Ion transport and focal properties of an ellipsoidal electrode operated at atmospheric pressure

Abstract

Ion focusing is achieved at atmospheric pressure using an elliptical ion mirror. Working at a fixed distance of several centimeters from the ion source to a mass analyzer entranceaperture, the elliptical electrode increases ion currents reaching in the mass spectrometer by a factor of a hundred. The ion transport efficiency, measured by soft landing of ionized dyes, collecting the resulting dye, and measuring its absorbance, is estimated to be 70% under typical focusing conditions. Simulations of ion motion using SIMION 8.0 reasonably predict the performance of the ion lens in air. Ion current measurements and spatial profiling of the focused beams were facilitated by use of a commercial detector operated in air, in which charge is measured as a function of position, and amplified electronically. This allowed the effects of operating parameters on beam shape and intensity to be measured. The results achieved indicate that analyte ions are directed by pneumatic forces, in the form of charged droplets, and focused electrically. Focusing can be done with good control, both for unsolvated (‘dry’) ions and also for charged microdroplets. The effects of turbulence in the primary ion beam were also characterized.