Irish researchers have developed a novel forensic technique that can recover fingerprints from ammunition casings even after exposure to the extreme heat of gunfire -- a capability long considered the "Holy Grail" of forensic investigation.
A common issue investigators face is the recovery of fingerprints from bullet casings because biological trace evidence, such as the oil from human skin contact, is often destroyed by the gases, high temperatures, and friction that occur after a gun is fired. Criminals will sometimes abandon their weapons or shell casings at crime scenes, with no fingerprint evidence left behind.
But a new test, detailed in a recent study published in the journal Forensic Chemistry, seems to be able to detect those biological traces.
The research was led by Dr. Eithne Dempsey and Dr. Colm McKeever from Maynooth University's Department of Chemistry, who successfully created an electrochemical method using specialized polymer combinations to visualize latent fingerprints on brass surfaces.
When someone touches a brass surface, like a bullet casing, they leave behind invisible traces of natural oils and sweat from their skin. These residues act like an invisible stencil or mask on the metal surface. According to the study, the scientists then apply a very small electrical current -- around 0.1 volts for 120 seconds -- to the brass surface while it's submerged in a solution containing special polymers. The electricity causes these materials to stick to the brass, but only in the areas where there's no fingerprint residue blocking the process.
"Using the burnt material that remains on the surface of the casing as a stencil, we can deposit specific materials in between the gaps, allowing for the visualization," explained Dr. Colm McKeever in a press statement.
In other words, the polymer deposits everywhere except where the fingerprint ridges touch the surface, creating a reverse or "negative" image of the fingerprint.
The breakthrough centers on a polymer secret sauce. Using co-monomer combinations of 3,4-ethylenedioxythiophene (EDOT) and thionine acetate, the team noticed that they received the highest quality visualizations. Moreover, this is the first reported use of electrochemical phenothiazine co-polymer deposition for forensic fingermark analysis on brass ammunition casings.
"Currently, the best case of forensic analysis of ammunition casings is to match it to the gun that fired it," said Dr. McKeever. "But we hope a method like this could match it back to the actual person who loaded the gun."
Much to the researcher's surprise, the test was incredibly successful, revealing fingerprint details that include individual pores within papillary ridges. Moreover, the technique demonstrated extraordinary longevity. It was able to visualize fingerprints aged up to 16 months at room temperature, and on casings that were exposed to 700°C, simulating the heat generated by gunfire.
This solves a significant problem in firearm forensics. Traditional fingerprint visualization on brass surfaces faces significant challenges due to the metal's reactivity, surface oxidation, and the need for aggressive chemicals that can destroy evidence. In contrast, the new electrochemical approach uses non-toxic, water-based reagents and requires minimal sample preparation while preserving evidence integrity.
And due to the nature of the test, the details of the fingerprints are incredibly high. At 1000x magnification, fingerprint ridge diameters were viewed at approximately 450 micrometers with pore diameters around 150 micrometers. That is more than enough detail for definitive forensic identification.
The test holds a lot of promise, but to pass muster -- especially in a court of law where guilt and innocence can hang in the balance -- more rigorous validation and testing are required. Still, the researchers believe their technique could extend beyond firearms to other metallic surfaces, expanding forensic applications from firearm-related crimes to arson investigations.