Residue forms identifying fingerprint for brands of bullets

Professor Paul Kirkbride with his PhD students, Nick Lucas and Kelsey Seyfang, in their lab at Flinders University(Credit: Flinders University)

In the forensic sciences, the discovery of a single fact can convict a criminal or let an innocent person walk free. This is particularly important in gun crimes and a team of scientists at Flinders University in Australia have developed a new technique for analyzing gunshot residue that is so sensitive that it's possible to match residue with a specific brand of ammunition.

As fans of the CSI shows know, ballistics is a major field of the forensic sciences. Beginning with such simple tests as using microscopes to show that a particular bullet was fired by a particular gun, scientists have moved on in the past century to such details as the distance the gun was fired at, the oil that lubricated it, and the residues left behind by the fired ammunition.

Firearm residue is a particularly important friend of the police forensic scientist because these byproducts of ammunition propellants can link a particular person to a particular weapon during a particular time period. Detecting and analyzing these residues once relied on simple chemical tests, but over the years more effective tests were developed using neutron activation, atomic absorption spectrophotometry, inductively coupled plasma mass spectroscopy, atomic force microscopy, and, most successfully, scanning electron microscopy with energy dispersive analysis by x-ray detector (SEM-EDX).

What all of these tests are looking for are the leftovers from a cartridge firing. Modern firearms use a complex mixture of smokeless propellants, primers, flash suppressors, deterrents, and others made up of various organic, mineral, and metallic compounds. When the propellant burns at a temperature of over a 1,000° C (1,832° F), these are mostly turned into carbon dioxide, carbon monoxide, and steam, but traces of the propellant metals, organics, and silicas, along with lubricants and metals from the cartridge casing are left behind – often in the form of microscopic nodules.

What the Flinders team is working on is the round glass fragments formed by the gunshot. Under the direction of Flinders University's Professor Paul Kirkbride, the team is studying how to determine how gunshot residues are deposited on suspects, and how to identify glass fragments and match them to particular brands of ammunition.

They do this by using two very sensitive techniques called Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and the Sensitive High Resolution Ion Microprobe (SHRIMP). As the names suggest, they use ions to scan samples and determine not only the elements present, but the isotopes. By measuring the ratio of these isotopes in the glass fragments, they can be matched against the isotopes in brands of cartridges with the precision of fingerprints. In this case, it's .22 ammunition, which is the most commonly used in Australian gun crimes.

"We've shown matching characteristics in the trace elements and isotopes found in glass fragments in the residue left on the shooter, in the wound and in the specific batch or brand of ammunition," says Professor Kirkbride. "This is like a fingerprint, which doesn't change before, during or after the gun is fired.

"Eventually we hope to provide law enforcement agencies with the ability to identify not only the brand of ammunition, but also the location of manufacture and points of distribution, which all contribute significantly towards identifying the purchaser."

The results of the team's research will be presented next month at an international conference in New Zealand.

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