Abstract:

Sample extraction is a crucial step in forensic analysis, especially when dealing with trace and ultra-trace levels of target analytes present in various complex matrices (e. g., soil, biological samples, and fire debris). Conventional sample preparation techniques include Soxhlet extraction and liquid-liquid extraction. However, these techniques are tedious, time-consuming, labor-intensive and require large amounts of solvents, which poses a threat to the environment and health of researchers. Moreover, sample loss and secondary pollution can easily occur during the preparation procedure. Conversely, the solid phase microextraction (SPME) technique either requires a small amount of solvent or no solvent at all. Its small and portable size, simple and fast operation, easy-to-realize automation, and other characteristics thus make it a widely used sample pretreatment technique. More attention was given to the preparation of SPME coatings by using various functional materials, as commercialized SPME devices used in early studies were expensive, fragile, and lacked selectivity. Examples of those functional materials include metal-organic frameworks, covalent organic frameworks, carbon-based materials, molecularly imprinted polymers, ionic liquids, and conducting polymers, all widely used in environmental monitoring, food analysis, and drug detection. However, these SPME coating materials have relatively few applications in forensics. Given the high potential of SPME technology for the in situ and efficient extraction of samples from crime scenes, this study briefly introduces functional coating materials and summarizes the applications of SPME coating materials for the analysis of explosives, ignitable liquids, illicit drugs, poisons, paints, and human odors. Compared to commercial coatings, functional material-based SPME coatings exhibit higher selectivity, sensitivity, and stability. These advantages are mainly achieved through the following approaches: First, the selectivity can be improved by increasing the π-π, hydrogen bonds, and hydrophilic/hydrophobic interactions between the materials and analytes. Second, the sensitivity can be improved by using porous materials or by increasing their porosity. Third, thermal, chemical, and mechanical stability can be improved by using robust materials or fixing the chemical bonding between the coating and substrate. In addition, composite materials with multiple advantages are gradually replacing the single materials. In terms of the substrate, the silica support was gradually replaced by the metal support. This study also outlines the existing shortcomings in forensic science analysis of functional material-based SPME techniques. First, the application of functional material-based SPME techniques in forensic science remains limited. On one hand, the analytes are narrow in scope. As far as explosive analysis is concerned, functional material-based SPME coatings are mainly applied to nitrobenzene explosives, while other categories, such as nitroamine and peroxides, are rarely or never involved. Research and development of coatings is insufficient and the application of COFs in forensic science has not yet been reported. Second, functional material-based SPME coatings have not been commercialized as they don’t yet have inter-laboratory validation tests or established official standard analytical methods. Therefore, some suggestions are proposed for the future development of forensic science analyses of functional material-based SPME coatings. First, research and development of functional material-based SPME coatings, especially fiber coatings with broad-spectrum applicability and high sensitivity, or outstanding selectivity for some compounds, is still an important direction for SPME future research. Second, a theoretical calculation of the binding energy between the analyte and coating was introduced to guide the design of functional coatings and improve the screening efficiency of new coatings. Third, we expand its application in forensic science by expanding the number of analytes. Fourth, we focused on the promotion of functional material-based SPME coatings in conventional laboratories and established performance evaluation protocols for the commercialization of functional material-based SPME coatings. This study is expected to serve as a reference for peers engaged in related research.

Key words: solid phase microextraction (SPME), forensic science, functional materials, review