Detection of gunshot residues using infrared photography: influence of ammunition type, surface color and blood contamination

Detecting gunshot residues (GSR) plays a crucial role in forensic science and forensic medicine by providing important insights into the shooting distance, the shooter, as well as the type of weapon and ammunition used. Detection of GSR on dark surfaces is often impossible on site, and traditional methods such as tape-lift techniques with adhesive films or scanning electron microscope tabs might destroy the GSR pattern at the crime scene during their application. Infrared (IR) photography has proven particularly effective in detecting GSR on dark surfaces, enabling the preservation of the GSR pattern before applying destructive methods. This study aimed to examine how the type of ammunition and the presence of bloodstains affect GSR detection and differentiation using IR photography. 15 types of leaded and 5 types of lead-free 9 mm Luger ammunition were fired onto white cotton fabric and dark-blue denim fabric using the same firearm model, with an additional 14 samples being stained with blood. Resulting GSR patterns varied depending on the ammunition type and fewer GSR particles were visible on denim fabric, even in IR photography, compared to white cotton fabric, but still achieving reliable results comparable to the standard tape-lift method. In addition to the known ability of IR photography to detect GSR from leaded ammunition on clean surfaces, our findings demonstrate for the first time that GSR from lead-free ammunition, as well as GSR on blood-contaminated surfaces can be reliably visualized. In conclusion, IR photography provides a robust, easy-to-use and non-destructive tool for GSR detection, with the distinct benefit of allowing immediate on-site visualization of GSR patterns prior to any laboratory processing. With the use of the custom-made Python script (openly accessible on Github), a quantitative evaluation of GSR comparable to the tape-lift method is a further enhancement in this aspect.