Tandem mass spectrometry and density functional theory of RDX fragmentation pathways: Role of ion-molecule complexes in loss of NO3 and lack of molecular ion peak

Abstract

Rationale: Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is an explosive compound that finds a wide range of military and civilian applications. RDX has been a target in environmental matrices by gas chromatography/tandem mass spectrometry (GC/MS/MS). MS/MS in negative chemical ionization (NCI) mode of RDX provides important fragmentation patterns that are useful for structural elucidation. The fragmentation patterns are needed for proper identification of precursor and product ions in analytical methods that depend on MS/MS approaches for a reliable identification of RDX. Methods: This study focuses on the MS fragmentation mechanisms of RDX in NCI mode using both MS/MS and density functional theory (DFT). The DFT studies were performed at the B3LYP/6-311G(d,p) level of theory. Results: The DFT results showed that NCI of RDX leads to the formation of an anion-molecule complex that was energetically more stable than the RDX anion. The fragmentation proceeds through two pathways, leading to the loss of NO2 and NO3. The loss of NO3 takes place in an anion-molecule complex leading to the formation of characteristic nitroso group fragment ions. Using the fragmentation schemes, important ion structures are proposed including structures for m/z 160, 129, 102, and 86. Conclusions: The results demonstrate the importance of both charge-induced and charge-remote dissociations in RDX pathways. The ion structures identified along the pathways could be used as targets in analytical methods for reliable identification purposes.