Analysis and Mechanisms of Cyclotrimethylenetrinitramine Ion Formation in Desorption Electrospray Ionization

The general ion chemistry of the explosive molecule cyclotrimethylenetrinitramine (RDX) was studied with an atmospheric pressure ionization mass spectrometer (API-MS) fitted with a desorption electrospray ionization (DESI) source. Explosive molecule chemistry within trace detection techniques such as ion mobility spectrometry (IMS) is an area of intense interest because of the widespread deployment of IMS-based explosive detectors for counterterrorism efforts. As in IMS, the DESI-MS experiments analyze material that starts in the solid phase and is detected in the gas phase. Using the unique chemical characterization inherent in mass spectrometry, information pertinent to the atmospheric ionization of RDX is obtained in order to help explain the behavior of explosive molecule signatures observed within IMS experiments. Qualitative and quantitative information was obtained over 3 orders of magnitude of deposited mass (nanograms to greater than micrograms). A method was developed to use the relative integrated mass spectral peak intensities of RDX monomer and dimer chloride adducts to determine the amount of explosive present on a surface. The ratio of RDX dimer chloride adduct to monomer chloride adduct ranged from 0.1 for 15 ng to 1.0 for 1.5 mu g of deposited explosive. The results are explained in terms of mechanisms reported in the literature for electrospray ionization (ESI), as well its by simple solution dynamics and the interaction chemistry between RDX molecules, On the basis of all available data, the RDX dimer chloride adduct becomes disproportionately favored over the monomer chloride adduct at larger amounts of explosive because of effects related to desorbed droplet charge, solvent declustering, and the strong intermolecular forces between RDX molecules in the solid, liquid, and gas phases. Additionally, considerations for optimization of the DESI-MS process are described in order to increase the practicality for this technique as an explosives detection tool in the public domain.