Gunshot Residue Analysis by Harrison-Gilroy’s Test
Harrison-Gilroy’s test is a method of detecting metallic constituents in the firearm discharge or gunshot residue(GSR). This test is specific for the presence of barium, lead, and antimony in the discharge. These metallic constituents are present in the primer charge and the bullet.
In 1959, Harrison and Gilroy developed the qualitative colorimetric test for the detection of the presence of antimony, barium, and lead in the hands of individuals who fired firearms. These metals which are discharged from the primer of the fired cartridge are deposited on the back of the firing hand as discrete particulate matter.
In revolvers, these metallic particles come primarily from the cylinder-barrel gap while in automatic pistols these particles come from the ejection port.
Earlier than the above-mentioned test, the Paraffin Test or Dermal nitrate test was used which detected propellant residues. But this test was somehow immaterial and non-specific. Therefore Harrison and Gilroy developed a relatively simple and inexpensive method for the detection of propellent residue. Unlike paraffin tests, Harrison-Gilroy’s test is more specific and suitable for field use.
One of the important features of Harrison-Gilroy’s test is that it is based on primer residue rather than powder residue. The test is found to be very useful in identifying the shooter or bullet hole present at the scene of the crime.
Primers are the components of gunshot residue that are propelled out of the firearm after firing the cartridge. They are chemicals that have a tendency to ignite when they are subjected to great pressure. The most commonly used primer charges are- lead azide, lead styphnate, mercury fulminate, barium nitrate, potassium chlorate, and antimony sulfide.
Primer is placed at the base of the cartridge case. When the trigger of a gun is pressed, the firing pin strikes the base of the cartridge case, which ignites the primer. The ignition of the primer produces flames which further ignites the propellant charge (black powder or smokeless powder or semi-smokeless powder). The propellant charge on burning produces a high amount of gases that finally propels the cartridge out of the firearm.
When the cartridge is fired out from the firearm, the combusting primer and propellant charge also come out of the firearm which is deposited on the hand of the person who fired the firearm. This deposited powder is easily collected from that person’s fingers and hand used to fire.
How is Harrison-Gilroy’s Test Conducted?
The test begins with the collection of the propellent residue from the hand of the person who fired the firearm.
- A square cotton cloth is moistened with hydrochloric acid and then used to swab the hand or the fingers containing the propellant charge.
- The swab is dried in air and then sprayed with a 10% solution of Triphenylmethyl Arsonium Iodide in alcohol. If antimony is present then there will be an orange coloration on the swab.
- The swab is again dried and sprayed with a freshly prepared 5% saturated solution of Sodium Rhodizonate which gives a red spot in the presence of barium or lead in the propellent residue.
- For the confirmation of lead in the residue, the swab is again dried and sprayed with dilute hydrochloric acid. Then the red spot changes to a purple spot, which is a positive test for lead. But in the absence of lead, the red color persists after spraying dilute hydrochloric acid.
Reactions
- Triphenylmethyl Arsonium Iodide (alcoholic) + Antimony = Orange colour
- Triphenylmethyl Arsonium Iodide (alcoholic) + Sodium Rhodizonate + Barium = Red colour
- Triphenylmethyl Arsonium Iodide (alcoholic) + Sodium Rhodizonate + Lead = Purple colour
Drawbacks of Harrison-Gilroy’s Test
Though this method is considered good for detecting the propellant residue, still it is not widely used because of the following limitations:
- The method is only qualitative and the results cannot be quantified.
- There is an interference in the color reactions within the three elements themselves.
- The colors developed after the reactions with reagents are unstable.
- The method is inadequately sensitive.
- The color reaction for the elements is not fully specified.
- It is not adequate to reliably detect the low concentrations found in actual firings.
Conclusion
Harrison-Gilroy’s test is a simple and inexpensive method to detect the presence of metallic components (antimony, barium, and lead) of the primer of the gunpowder residue. It is basically a color spot test based on the chemical reactions between the metallic constituents of the primer and the chemical reagents (Triphenylmethyl Arsonium Iodide and Sodium Rhodizonate) used for the tests.
The test shows positive results by developing colored spots for each of the three metals present in the primer. It is a very useful test to identify the shooter or the bullet hole present at the crime scene. However, it is not widely accepted in forensic science laboratories because of its inadequate sensitivity, unstable color development, interference of the metallic elements with each other and reagents, and lower specificity. But this test is more specific than the Paraffin test.
Also Have a Look at:
- Post Blast Explosive Residue Collection and Analysis
- Mechanism of formation of Gunshot Residues
- Synthesis And Characteristics of TNT, PETN & RDX
