Characterization of phosphorous impurities originating from the synthesis of Sarin

Forensic chemical analysis aims not only to confirm the presence of an illicit chemical agent but also to determine its origin, synthesis route, and production conditions to bring evidence for a criminal investigation. In the production of organophosphorus-nerve agents and their precursors, phosphorous-containing species arise at different stages of synthesis and reflect specific reaction conditions. However, current methods for analysing methylphosphonic dichloride (DC) (precursor of sarin) impurities are often limited by the high reactivity and low concentrations of key phosphorus-containing species, which frequently evade detection using standard analytical protocols. Reactive phosphorus-containing impurities within the nerve agent precursor DC were investigated and we demonstrated how these species account for impurity profiles and are observed throughout the synthesis chain of sarin. A set of twelve sarin samples, including crude and distilled materials and precursors from each synthetic step, was analysed using GC- and LC-HRMS, NMR spectroscopy and GC-FPD, with particular emphasis on phosphorus-containing compounds. Many impurities in final products are shown to be reaction products of impurities already present in DC, enabling chemical linkages between starting materials, intermediates, and end products. This work enhances forensic attribution through impurity profiling and introduces butanethiol derivatisation as a novel tool for the detection and interpretation of reactive phosphorus-containing impurities. By converting highly reactive species to more stable analytes, this derivatisation method enables the detection of previously elusive impurities, providing critical information for the forensic attribution of chemical weapons production.