RESUMO
The automated search software integrated with a scanning electron microscope (SEM/EDS) has been the standard tool for detecting inorganic gunshot residues (iGSR) for several decades. The detection of these particles depends on various factors such as collection, preservation, contamination with organic matter, and the method for sample analysis. This article focuses on the influence of equipment resolution setup on the backscattered electron images of the sample. The pixel size of these images plays a crucial role in determining the detectability of iGSR particles, especially those with sizes close to the pixel size. In this study, we calculated the probability of missing all characteristic iGSR particles in a sample using an SEM/EDS automated search and how it depends on the image pixel resolution setup. We developed and validated an iGSR particle detection model that links particle size with equipment registers and applied it to 320 samples analyzed by a forensic science laboratory. Our results show that the probability of missing all characteristic iGSR particles due to their size is below 5% for pixel sizes below 0.32 µm2 . These findings indicate that pixel sizes as large as twice the one commonly used in laboratory casework, that is, 0.16 µm2 , are effective for initial sample scanning, yielding good detection rates of characteristic particles that could exponentially reduce laboratory workload.
RESUMO
Using a video tape and three still photographs, we analyze recorded sound to attribute authorship in a fatal shooting incident. We measure the acoustic signature of the scene with four test detonations, and we discuss two methods of processing putative echoes in the audio band of the video. They allow us to locate 11 out of 17 shots, with a precision of 0.4 x 1.0 m2. Combining the location of the shots with the known positions of the participants, we arrive at the clear attribution of eight shots; three other shots (among which is the fatal one) probably issue from the same shooter, who remains unidentified within a small group of men. Our results lead to the exculpation of the main suspect. We analyze the statistical significance of the results, use Monte Carlo simulations to set an upper bound to the probability of false positives, and discuss areas of improvement of the method.