Gunshot Residue Analysis Flashcards
critical time for investigation of a firearms crime scene
first 72 hrs
Discharge called
Gunshot Residue, Cartridge discharge residue, Firearm discharge residue
Gunshot residue
- residue products are expelled from the muzzle (gases, vapours, particulate material, norm and unburnt gunpowder , metallic chips)
- gunshot residue are dispersed forward (in and around bullet hole), backward and laterally (hand, face and clothes of shooter contaminated)
Anatomy of a bullet
a round of ammunition- single, live, unfired, cartridge comprising the missile, cartridge case, propellant and some form of primer
primer
- the means of igniting the propellant
- consists of initiating explosive, oxidising agent, fuel and sensitiser
- primers such a lead styphanate are shock sensitive material
- used to detonate a further explosive material in the bulled through a complex chain reaction
common primer elements
lead, barium, or antimony
propellant
- can contain up to 23 organic compounds which when ignited produce very large quantities of gas
- modern gunpowders are single based consisting of mainly nitrocellulose, double base is when there is an additionally 1 to 40% nitroglycerine added
what does a propellant consist of?
black gunpowder, nitrocellulose based powders, cordite, smokeless powder, ballistite and solid oxidisers
Cartridge
refers to the ammunition case and primer but not the bullet
- made from brass with 75:25 copper/zinc alloy content
- shotgun cartridges generally have a brass base with plastic case
- they have head stamps that will identify manufacturer
bullet
- refers to the missile alone
- most common material is lead and antimony
- lead bullets need lubrication jacket bullets don’t
- lubricant = beeswax, graphite, vaseline, or silicone based waxes
gun powder residue and distance determination
- ammunition is propelled by expanding gases produced by ignition of powder
- unburnt and burnt powder and smoke are propelled out of the barrel with bullet towards target
- if weapon is close products will be deposited on target
- distribution of gunpowder residue and other residues permits assessment of distance
detecting firearms residue
spot tests for firearms residue must have the following characteristics
- be simple enough to be performed by police officers with little training
- test outcomes must be simple and decisive with relatively stable colour forms
lead and barium in firearm discharge residues
- the presence can be used to characterise the primer and leakage residues from GSR
- crude but effective photography paper method
spot test for lead in GSR
sodium rhodizonate
- if lead is present it goes from yellow to violet or red colour
spot test for barium in GSR
sodium rhodizonate
- produces a reddish brown ppt
- strontium ions give a similar colour
- under acidic conditions Ba salts are converted into an insoluble bright red complex. the red ppt of strontium dissolves under these acidic conditions
spot test for antimony
triphenylmethylarsonium iodide
- ppt reaction
- the presence on antimony is confirmed with 30 seconds by an orange ppt
Particle shape
- inorganic particles or primer residues are unique in there morphology and elemental composition
- both titanium and zinc are commonly used in paints and can be contaminants but the appearance of particles containing them can be distinguished from GSR by SEM
Identification of GSR
- surface morphology (predominantly spherical also irregular, cluster and flake)
- elemental composition
GSR analysis techniques
bulk analysis techniques (neutron activation analysis and atomic absorption spectroscopy)
- total quantity of elements estimated
- the measured amount should be more than the threshold limit
- very low success rate of about 10%
- lack of specificity, time consuming and costly
GSR analysis techniques
particle analysis technique- SEM and EDX
- morphological analysis and elemental analysis simultaneously - make particles unique so no threshold limit
- superior success rate
Detection of residues on the hand
detection of nitrate and nitrate on the hand of the suspect is a good indication of the recent use of a firearm. establishing the spatial distribution of the powder residue is an important part of the investigation
collection techniques
stubs- dabbed over surface, little sample prep prior to analysis, good particle retention, all particles retained are analysed
swabs- alcohol dampened cotton wiped across surface, designed to retain particles, useful for propellant analysis, technique requires large amount of physical contact during extraction
garments examined for gunpowder residue
- colour shape and size
- IR photographs help to enhance contrast revealing vaporous lead and powder
- greys test- nitrite particle
spot test for nitrates and nitrites in GSR
3 types of powder used in small firearms- smokeless, black and semismokless
- smokeless powder is composed of cellulose nitrate and potassium nitrate
- certain organic and in organic compounds are added as stabilisers to regulate firing rate of the powder and minimise corrosion to the gun barrel
paraffin - diphenylamine test for nitrates
- melted paraffin is used to gather the nitrate particles from the surface and intradermal layers of skin
- the heat of paraffin opens the pores to get trapped particles
- these are then identified by the diphenylamine test ( presence of nitrate causes a violet colour)
spot test for nitrites
- walkers test- positive is confirmed by red/orange coloration
- greiss test- naphtha- amine is used which gives an orange pattern of spots
