Chemical forensic profiling of compounds related to the chemical weapons convention using 2D and 3D diffusion-ordered NMR spectroscopy

Abstract

The ongoing use of chemical warfare agents (CWAs) in conflicts, assassinations, and terrorist attacks means that the detection and identification of these compounds are crucial. The forensic identification of organophosphorus nerve agents (OPNAs) and their precursors and degradation products remains challenging due to the destructive nature and extensive preparation required for conventional chromatographic methods. In this study, we characterise precursor and degradation products of Novichok analogues, including 1,1,3,3-tetramethylguanidine, N,N-diethylpentanimidamide, N,N-dipropylbutanimidamide, using 2D ¹H–¹³C heteronuclear multiple quantum coherence (HMQC) NMR. We further investigate mixtures of phosphonate compounds [dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), diethyl 2,2-diethoxyethylphosphonate (DEOP), and dibutyl butylphosphonate (DBBP)] and a fully degraded VX sample using ¹H diffusion-ordered spectroscopy (DOSY) and 3D ¹H–¹³C DOSY-HMQC NMR. The ¹H DOSY experiments successfully separated components of both amine and phosphonate mixtures, while analysis of the degraded VX sample revealed key degradation products, including ethyl methylphosphonic acid and bis-2-(diisopropylaminoethyl)disulfide. The 3D DOSY-HMQC method provided improved resolution of overlapping signals compared to 2D approaches, representing the first application of this experiment to CWA-related compounds. These results demonstrate that DOSY-based NMR can virtually separate complex mixtures non-destructively, providing complementary capability to GC-MS and LC-MS in forensic CWA investigations.