Qualitative and Quantitative Differences between GSR and non-GSR Source Particles Revealed by Raman Spectroscopy

Analysis of Gunshot Residue (GSR) is relevant to forensic science by providing a correlation between an individual and a firearms incident. Detection of GSR is usually made by using Scanning Electron Microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (SEM-EDX). Particles originating from some manufacturing trades could represent false positive threats to SEM-EDX analysis of GSR. In particular, GSR-like particles arising from brake pads are very similar in composition. In this study, we propose a non-destructive complementary method for the discrimination of GSR samples by using Raman Spectroscopy. The analysis of Raman spectra obtained from GSR and brake pads is able to differentiate the origin of the particles in a qualitative and quantitative manner. In particular, the center frequency of the G-band is shifted to lower frequencies for brake pad dust; on the other hand, the center frequency of the band is shifted to higher frequencies for GSR samples. In addition, to assess quantitative changes due to chemical structural modifications, we computed the intensity ratio R between the detected band centered around 1350 cm-1 and 1585 cm-1, respectively. The R values for GSR samples were, in all cases, lower than the R values computed for brake pad samples. In addition, scanning SEM was employed to characterize the morphology of the samples, coupled with EDX to determine the elemental composition of the sample surface. Since elemental compositions detected via traditional SEM-EDX can overlap between brake and GSRs, detailed morphological and structural analyses are mandatory for an unambiguous attribution. Although SEM-EDX remains the gold standard for GSR analysis, this study demonstrates that its combination with Raman spectroscopy provides a crucial advantage. Leveraging its non-destructive nature, Raman spectroscopy offers vital complementary characterization, establishing a promising framework to enhance the reliability of traditional forensic technical investigations.