# Scanning Electron Microscopy Techniques in the Analysis of Gunshot Residues: A Literature Review
### SEM Detectors and Imaging Techniques in GSR Analysis  
SEM employs different types of detectors to capture various signals generated when the electron beam interacts with the sample. The secondary electron (SE) detector captures low-energy electrons emitted from the sample surface when bombarded by the electron beam [1; 2; 3; 4; 5; 6; 7]. These electrons originate from the top layers of the material, providing high-resolution images with excellent surface detail. This type of imaging is particularly useful for examining the morphology of GSR particles, allowing forensic scientists to identify characteristic spherical shapes that result from the high-temperature vaporization and condensation of metals upon firearm discharge [1; 2; 3; 4; 5; 6; 7]. Since secondary electrons are highly sensitive to topographical variations, they are effective in distinguishing the fine surface features of GSRs from environmental contaminants. The backscattered electron (BSE) detector operates by detecting high-energy electrons that are elastically scattered from the sample. Unlike secondary electrons, backscattered electrons originate deeper within the material, and their signal intensity is directly related to the atomic number of the elements present. This characteristic makes BSE imaging particularly valuable in forensic applications, as it enables the differentiation of GSR particles based on their elemental composition. Heavier elements, such as lead (Pb), barium (Ba), and antimony (Sb), appear brighter in BSE images, while lighter elements, commonly found in non-GSR particles, appear darker. This contrast allows forensic scientists to quickly assess the presence of GSR particles and distinguish them from naturally occurring environmental debris [1; 2; 3; 4; 5; 6; 7]. In addition to electron-based imaging, SEM is frequently combined with energy-dispersive X-ray spectroscopy (EDS) to perform elemental analysis of GSR particles. When the electron beam interacts with the sample, it excites atoms within the material, causing them to emit characteristic X-rays that correspond to specific elements. By analyzing the emitted X-ray spectrum, EDS allows for the precise identification of the chemical composition of the particles. This technique is crucial in forensic investigations, as it confirms the presence of elements commonly associated with firearm discharge, such as lead, barium, and antimony. Combining SEM imaging with EDS analysis enhances the reliability of GSR identification, reducing the risk of misclassification due to environmental contamination [1; 2; 3; 4; 5; 6; 7]. Different imaging techniques can be employed in SEM to optimize the visualization of GSR particles. Low-voltage imaging improves resolution and reduces sample charging effects, which is particularly beneficial for analyzing non-conductive GSR particles without requiring a conductive coating. Environmental SEM (ESEM) allows imaging under low-vacuum or variable-pressure conditions, enabling the analysis of uncoated forensic samples while preserving their original state. Additionally, the integration of automated SEM-EDSparticle recognition software has significantly enhanced the efficiency of GSR analysis by streamlining particle detection, classification, and chemical composition assessment. This article explores, through a narrative review, the application of SEM techniques in GSR analysis to clarify the methodological approaches supporting effective forensic investigations, highlight the advantages and limitations of SEM, and enhance the accuracy of GSR particle identification. Understanding the morphology and composition of GSRs provides valuable information about various aspects of the crime scene and the firearm used. Moreover, this study addresses analytical and methodological challenges to reduce artifacts, improve the sensitivity and specificity of SEM-based GSR analysis, and develop and validate new forensic techniques based on cutting-edge technologies.