Gunshot Residue Analysis

Question 1

critical time for investigation of a firearms crime scene

Answer 1

first 72 hrs

Question 2

Discharge called

Answer 2

Gunshot Residue, Cartridge discharge residue, Firearm discharge residue

Question 3

Gunshot residue

Answer 3

• 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)

Question 4

Anatomy of a bullet

Answer 4

a round of ammunition- single, live, unfired, cartridge comprising the missile, cartridge case, propellant and some form of primer

Question 5

primer

Answer 5

• 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

Question 6

common primer elements

Answer 6

lead, barium, or antimony

Question 7

propellant

Answer 7

• 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

Question 8

what does a propellant consist of?

Answer 8

black gunpowder, nitrocellulose based powders, cordite, smokeless powder, ballistite and solid oxidisers

Question 9

Cartridge

Answer 9

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

Question 10

bullet

Answer 10

• 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

Question 11

gun powder residue and distance determination

Answer 11

• 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

Question 12

detecting firearms residue

Answer 12

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

Question 13

lead and barium in firearm discharge residues

Answer 13

• the presence can be used to characterise the primer and leakage residues from GSR
• crude but effective photography paper method

Question 14

spot test for lead in GSR

Answer 14

sodium rhodizonate

- if lead is present it goes from yellow to violet or red colour

Question 15

spot test for barium in GSR

Answer 15

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

Question 16

spot test for antimony

Answer 16

triphenylmethylarsonium iodide

• ppt reaction
• the presence on antimony is confirmed with 30 seconds by an orange ppt

Question 17

Particle shape

Answer 17

• 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

Question 18

Identification of GSR

Answer 18

• surface morphology (predominantly spherical also irregular, cluster and flake)
• elemental composition

Question 19

GSR analysis techniques

Answer 19

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

Question 20

GSR analysis techniques

Answer 20

particle analysis technique- SEM and EDX

• morphological analysis and elemental analysis simultaneously - make particles unique so no threshold limit
• superior success rate

Question 21

Detection of residues on the hand

Answer 21

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

Question 22

collection techniques

Answer 22

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

Question 23

garments examined for gunpowder residue

Answer 23

• colour shape and size
• IR photographs help to enhance contrast revealing vaporous lead and powder
• greys test- nitrite particle

Question 24

spot test for nitrates and nitrites in GSR

Answer 24

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

Question 25

paraffin - diphenylamine test for nitrates

Answer 25

• 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)

Question 26

spot test for nitrites

Answer 26

• walkers test- positive is confirmed by red/orange coloration
• greiss test- naphtha- amine is used which gives an orange pattern of spots