Voltammetric Determination of New Chemical Markers in Gunshot Residues from Conventional and Ecological Ammunitions: Importance of Forensic Electroanalysis

Abstract

Gunshot residues (GSR) are chemical traces present in the shooter's body. GSR are primarily deposited on the dorsal region of the hands. Metallic microparticles formed by Sb, Ba, and Pb are the main inorganic residues produced by conventional ammunition. These residues are commonly analyzed by SEM-EDS for forensic evidence. Currently, two experimental limitations for forensic GSR analysis exist: GSR (1) are produced indistinctly (revolvers, pistols, and shotguns, among other firearms, produce the same GSR.) and (2) are not produced when metal-free (ecological) ammunition is fired. To overcome the issue of absent metallic traces and to provide proper ammunition traceability, we suggest using luminescent chemical markers based on rare earth ions. These markers can be synthesized with different elements, to produce distinct colors depending on the firearm caliber. In addition, these markers present good visual detectability under forensic flashlights, and they can be detected and quantified by sensitive, low-cost instrumental techniques, such as voltammetry. In this context, we aimed to develop a composite containing Eu and Zr and to incorporate it in conventional and ecological ammunition propellants to test whether voltammetric detection of GSR traces collected after the shots is feasible. Conventional and ecological .380-caliber pistol ammunition was used. Both types of ammunition were previously disassembled with a commercial inertia hammer. Aliquots of the composite containing Eu and Zr were mixed at 10% ratio (w/w) with the propellants of these types of ammunition. Later, ammunition was reassembled with a mechanical press from Lee, model Lock Pro. The firing step was carried out in a professional shooting range; a .380 caliber pistol from Taurus, model PT 638 PRO, was used. Shots were fired on boards of pressed wood placed 1.0 m away from the end of the gun barrel. After the shooting stage, a Spectroline UV flashlight, model ENF-280C, was used to detect the luminescent chemical marker in the ammunition capsules and shot residues deposited on the hands, target, and weapon. GSR for SEM-EDS analysis were collected with aluminum stubs equipped with conductive adhesive tape. GSR aliquots for GC-MS analysis were collected by using swabs soaked with chromatographic grade ethanol solution and placed in 2.0-mL Eppendorf tubes. GSR aliquots for voltammetric analysis were collected by using carbon paste electrodes, with their surface being lightly pressed on the sample collection region. GSR detection by UV flashlight showed that the luminescent chemical marker was present on the target, hands, and weapon and in the deflagrated ammunition cases, as evidenced by red dots spread over the samples. This indicated that the method was practical, and that the luminescent chemical marker was effective for the intended purpose. Analysis of GSR from conventional and ecological cartridges by GC-MS showed that they had distinct chemical profile, but the chemical components present in the luminescent chemical marker could not be detected. MEV-EDS analysis allowed particles containing the luminescent chemical marker to be detected in the cases of GSR from conventional and ecological munitions. The characteristic peaks of Zr and Eu were observed during EDS analysis, indicating that this technique can also be used to detect these markers. The voltammetric results indicated that the luminescent chemical marker can be detected through Zr detection at an anodic peak potential of 0.45 V vs. Ag/AgCl, consistent with the expected signal for this element. Additional substances present in these residues did not interfere in voltammetric Zr detection, making the new methodology a fast and viable alternative for forensic analysis. Figure 1