Hydroxylammonium nitrate species in a monopropellant electrospray plume

A mixture of 1-ethyl-3-methylimidazolium ethyl sulfate (EmimEtSO₄) and hydroxylammonium nitrate (HAN) is an energetic monopropellant potentially suitable in a multimode chemical-electric microtube-electrospray micropropulsion system. In this work, electrospray plume mass spectra are compared between the monopropellant mixture and neat EmimEtSO₄. This comparison clearly indicates new and additional species present in the plume due to the addition of HAN. Mass spectra from 20 to 600 amu∕q were obtained over a variety of angles and flow rates from 2 pL∕s to 3 nL∕s in both cation and anion extraction modes. Mass spectra dependence on flow rate and angular orientation agree qualitatively well with literature. Results indicate the presence of HAN-based species in anion mode, but no HAN-based species in cation mode. Three of the four monomer species from the monopropellant are apparent in the plume; Emim, EtSO₄⁻, and NO₃⁻are observed, whereas HA is noticeably absent. Results also show emission of both proton-transferred covalent forms of HAN: [HA-H] and HNO₃. Swapping of anion and cation species between the constituents of the monopropellant mixture is also observed.