Equilibrium Partitioning Model Applied to RDX-Halide Adduct Formation in Electrospray Ionization Mass Spectrometry

An equilibrium partitioning model is applied for the first time to the sequential formation of 1:1 and then 2:1 adducts between the high explosive cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX) and halide anions fluoride, chloride, bromide, and iodide in electrospray ionization interface (ESI) mass spectrometry. The equilibrium partitioning model is developed and model calculations are presented to demonstrate the generic behavior of the system, which is in qualitative agreement with the observed changes in 1:1 (RDX-halide) and 2:1 (RDX-halide) responses in ESI-MS. The model is successfully applied to the experimental data with the use of octanol-water partitioning coefficients to predict interior-to-surface partitioning behavior of the complexes in droplets formed in the ESI. The data and model suggest that the significantly more hydrophobic 2:1 complexes are readily observed in ESI-MS, even though their formation constants may be several orders of magnitude less than that of the 1:1 complex. Structures for RDX-halide 1:1 and 2:1 complexes are proposed based on ion-dipole attractions and destabilizing dipole-dipole interactions.