# Scanning Electron Microscopy Techniques in the Analysis of Gunshot Residues: A Literature Review
### Analytical Workflow and Methodological Considerations in SEM-Based GSR Detection  
Scanning electron microscopy (SEM) is an advanced technique that enables the acquisition of detailed images of a sample's surface using an electron beam. The operating principle of SEM involves scanning the sample's surface with an electron beam and collecting the emitted signals to form high-resolution images. SEM is a fundamental tool in various fields of scientific and technological research due to its ability to obtain detailed morphological and compositional data.  
SEM begins with the emission of electrons from a source, typically a tungsten filament or a cold field-emission gun (FEG). These electrons are then accelerated and focused into a fine beam through the use of electromagnetic lenses. This focused electron beam allows for high-resolution imaging [1]. The electron beam scans the sample in a zigzag pattern, covering its entire surface. During this process, interactions between the electron beam and the sample generate various signals [2]. When the beam strikes the sample, multiple interactions occur, including the emission of secondary electrons (SEs), which provide detailed images of surface topography and are highly sensitive to morphological variations [3]. Backscattered electrons (BSEs), on the other hand, are reflected from the sample and provide information about its composition and structure [4]. Additionally, characteristic X-rays are emitted during electron-sample interactions, facilitating spectroscopic analysis to determine the chemical composition [5].  
The signals generated (SEs, BSEs, and X-rays) are collected by detectors within the SEM. These detectors convert the signals into electrical impulses, which are processed to create an image of the sample's surface. The resulting image represents the surface morphology and compositional characteristics of the sample [6]. To obtain high-quality images, non-conductive samples must be coated with a thin layer of conductive material to prevent the accumulation of surface charges, which could distort the image. This preparation step is essential to ensure the accuracy and quality of the images obtained [7].  
The forensic applications of SEM have gained considerable traction due to their ability to provide conclusive evidence in criminal cases. SEM plays a crucial role in GSR analysis, where it is used to characterize the morphology and elemental composition of particles originating from firearm discharge. The integration of advanced spectral techniques, such as wavelength-dispersive X-ray spectroscopy (WDS) and X-ray photoelectron spectroscopy (XPS), has further broadened its scope, enabling forensic experts to distinguish between authentic GSRs and environmental contaminants. By leveraging these enhanced analytical capabilities, forensic scientists can provide more reliable interpretations of crime scene evidence, strengthening the role of SEM in judicial proceedings. With ongoing research and technological improvements, SEM is expected to expand its forensic applications, facilitating more accurate and efficient criminal investigations.