Integrating Chromophoric and Instrumental Methods (SEM-EDS and FTIR) for Accurate Shooting Distance Estimation: An Inter-laboratory Study in Forensic Chemistry

Background: Accurate shooting distance estimation is critical in forensic investigations. Traditional color tests like the Modified Griess Test (MGT) and Sodium Rhodizonate Test (SRT) are commonly used but lack sensitivity and objectivity, necessitating improvements in reproducibility and validation. Methods: A 7.62x39 mm cartridge was fired using an assault rifle (Sarsılmaz® SAR 308) at fabric samples from distances ranging from 0 to 100 cm. MGT and SRT were employed to estimate shooting distances, and the presence of inorganic and organic gunshot residues was confirmed using Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEMEDS) and Fourier Transform Infrared Spectroscopy with Attenuated Total Reflection (FTIRATR). Inter-laboratory reproducibility was assessed through Relative Standard Deviation (RSD), paired t-tests, and Pearson Correlation Coefficient (PCC). Results: MGT and SRT showed high reproducibility at short distances (RSD <11% up to 30 cm), but sensitivity decreased at longer distances, with an RSD of 28.6% for SRT at 100 cm. Paired t-tests revealed statistically significant consistency between laboratories (p < 0.05), and a strong negative correlation (PCC = -0.72) was found between shooting distance and residue area. Conclusion: Integrating SEM-EDS and FTIR-ATR with MGT and SRT enhances the sensitivity, objectivity, and reproducibility of shooting distance estimations. This multi-method approach provides more reliable forensic evidence for criminal investigations.