When a cartridge is fired from a firearm, gunshot residue is produced. They are also known as powder residues or firearm discharge residues. The acronym GSR has become popular for the gunshot residues.
The GSR is found on:
► The hands and cloth of the person who fires a handgun, also on the face in shoulder arm firing.
► The target, around the projectile holes, including clothes, exposed skin, etc when the shot is fired from close range.
► The firearms, their inner and outer surface, the fired cartridge cases, and the projectiles involved.
► The articles around the target or the shooter.
► The intermediate targets
► Floating in the air.
The examination and the evaluation of the GSR help to solve several problems:
► Has the given gun been fired?
► Has the given projectile or the cartridge case been fired recently?
► Did the suspect shoot?
► Is the given wound a firearm injury?
► Is the given hole a gunshot hole?
► What was the firing range?
► When was the given gun, projectile, or cartridge case fired?
► Is the given wound or the hole an entrance wound or hole?
► Did the victim commit suicide or was it murder?
Also Read: Abbreviation in Forensic Ballistics
Gunshot residues are a complicated mixture of elements produced when a gun is fired. Their composition depends upon:
► The nature of the formulations of the propellants and primer mixtures.
► The composition of the projectile materials.
► The barrel fouling is scraped by the shot.
The detectable inorganic materials in the modern smokeless powder GSR are:
► Lead, from bullets and primers
► Barium, from the primers
► Antimony, from the primers
► Copper, from the jacket
► Iron, from the barrels
► Nitrates, from the inorganic oxides and the propellants
► Nitrites, from the propellants
► Zinc, from the jacket
► Carbon monoxide, absorbed in skin and flesh comes from the propellants, if fired from close range.
The detectable organic components of GSR are:
■ Nitrocellulose, the main component of the propellant is the unburnt or the semi-burnt part.
■ Nitroglycerine, the unburnt or semi burnt part of the double-barrel propellents if used.
■ DPA (diphenylamine), used as a stabilizer in all single base propellants. It constitutes roughly 1% of the propellant weight.
■ Ethyl centrality, used as a stabilizer in double base powders.
■ Phthalates, used as plasticizers.
■ Glyceryl triacetate, used as the plasticizer. Nitroguanidine, used as a plasticizer.
■ DNT (dinitrotoluene), used as a flash suppressant.
■ Bullet lubricants.
■ GSR constituents, from the barrel fouling of the previous firing.
The GSR is produced in the firing process. They are light in weight. They, therefore, do not travel far. They are deposited on the object which is close to the firearm.
The GSR particles are dispersed in the air. They continue in the air for a long time because these particles are light in weight. Besides, they are also transferred to a surface which comes in contact with the air or surface bearing GSR.
The gunshot residues are collected by several methods.
The importing methods are given below:
For the collection of the gunshot residues on the hands, molten wax of suitable melting point is gently brushed over the hands till it acquires sufficient thickness. It is peeled off when the wax is set. It picks up the gunshot residues from the hand.
A solution of cellulose acetate is applied to the site bearing the gunshot residues. It is peeled off on drying. The gunshot residues are picked up by the cast.
A collodion solution is sprayed on the area containing the powder marks. Nylon fibers are used to reinforce the film. On drying, the reinforced layer that gathers up the powder residues is peeled away.
The site bearing the gunshot residues is pressed with an inert adhesive tape or with an adhesive aluminum foil, repeatedly. The tape is kept in a vial with an adhesive surface inside the vial, before and after the use. For examination, the tape is mounted on a specimen stub. A cellophane sheet impregnated with acetic acid and is pressed against the site. It picks up lead.
