The Gun Shot residues (GSR) are type of trace evidences which are produced, when a cartridge discharged from a firearm. They are also known as Firearm discharge residues or powder residues. GSR includes various residues from projectiles, primer residues, and partially burnt and unburnt gun powder particles. Particles from primer and gun powder are most significant in forensic examination to solve huge cases related to firearms. Collect and preserve GSR properly as soon as possible, if not collected then it can be easily lost.
The GSR are found on:
▪️ The hand(s) of the person who fires a gun.
▪️ The target around the projectile holes (clothes, skin, etc.)
▪️ The firearms- its inner and outer surface, the fired cartridge cases.
▪️ The intermediate targets.
GSR composition depends upon:
▪️The nature of formulations of the propellants and primer mixtures.
▪️The composition of the projectile materials.
▪️The barrel scrapings.
Collection Methods of GSR:
📌 Molten Wax- Gently apply wax with the help of brush over the hands till it acquires sufficient thickness. It allowed to set and then peeled off. It picks up the GSR particles from the hand.
📌 Cellulose Acetate- Its solution is applied to site bearing GSR. When solution dried and set then peeled off. The cast picks up the GSR.
📌Nylon Fibre- Collodion solution is sprayed on sample site bearing GSR. The film formed is reinforced with nylon fibers which picks up residues when peeled off.
📌Adhesive Tape- The tape is mounted on the GSR bearing site and when peeled off it picks up the GSR for examination. This method is most popular and convenient method to collect GSR in every condition. It also collects GSR from cadavers. It can collect both organic and inorganic components of GSR.
A cellophane sheet impregnated with acetic acid is pressed against the site. It picks up lead.
▪️A filter paper moistened with dil. acetic acid, presses against GSR particle on the sample site. Paper picks up the particles and used for further examination.
▪️The site bearing GSR is swabbed with a piece of cotton cloth, moisten with HCl (10%) or with nitric acid (5%). It picks up GSR particles.
Washing– Wash the sample area (barrel or hand of the shooter) with hot distilled water or with acid (dil. HCL and dil. HNO3) and then collect in a plastic bottle.
Swabbing– Cotton ball moistened with solvent (acetone, ethanol, ether) swab sample bearing site. The swabs are collected and then extracted for further examination.
Vacuum Lifting– The material deposited on the filter disc is extracted with a suitable solvent for further processing. To collect organic components of GSR two types of filters are used- Fiberglass and Teflon. Vacuum lifting method is best for lifting samples from above and depth of clothe piece.
From Hairs– After firing, particles settled on hair and helps to determine whether the person was present near firing or not. GSR collected from hair by using comb method, swab method and by tape lifting method.
Components of GSR are:
Gun shot residues including inorganic and organic constituents. Inorganic residues are from propellant, cartridge case, primer, core of the jacket and ammunition barrel while organic residues are from propellant, primer and lubricating materials.
Size of GSR particles varies from 0.5 to 10μm.
Black powder residues:
✔40-60% of combustion products of black powders are gases and rest are solids.
Gaseous products are CO,CO2,H2S,N2,O2 and water vapours.
Solid residues consist of C, CO3,SO3,SO2, thiosulphate, unconsumed charcoal and S2.
✔The black powder is not used in modern ammunition. Muzzle -loading guns have been used black powder.
✔ The primer residues contain mercury Fulminate, Antimony Sulphide, Antimony oxide and potassium chlorate.
✔ In some primer mixture potassium chlorate is replaced by BaO, BaNO3 and PbO. Mercury Fulminate in some primers is replaced by a mixture of Sulphocyanide of copper or lead ,TNT, tetryl and Sulphur.
✔ The primes contribute oxides of Barium, Antimony, copper, lead, potassium chloride and mercury to the gunshot residues.
Smokeless Powder Residues:
The detectable inorganic materials in smokeless powder residues are:
◼ Lead from bullets and primers.
◼ Barium from primers.
◼ Antimony from primers.
◼ Zinc, Copper from jacket.
◼ Iron from barrel.
◼ Carbon monoxide absorbed in the skin and flesh from propellants.
◼ Nitrites from the propellants.
Detectable organic components of GSR are:
◼ Nitro-cellulose, unburnt and semi-burnt part of propellant.
◼ Nitro-glycerin, unburnt and semi-burnt part of propellant.
◼ DPA (diphenyl amine) used as a stabilizer in single base propellants.
◼ Ethyl centralite, used as a stabilizer in double base propellants.
◼ Phthalates, used as the plasticizers and Glyceryl tri-acetate, Nitroguanidine.
◼ DNT (di-nitro toluene) used as flash suppressant.
◼ Bullet lubricants.
Smokeless propellants produce mostly gases like CO, CO2, N2, O2 and water vapours and oxides of N2.
Detection of GSR:
To examine these residues, firstly color tests were performed but these tests destroy the sample. To overcome this problem with developing technology, instrumental method have taken place of color test. Later, the electrochemical methods are developed to analyze GSR. The GSR are detected visually with a magnifying lens and stereomicroscope.
If the firearm is discharged from a close range, the visual examination may reveal by burning, blackening, tattooing etc. Their presence indicates a gunshot injury or a gunshot hole. The GSR are detected visually with a magnifying lens and stereomicroscope. A hand magnifier or low power stereomicroscope with an intense light source improves the detection of GSR.
Color Tests To Analyze GSR:
Dermal Nitrate test- This test was introduced by Teodoro Gonzalez in 1933. The residues on the cast are treated with diphenylamine dissolved in strong Sulphuric acid. The formation of blue color spots indicates presence of nitrates from burnt and unburnt propellant particles.
