSR was detected on a given sample stub.

Documentation of analytical results Documentation of analytical result is very important

for court purposes, or it may involve legal problem. The systematic approaches in documentation will also

ensure to reliability of the analyst and the ballistic expert.

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Forensic Significance Once the presence or absence of GSR on a sample stub is

established through SEM-EDXA analysis. By examining morphology and elemental content of

individual particles and by consideration of particle population as a whole.

The established population may consist of GSR particles in addition to particles from other known and unknown sources.

The particle population detected must be consistent with what would be expected of a normal firearm discharge.

The variables that influence How the GSR was deposited, What type of surface it was deposited upon, and Conditions that the deposit was subjected to during the time

frame between deposition and sampling, will be subjected to change on a case to case basis.

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Reports Positive face and hand sample or positive face and negative hand

sample The person either fired a firearm, was in proximity to a firearm during

discharge, or was in contact with a source of GSR i.e. a recently fired firearm

Positive hand sample only or positive hand and negative face sample The person either fired a firearm, was in proximity to a firearm during

discharge or was in contact with a source of GSR Negative hand sample only or negative hand or face sample

It cannot be possible to determine whether the person fired a firearm or was in proximity to a firearm being discharged

Positive clothing sample Sampled clothing of suspect was in proximity to a firearm being discharged

or was in contact with a source of GSR such as a recently fired firearm Negative clothing sample

It can not be determined whether the clothing was in proximity to a firearm being discharged

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Conclusion

Detection of even a single unique particle on hand, face or clothing of a suspect using the particle analysis technique allows the investigator to establish conclusively whether the person has recently discharged a firearm or was in the proximity during firing or has come in contact with the discharged firearm.

GSR particles detection helps in the bullets holes identification and the GSR particle dispersion pattern helps in estimation of range of fire.

Thanks