Dilute acetic acid is used to wet filter paper. It is pressed against the area where gunshot residues are suspected. The filter paper picks them up. The gunshot residues are swabbed with a piece of cotton cloth or a cotton swab moistened with dilute hydrochloric acid or nitric acid, and the spot bearing the gunshot residues is swabbed with this piece of cloth or swab. It picks up GSR. The swab from various parts of the hand is collected separately. The hands are rinsed thoroughly in dilute nitric acid and it is placed in a plastic bag. The resulting solution has been freeze-dried and is ready for testing. By cleaning the barrel with hot distilled water, the residue in the barrel is collected. The washings are also tested for the constituents of the gunshot residues.
Collection of Organic Residues
The evaluation of the organic component of the powder residues has acquired tremendous importance in recent times. The following are the important collection techniques:
A small cotton wool ball, a clean cloth piece, or a filter paper is moistened with an organic solvent and the site is swabbed. The swabs are collected and extracted.
- Tape lifting
An inner tape of suitable width is taken and the site is taped. The tape is used as such or extracted and the extract is utilized for testing. Tape lifting is becoming popular for its convenience and application in all situations including lifting GSR from cadaverous. The tape picks up both organic and inorganic constituents of GSR.
- Vacuum Lifting
Vacuum lifting is especially useful to collect the GSR from clothes. The material deposited on the filter disc is extracted with a suitable solvent for further processing.
Identification and Evaluations
A lot of research has been carried out in recent times to evaluate the powder residue. Excellent techniques have come up to identify and give both qualitative and quantitative estimates of the constituents of the gunshot residues even in micro and nanogram quantities.
The GSR is detected visually, with a magnifying lens and a stereomicroscope. But the visual examination even with the instrumental help has limited utility in these cases.
The following techniques for the detection are in vogue to supplement the visual examination.
Infrared rays photography is useful especially in the case where GSR are on colored clothes. They reveal the pattern. X-ray microfluorescence reveals lead. Another metallic particle is present in deposited GSR from the primers.
The scanning electron microscope has been found exceptionally useful not only to detect the GSR but also it can give the size and number of particles. Its quantitative aspect has made it indispensable in GSR detection and evaluation.
Also Read: Gun Shot Residue
- Visual examination
The visual examination may reveal burning, blackening, tattooing, etc. if the firearm is discharged from a close range. The presence of these particles indicates a gunshot injury or a gunshot hole. A hand magnifier, a low-power stereo microscope, and a strong light improve the detection.
- Infrared photography
If the bullet holes are on colored clothes, the presence of gunshot residue is masked. Infrared photography proves useful in such cases. The gunshot residue becomes visible in the photograph.
- Soft X rays radiography
The lead metal is opaque to x-rays. A soft x-ray radiograph of the hole, reveals the radio-opaque deposits around the hole in a characteristics pattern, indicative of a gunshot fire.
- Dermal nitrate Test
The dermal nitrate test was frequently used before 1950 to identify the shooter. The test is simple. The nitrates are picked up on a paraffin wax cast. The picked-up residue on the cast is treated with diphenylamine dissolved in strong sulfuric acid. The appearance of a blue color spot indicates nitrates. They are, in turn, indicative of the presence of the gunshot residues.
- Walker’s Test
Walker test is widely used even today. It is a simple test and has greater specificity, making it a convenient test.
A desensitized glossy bromide paper is taken. It is treated with 2 naphthyl amine 4,8 sulphonic acid. The bromide paper is placed over a table with the treated surface upward. The cloth bearing the gunshot residues is placed over the bromide paper. It is covered with a towel moistened with 20% acetic acid and the set up is pressed with the hot electric iron for about 5 to 10 minutes. Dark red spots on the bromide paper indicate the gunshot residues.
A modified version of the test is as under A piece of filter paper is moistened with acetic acid. It is pressed against the target surface. It picks up the gunshot residue. The filter paper is then sprayed with a solution of 2 naphthyl amine sulphanilic acid and citric acid. The pink-colored speck indicates nitrite particles. They, in turn, indicate the gunshot residues.
The test is useful too:
- Identify the bullet entrance holes and wounds.
- Determine the range of fire.
