As forensic evidence goes, gunshot residue can be full of holes. But a new technique could provide a more definitive link between suspect and gun. A tool that employs lasers may allow investigators to match residue alone to a specific caliber of firearm, scientists report in two recent papers in Analytical Chemistry.
“Anything that’s going to enhance or expedite the detection of gunshot residue and provide stronger evidentiary value is a way forward,” says analytical chemist Jason Birkett of Liverpool John Moores University in England. “This work is very good and very novel and will do nothing but assist.”
Along with the bullet, a cartridge or round also contains a propellant (usually gunpowder) and a primer. When the firing pin strikes the primer cap, the primer ignites, igniting the propellant and expelling the bullet from the gun. “It’s a very complicated chemical reaction involving extremely high temperatures and pressures,” says Birkett. This reaction results in a spray of residue that can land on clothing, skin or anything nearby.
The resulting gunshot residue includes burnt and unburnt powder, primer, smoke from the combustion and grease and metals. Gunshot residue analyses typically use scanning electron microscopy to assess particles of barium and antimony (from the primer) and lead (from primer and bullet). Finding these particles indicates that a gun was fired and can help determine the distance the bullet traveled, but the approach is time-consuming and often not specific enough to link the residue to a gun of a particular caliber. These particles can also come from other sources, such as fireworks or the bits of dust some machine-shop workers might encounter. And to further complicate things, lead and other metals are being phased out of some cartridges for environmental reasons.
Now two teams have tackled gunshot residue with Raman spectroscopy, in which laser light of a specific wavelength is shined on a sample, sending its molecules vibrating.
Researchers in Spain analyzed the Raman spectra of six kinds of ammunition, assessing organic ingredients such as gunpowder stabilizers rather than metal signatures. The team assessed propellant from unfired cartridges, then compared these spectra to the signature in residue from fired guns. The technique revealed a particular residue signature for each type of ammunition, the scientists reported March 15. These findings suggest that Raman spectroscopy could be used even in situations where there is no smoking gun or the bullet is too mangled to match to a weapon.
Another team, at the University of Albany in New York, also employed Raman spectroscopy to look for an ammunition signature in gunshot residue. Their analyses compared residue from a 9 mm and from a 0.38 special. While the spectra looked deceivingly similar, additional statistical analysis allowed the scientists to differentiate between the two calibers.
“It worked extremely well,” says laser spectroscopist Igor Lednev, who published the work March 13 with graduate student Justin Bueno and postdoc Vitali Sikirzhytski. The approach could be used to quickly rule out specific firearms found on a suspect or at a crime scene, Lednev says.
Many more analyses need to be done before CSI teams employ the method and it’s tested in a courtroom, but the approach looks very promising, says Lednev. One day, investigators might even be able to flip through a database of Raman spectra of different ammunitions to more quickly link that crime-scene residue to a specific kind of gun.