Forensic Analysis of Gunshot Residues (GSR)
Gunshot residues are produced when a cartridge is fired from a firearm. They are also known as firearm discharge residue or powder residues.
Gunshot Residue is usually found on hands, clothes, target, on the articles found around the targets, and is also found floating in the air. The gunshot residue is collected in several ways, including the dry and the wet methods. Analysis of gunshot residue is done using different chemical and instrumentation methods. The various methods of analysis using Walker’s test, Greiss test, and instrumentation such as SEM, EDX, and NAA are discussed in this article.
GSR analysis is a standard method to determine if a firearm has been used. Particles for GSR analysis typically range from 0.5µ to 10µ. Primer particles containing lead, antimony, barium are detected and analyzed using EDX in Scanning Electron microscopes.
Gunshot Residue Examination
1. Visual examination
In cases where the firearm is discharged from a close range, the visual examination may reveal burning, blackening, tattooing, etc. Their presence indicates a gunshot injury or a gunshot hole.
2. Infrared photography
If bullet holes are on colored clothes, the presence of gunshot residue is masked. Infrared photography proves helpful in such cases, and the gunshot residue becomes visible in photographs.
3. Chemical analysis
DERMAL NITRATE TEST
The nitrates are picked up on a paraffin wax cast. The picked-up residues on the cast are treated with diphenylamine dissolved in strong sulphuric acid. The appearance of blue color spots indicates nitrates which in turn are indicative of gunshot residues.
Its simplicity and greater specificity make it a convenient test. A desensitized glossy bromide paper is taken. It is treated with two naphthylamines 4:8 Sulphonic acid (5%). The bromide paper is placed over a table with the treated surface upward. The cloth bearing the gunshot residue is placed over the bromide paper. It is covered with a towel moistened with 20% acetic acid, and this set-up is pressed with a hot electric iron for about five to ten minutes. Dark red spots on the bromide paper indicate gunshot residue.
This test is helpful to identify bullet entrance holes and wounds, determine the range of fire, identify a shooter, and estimate the approximate time of the fire.
HARRISON AND GILROY’S TEST
his test determines the presence of lead, antimony, and barium through a spot test. The gunshot residue containing the metallic constituents is collected on a piece of cotton cloth that is moistened with dilute 0.1 molar hydrochloric acid.The cloth is then tested under a dried piece of test cloth is treated with one drop of triphenyl methylarsonium iodide alcoholic solution (10%). The appearance of an orange ring, in about two minutes, indicates the presence of antimony.
The test cloth bearing the orange ring is dried, and two drops of sodium rhodizonate solution(5%) are put in the center of the ring. Red color development inside the ring indicates lead, barium, or both. The test cloth is dried again. A drop of dilute hydrochloric acid (1:20) is placed on the respot developed. If the color changes to blue lead are indicated. If the color does not change, barium is indicated.
PRICE’S SPOT TEST
This technique involves identifying lead through a standard spot test using sodium rhodizonate solutions. The gunshot residues are collected from the hands-on a piece of cloth moistened with 10% hydrochloric acid.
This test is done to detect the presence of nitrates. The reagent gives rise to GSR as nitrates are not a common commodity in everyday life. The test is relatively specific for GSR.
4. Instrumentation methods.
NEUTRON ACTIVATION ANALYSIS
The examination of gunshot residue by neutron activation analysis detects the presence of antimony and barium. When the gunshot residues are exposed to neutron bombardment in a nuclear reactor, antimony and barium capture one neutron to give radioactive antimony and radioactive barium.
FLAMELESS ATOMIC ABSORPTION SPECTROMETRY
Atomic Absorption Spectroscopy is a convenient technique for the evaluation of gunshot residues. It can detect the elements in nanogram and picogram ranges. It works on a straightforward principle. An element absorbs the radiation of the same wavelength as it emits when excited. The suspected element is 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. The loss of intensity in the radiation gives the measure of the metal in question.
Analysis of gunshot residues helps in determining the various components present, such as barium, antimony, nitrate, and lead that plays a vital role in determining the primers and propellants used in the weapon, which in turn helps in the identification of weapon used, the range of firing and also identify the shooter.