- A certain whether a firearm, bullet, cartridge, etc. has been fired or not. Estimate the approximate time of the fire.
- The diminishing intensity of the color formed is the indicator of the time elapsed.
- Identify a shooter.
- Harrison and Gilroy’s Test
Harrison and Gilroy’s test determines the presence of lead, antimony, and barium through spot test. The test gives good results involving revolver shooting.
The gunshot residues containing the metallic constituents are collected on a piece of cotton cloth. The cloth is, then, tested as under:
A dried piece of test cloth is treated with one drop of triphenyl methylammonium iodide alcoholic solution. The appearance of an Orange ring, in about 2 minutes, indicates the presence of antimony. Then, the test cloth bearing the Orange ring is dried and two drops of sodium rhodizonate solution are put in the center of the ring. Red color development, inside the ring, that indicates lead, barium, or both.
The test cloth is dried again. A drop of dilute hydrochloric acid is placed on the red spot developed. If the color changes to blue, lead are indicated. If the color does not change, barium is indicated.
The test is useful to identify a shooter or a bullet hole. The test is, however, not popular. It gives unstable colors. The results are qualitative only.
- Tewari Test
In 50 mL of water, one gram of Antazoline Hydrochloride (2 – N-benzyl aniline methyliminazoline hydrochloride) is dissolved. 45 mL concentrated HCl is added, and the white precipitate is stirred until it dissolves. An acetone-soaked filter paper was put against the target. After that, it’s air-dried. The prepared Antazoline solution is then sprayed heavily on it. Nitrite compounds will produce yellow spots as a result of a positive reaction. The visualization of propellant particles in this test is primarily used for a variety of firing estimations.
The examination of gunshot residues by Neutron activation analysis detects the presence of antimony and barium. The technique is useful for research. But it is more or less useless for fieldwork. The facility for neutron activation analysis is available at Bhaba Atomic Research Center in Bombay.
- Flameless Atomic Absorption Spectrometry
Atomic absorption spectrometry is an extremely sensitive and convenient technique for the evaluation of gunshot residues. It can detect the elements in the nanogram and picogram ranges.
Atomic absorption spectrometry works on a very simple principle. When an element is stimulated, it absorbs the same wavelength of light that it emits. Heating is used to stimulate the suspected element in an atomizer. The atomizer receives radiation from a discharge tube containing the suspected metal electrodes. The loss of intensity in radiation gives the measures of the quantity of the metal.
The technique is useful and popular for the examination of gunshot residues. It is less costly, consumes less time, and is capable of lead estimations also.
- X-ray Fluorescence
This technique has been extensively used to carry out both qualitative and quantitative elemental analysis of the residues. The comparatively low price of the needed equipment, the simplicity of the technique, and the operation and efficient results make it one of the most convenient methods for the evaluation of powder residues.
Auto radiography, photoluminescence, lead isotopy, emission spectroscopy, and polarography have also been employed.
Gun Shot Residue, https://forensicfield.blog/gun-shot-residue/
Shrivastava, P., Jain, V.K. & Nagpal, S. Gunshot residue detection technologies—a review. Egypt J Forensic Sci 11, 11 (2021). https://doi.org/10.1186/s41935-021-00223-9
Max M. Houck,Terry McAdam, in The Science of Crime Scenes (Second Edition), 2018
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One Word QnA:
1.) The mechanism in a firearm by which a cartridge of a fired case is withdrawn from the chamber is called a(n):
2.) Tool marks are most often encountered at which type of crime scene?
3.) What is the primary consideration in collecting impressions at a crime scene?
Answer: Preservation of the impression or its reproduction for later use
4.) When tire impressions have been left at a crime scene, it is possible for the forensic examiner to determine the:
Answer: style and/or manufacturer of the tire.
5.) The best way to preserve shoe and tire marks that have been impressed into soft earth is:
Answer: Photography and Casting.