As certain other common materials also give positive reaction with the test. That’s why this method is no longer in use.
Walker’s Test or Griess test–
It was first described in1858 by Peter Greiss.
A desensitized glossy bromide paper is treated with 2Naphthylamine 4:8 disulphonic acid. The bromide paper is placed over the cloth sample bearing the sample GSR. It is covered with a towel moistened with 20% acetic acid and then pressed with hot electric iron for 5-10 min. Dark red spots appear on Bromide paper. It is a convenient test due to its simplicity and greater specificity.
Modified version of Griess test–
Piece of filter paper is moistened with acetic acid pressed against surface bearing GSR and picks up GSR. The filter paper is then sprayed with a solution of 2Naphthylamine sulphonic acid and citric acid. Pink color indicates nitrite particles of GSR.
Test is useful to :
📌 Identify the bullet entrance holes and wounds.
📌 Determine the range of fire.
📌 Whether a firearm, Bullet, cartridge, etc. has been fired or not.
📌 Estimate approx time of fire.
📌 Identify a shooter.
Harrison and Gilroy’s Test–
This method was developed in 1959. This test determines presence of lead, antimony and barium.
Cotton swab moistened with 0.1N HCl and surface bearing GSR especially hand and fingers of the shooter swabbed with it to collect sample. The swab is allowed to dry and then treated with 2-10% of triphenylmethylarsonium iodide. Appearance of orange colour spot indicates the presence of antimony.
After 2-3min, add 2-3 drops of freshly prepared sodium rhodizonate solution and appearance of red colour indicates presence of barium or lead in the sample.
Again, after few minutes add 1:20 HCl then if red colour convert to blue indicate presence of lead and if red colour remains then barium is present.
The test is useful to-
📌 Identify a shooter or bullet hole.
Instruments used for detection of inorganic and organic gunshot residues:
● Neutron Activation Analysis:
Most sensitive, effective and non-destructive method. This method indicates the presence of antimony and barium in GSR.
When GSR are exposed to neutron bombardment in a nuclear reactor, antimony and barium capture one neutron each to give radioactive antimony and barium.
Radioactive nuclides emit gamma rays. Evaluation of energy and half-life identify and estimate the isotopes.
🔸️ It needs a reactor for the neutron source.
🔸️ It is time consuming.
🔸️ It needs a lot of pre and post-activation chemical treatment of the samples.
🔸️ Not suitable for analysis of lead, which is most important metallic constituent of GSR.
● Flame-Less Atomic Absorption Spectrometry:
AAS is sensitive and convenient technique for evaluation of lead, metallic compound of GSR. It can detect the elements in nanograms and picogram range.
Sample particles are taken into an atomizer and is excited by heating. The radiation from a discharge tube with the suspected metal electrodes is passed through the atomizer. Sample particles absorbs the radiation of same wavelength as it emits when get excited. The loss of intensity in radiation gives the measure of the quantity of metal in GSR.
It is less costly, consume less time and applicable for lead estimates also.
Inductive coupled plasma– mass spectrometry (ICP-MS)- It is used for analysis of elements present in the primer and it also allows very fast and multi-elemental analysis. This method is more advantageous when extracted sample is analyzed.
● Scanning Electron Microscopy:
▪️It is most convenient and non-destrcutive method for elemental analysis of GSR particles.
▪️Specific shapes of lead particles indicate GSR.
▪️It gives the number of particles/unit area.
In SEM, a fine beam of electron is focused on the sample. Electron beam has following effects:
► It generates X-rays: Generated X-rays are used to find out the elemental composition of material with an X-ray analyzer as X-rays are characteristic of element.
► It gives cathode luminescence: When electron beam falls on certain material, it creates luminescence, specific of elements.
► It creates back-scatter: Some electrons are reflected back. The photographs obtained from these electrons have 3-D effect which permit the topographic evaluations of the surface GSR particles.
► It provides high magnifications: High resolution helps in study of structure of GSR particles.
The paraffin lifts, adhesive tapes, cellulose acetate or the collodion lifts are used to evaluate the GSR under SEM.
● X-ray Fluorescence:
It is most convenient and simple technique. Used for both qualitative and quantitative analysis of the residues. This method exposes lead and metallic particles in the deposited GSR sample from primers.
● Infra-Red Rays Photography:
This method is useful especially when GSR samples and bullet holes are on coloured clothes. This method also reveal pattern.
● Soft X-Rays Radiography:
The lead metal is opaque to X-rays. Soft X-rays radiography of the hole shows the radio opaque deposits around the hole in a characteristic pattern which indicates a gunshot fire.
● Gas Chromatography-Mass Spectrometry:
This method used to examine components in smokeless powder that comes out of the barrel when a firearm is fired.
● Raman Spectroscopy:
This method helps to analyze and identify various propellant components. Combination of Infrared with Raman enhances the variation between GSR.
● Thin Layer Chromatography:
This method helps to analyze flakes of smokeless powder by separating nitrocellulose from nitroglycerine.
● High Performance Liquid Chromatography:
This method analyzes the mixture of organic residues present in GSR by using different detectors.
● Electrochemical- based sensors to detect GSR:
As instrumental techniques are time consuming, costly and require high- profile expert. To overcome these limits this method is used to detect GSR. It is both qualitative and quantitative technique which can detect even trace amount of metal components of GSR. But this method can not be applied to barium metal due to its high electrochemical potential.