[Recent advances in stable isotope ratio analysis of common explosives]

The ratio of stable isotopes of the elements in explosives differs depending on the raw materials obtained from different geographical sources or the production processes adopted. Hence, this ratio can be used as an important index for the comparison and trace of explosives. Isotope ratio mass spectrometry (IRMS), a high-precision method for the analysis of stable isotope ratios, has evolved into a mature tool in this regard. In combination with elemental analysis, gas chromatography, liquid chromatography, etc., IRMS is widely used in food safety, environmental protection, forensic science, and other fields. IRMS also plays an important role in the comparison and trace of explosives. Since its application to distinguish trinitrotoluene (TNT) produced in different countries in 1975, IRMS has been successfully used in the analysis of various explosives. However, there is no systematic summary on the research progress on the stable isotope ratio analysis of common explosives. This paper provides a brief description of the related principle, instrumental composition, and characteristics of stable isotope ratio analysis. Methods for the stable isotope ratio analysis of common explosives such as ammonium nitrate, black powder, TNT, pentaerythritol tetranitrate (PETN), and cyclotrimethylene trinitroamine (RDX) are reviewed. The bulk stable isotopic ratio analysis method was used in most of the studies to determine the total isotope ratio of the sample. A compound-specific isotope analysis method was also employed to determine the isotope ratio of organic explosives in a complex matrix. The reported stable isotope ratios of explosives such as ammonium nitrate, black powder, and TNT produced in different countries are summarized. The discrimination ability of the stable isotope ratio for explosives is discussed. Based on the stable isotope ratio, explosives from different sources can be distinguished effectively. By combining the results of elemental analysis with the ICP-MS results, the discrimination efficiency of different samples could be further improved. The influence of relevant factors on the isotope ratio during the production and storage of explosives are collated. There is a strong correlation between the stable isotope ratios of explosives and raw materials. The stable isotope ratios of TNT, PETN, and other explosives are related to that of nitric acid used in the production. The stable isotope ratios of nitrogen and oxygen in the explosive are relatively stable and almost unchanged within one year of production. The complexity of the environmental matrix at the explosion site and the low concentration of explosive residues make the stable isotope analysis of explosive residues challenging. However, the changes in the stable isotope ratio before and after the explosion are discussed. Since there is no information on the application of stable isotope analysis to the traceability of explosives, the paper mentions that the standardized explosive sample pretreatment, stable isotope analysis method, collection and analysis of large amounts of explosive samples, and explosive stable isotope database are the basis of explosive traceability. This paper also outlines the existing challenges in the analysis of the stable isotope ratios of explosives, including the small number of explosive samples, lack of a stable isotope explosive analysis database, and difficulty in the stable isotope analysis of explosive residues. Possible solutions to these problems are proposed, followed by suggestions for the future development of the stable isotope ratio analysis of common explosives. The suggestions include establishing an effective extraction and enrichment method for explosive residues, combining IRMS with GC or LC for analyzing explosives, establishing a comprehensive process for the analysis of the stable isotope ratios of inorganic and organic explosives, and comparison and analysis of the stable isotope analysis data using statistical methods.