Mass spectrometry of selected ionic liquids in capillary electrospray at nanoliter volumetric flow rates

Capillary-based electrospray thrusters allow user-enabled control of the volumetric flow rate of the propellant. This control, coupled with mass spectrometry techniques spanning a large mass-to-charge range, enables elucidation of the composition of the electrospray beam and through further analyses, a better understanding of the physics occurring at the liquid/vacuum interface. In this work, mass spectra of selected ionic liquids electrosprayed from a capillary emitter are measured, using time-of-flight mass spectrometry, over a wide range of volumetric flow rates. The time-of-flight mass spectrometer enables simultaneous acquisition over a mass-to-charge range of 20 amu/q to ~500,000 amu/q in a single pulse cycle. Additionally, the use of orthogonal extraction enables direct determination of the kinetic energies of ions present in the electrosprayed beam. The presented data reveal a complex emission process occurring for ionic liquid capillary-based electrospray at nanoliter volumetric flow rates. The electrospray beam mass-to-charge composition includes a sequence of singly-charged ions and doubly-charged ions at nearly all sampled flow rates for all electrosprayed ionic liquids. In addition to the small ion-clusters, two Maxwell-Boltzmann distributions from approximately 10,000 amu/q to 500,000 amu/q exist in the spectra each with mass-to-charge distribution sensitive to the volumetric flow rate. The volumetric flow rate dependence of the largest charge-to-mass ratio droplets appears to be consistent with previously measured scaling laws, although exhibiting a wide mass-to-charge distribution. The flow rate dependence of the lower mass-to-charge distribution is more complex and is discussed in some detail. Measurement of the time-of-flight mass spectrum for a number of different ion kinetic energy defects over a number of different volumetric flow rates results in a more thorough understanding of kinetic energy losses experienced in the jet structure of the Taylor cone, with direct impacts to the thrust performance of these types of devices. The appearance of these ions and distributions alter the thought on the emission mechanism at work in the electrospray system. No longer is electrospray defined and easily explained by the Iribarne emission mechanism and droplet generation by the Rayleigh instability.