Injecting electrospray ions into a Fourier transform electrostatic linear ion trap

Abstract

A Fourier transform electrostatic linear ion trap (FT-ELIT) is a mass analyzer consisting of a field free region with a reflectron on each side. Ions bounce back and forth and a signal is generated using a centrally located image charge pickup electrode. In this report we describe a technique for injecting packets of ions produced by dim sources such as electrospray ionization (ESI) into an FT-ELIT. The technique involves accumulating and thermalizing ions in a collision cell. The collision cell is equipped with a set of electrodes that enables the creation of an axial electric field that is used to concentrate the accumulated ions near the exit. Further concentration is achieved by reducing the potential on the exit lens of the collision cell prior to ion ejection. We demonstrate that these concentration techniques significantly increase signal intensity. We also use ion optical simulations to show that the signal intensity increases because the concentration increases the spatial charge density of the injected ion cloud not only by compressing the ion cloud in the collision cell, but also by decreasing the time required to eject the ions from the collision cell. We also demonstrate that the concentration techniques do not broaden the kinetic energy distribution of the injected ions; therefore, the concentration does not degrade resolution. Using these injection techniques, we are able to analyze ions produced by ESI ranging from 300 to 2200m/z in a single injection with high signal-to-noise ratio using FT-ELIT.