Photoluminescent detection of gunshot residue

Crime scene investigation may soon become more accurate and efficient thanks to a new method for detecting gunshot residue — one in which the lead particles found in the residue are converted into a light-emitting semiconductor.

The new method, published in the journal Forensic Science International, is understood to be faster, more sensitive and easier to use than current alternatives. Furthermore, forensic experts in the Amsterdam police force are already testing it in actual crime scene investigations.

Currently, when a weapon is fired, it leaves gunshot residue containing traces of lead in the surrounding environment, including clothing and skin. Obtaining an indication of gunshot residue at the crime scene helps to answer key questions about shooting incidents, so police will test for lead traces on possible bullet holes and any suspect or victims’ clothing and hands, sending all samples to the lab for analysis. However, the methods used there are often time-consuming, labour-intensive, and require expensive equipment.

A few years ago, the research group of Wim Noorduin at AMOLF in the Netherlands developed an easy-to-use lead detection method based on perovskite technology (perovskite being a promising material used in applications ranging from solar cells to LEDs). In this method, a reagent converts lead-containing surfaces into a perovskite semiconductor. A UV lamp will make the newly formed semiconductor emit a bright green glow visible to the naked eye — making even small traces of lead easily detectable.

In 2021, Noorduin and Lukas Helmbrecht — a former PhD student in Noorduin’s group — established a startup company to develop this lead testing method into a practical lead detection kit: Lumetallix. In recent years, people worldwide have been investigating their surroundings using the Lumetallix test kit, reporting positive tests in all sorts of objects — from dinner plates, to beer glasses, to paint dust at construction sites. Helmbrecht has now developed an altered version of the Lumetallix reagent for forensic applications: one that reacts especially well with lead atoms in gunshot residue and produces a long-lasting green glow.

To validate the effectiveness of this method, the researchers conducted a series of controlled experiments, which saw PhD students Kendra Adelberg and Arno van der Weijden visit an Amsterdam shooting range. Adelberg explained, “We used standard 9 mm full metal jacket bullets and fired them from two different pistols at cotton cloth targets placed at various distances. After applying the reagent, we visualised the gunshot residue patterns. The results revealed well-defined luminescent patterns that were clearly visible to the naked eye, even at extended distances.”

During their experiments, Adelberg and her colleagues made two other remarkable discoveries. First, unlike other methods, the light-emitting technique remains effective even after extensive washing of the shooter’s hands. This is important for forensic investigations, as suspects often try to tamper with or remove evidence of their involvement. Second, bystanders standing approximately two metres away from the shooter also tested positive for lead traces on their hands.

“'These findings provide valuable pieces of the puzzle when reconstructing a shooting incident,” Adelberg said. “But a positive test also needs to be carefully interpreted; it does not automatically mean that you fired a gun.”

The researchers believe their method will be especially beneficial to first responders, such as police officers, who can use it to rapidly screen potential suspects and witnesses to secure crucial evidence. Beyond forensic applications, the team is also exploring the potential of this light-emitting method to detect lead contamination in environmental samples such as water and soil. Since lead is toxic and harmful to the environment, this research could have broader implications for environmental monitoring and public health.

Image caption: The combustion plume and bullet wipe contaminate objects such as clothing, hands and targets with gunshot residue, which is rapidly visualised with photoluminescent lead detection to assess suspect involvement and to assist crime scene reconstruction. Image has been cropped from the original and is courtesy of the study authors under CC BY 4.0