Scientists from the University of Nottingham say they have developed a spectroscopy-based technique that can retrieve high-resolution fingerprint images from surfaces where conventional fingerprint imaging typically fails.
James Sharp, an associate professor in the School of Physics and Astronomy, and his team boosted the time-of-flight secondary ion mass spectroscopy (ToF-SIMS) method with a key creation—a rotation stage that caters to the imaging of challenging shapes, like curved bullet casings, as well as difficult materials, such as metal bullet casings.
ToF-SIMS is a sensitive surface-analysis technique that provides very detailed information about the locations of different chemical species on a surface. The technique uses high-energy beams of positive ions directed at the sample’s surface to free secondary ions from any material that they collide with. These ions are then accelerated into a time-of-flight analyzer and separated according to their mass-to-charge ratio, producing a spectrum that is indicative of the sample’s chemical composition.
When a bullet is fired, the casing is subjected to a multitude of environments that can make retrieval of fingerprints exceedingly difficult, including high temperatures, pressures and friction. Bullet casings can also be contaminated by propellant and powder residues that are used to generate the reaction that forces the bullet out of the chamber.
“These combined effects often result in the removal, evaporation or degradation of the more volatile components of fingermark residue—such as water, amino acids and low molecular weight organics such as lipids—as well as potential smudging or obscuring the mark. These factors can make it difficult for conventional methods of fingermark retrieval such as cyanoacrylate fuming and fluorescent staining approaches to work,” explain the University of Nottingham scientists.
In experiments performed over seven months, Sharp and his team were able to extract images from bullet casings using ToF-SIMS and the rotation stage that showed a fingerprint’s friction ridge and sweat pore level detail. When tested using a more conventional technique involving cyanoacrylate and Basic Yellow 40 dye, the researchers say these characteristics were not visible.
Importantly, the team showed that the ToF-SIMS technique is non-destructive, recording no evidence of image degradation over the seven months, even when samples were repeatedly exposed to UHV conditions.
“We already proved in our previous research that ToF-SIMS imaging provides much more accurate and detailed fingerprint images on different types of surfaces,” said Sharp. “It’s really exciting to be taking this research a step further by adding the rotational stage. This new rotational capability allows us to image in even more detail and over whole surface areas of difficult materials and shapes while keeping the evidence intact. This could really pave the way for a new reliable way to analyze evidence, identify persons of interest and link them to the ammunition in a firearm.”
In 2019, Sharp was a co-author on the first study that tested ToF-SIMS on fingerprints placed on disks of brass, aluminum and stainless steel. The results showed that the fingerprints developed using traditional techniques were unreliable, with quality degrading after 8 days. Some prints were even unrecognizable within 3 days, and one sample showed no results at all after the 14 days. With the ion beam from the ToF-SIMS method, researchers were able to detect and visualize fingerprint secretions in samples up to 26 days after they were taken, a first in the field.
Sharp’s 2021 study is a continuation of this one, which has already piqued the interest of the local East Midlands Special Operations Unit.
“We are excited about the opportunities that this new technique offers in developing fingermarks on challenging surfaces,” said Vickie Burgin, Deputy Head of Forensic Services, East Midlands Special Operations Unit, at the time. “This innovative approach could help us identify suspects and revisit historical crimes where success has—in the past—been low.”
Photo: The newly developed rotation stage. Credit: University of Nottingham.