Module 4 - Forensic Ballistics

Whenever a gun is fired, the shooter gets sprayed with an invisible blast of chemical residue that consists of the by-products of the incomplete combustion of gunpowder and primer. These products of combustion are deposited on areas of the exposed hand, arm, face, and clothing—right or left side, depending on how the gun was held. The deposited material is known as gunshot residue (GSR), detectable on a person’s skin and clothing to determine if that person has fired a gun.

Because many different types of chemical compounds are in both gunshot residue and primer residue, many testing techniques can be used to detect them. Each of the GSR chemical tests identify unique elements, ions, or compounds. Gunpowder consists of charcoal, sulphur, potassium nitrate, nitrocellulose, nitro-glycerine; primers consist of lead(II) azide, lead(II) styphnate, mercury(II) fulminate, barium nitrate, potassium chlorate, and antimony(III) sulphide.

Paraffin Test

This chemical test is also known as the dermal nitrate test or diphenylamine test. In this procedure, the hands of the suspect are coated with a layer of paraffin. After cooling, the paraffin casts are removed and treated with an acidic solution of diphenylamine. This solution detects two polyatomic ions (nitrites and nitrates) that originate from gunpowder or primer. A positive test produces blue flecks in the paraffin in response to deposits on the hands of the shooter.

Sodium Rhodizonate Test

The sodium rhodizonate test is designed to determine if barium or lead residue are present on the hands. In this test, the hands of the suspect are swabbed with gauze moistened with dilute hydrochloric acid. Then, the gauze is cut into small pieces and drops of sodium rhodizonate, distilled water, and a buffer solution are added to each piece.

A colour change indicates that a salt has formed from rhodizonate and either barium or lead. If the gauze turns a brown-pink, barium was present on the hand. If the gauze turns a scarlet red, then lead was present. A mixture of both these colours indicates that both elements were present.

Harrison-Gilroy Test

The Harrison-Gilroy Test checks for the presence of barium, antimony, and lead on the hands. In this chemical test, the hands of the suspect are swabbed with gauze moistened with dilute hydrochloric acid. The gauze is allowed to dry and then treated with the chemical reagent triphenylmethylarsonium iodide. If antimony is present, the triphenylmethylarsonium iodide turns an orange colour.

Modified Greiss Test

The Modified Greiss Test detects the presence of nitrite residues near suspected bullet holes on a surface such as wood, metal, furniture, clothing, belt, hat, shoes, or purse. Nitrite residues are a common by-product of the combustion of smokeless gunpowder.

In this test, a piece of photographic paper is treated with a chemical mixture of sulfanilic acid and alpha-naphthol in methanol. As a result, the photographic paper is no longer light-sensitive, but it is reactive to the presence of nitrite residues. Then, the paper is placed face down against a suspected bullet hole. The back of the photographic paper is steam ironed with vinegar instead of water in the iron. Vapours from the vinegar penetrate the paper, and a reaction occurs between any nitrite residues on the suspected bullet hole and the chemicals in the treated paper. One of the products of the resulting reaction appears as red and/or orange specks on the photographic paper.

X-ray Analysis with a Scanning Electron Microscope (SEM)

The scanning electron microscope method using X-ray analysis is considered the most reliable and useful way to test for gunshot residue because it can detect any type of element, ion, or compound. This testing procedure is especially helpful when used to confirm the results of one of the other GSR chemical tests. The false negative rate is only 10% for pistols, but it is 50% for long guns.

Samples taken from the hands are analyzed with a scanning electron microscope. X-rays within the microscope are capable of identifying individual particles. Because the scanning electron microscope is an expensive piece of equipment requiring highly specialized experts, not all police departments are able to use X-ray analysis. However, using this method to analyze a sample is possible up to twelve hours after the shooting.

Validity of Gunshot Residue Analysis

Positive GSR test results alone never prove the guilt of a suspect in a court of law. However, when these results are combined with other valid pieces of evidence, gunshot residue test results are powerful.

The strongest conclusion that can be made using only gunshot residue test results is that a suspect recently fired a gun, handled a recently discharged gun, or was close to a gun when it was fired. If the suspect’s fingerprints are also found on a gun, a positive GSR test is compelling evidence that the suspect fired that gun. However, a negative GSR test does not necessarily prove that a suspect did not fire a weapon.

The elements, ions, and compounds such as nitrate, nitrite, or lead that GSR tests identify are not unique to guns and cartridges. Although it is unusual, these substances can also be found in the environment. A person who has not fired a gun but has been near the shooter or the gun after it has been fired can become contaminated with gunshot residue. This is known as secondary transfer. Based on these issues, defence lawyers debate the reliability of GSR evidence in court.

Forensic experts conducting GSR testing are aware of potential contamination problems. Therefore, a positive test for GSR is typically not seen as conclusive unless three or more unique elements or compounds have been detected.