Methodology and Characterization of Isolation and Preconcentration in a Gas-Filled Digital Linear Ion Guide

Digital operation of linear ion guides allows them to operate as traps and mass filters by modulating the duty cycles of the two driving waveforms. A gas-filled (5 mTorr) digitally driven quadrupole ion guide was used to demonstrate ion isolation and preconcentration. These abilities allow ion trapping mass spectrometers to be filled to capacity with only ions in the range of interest at essentially any value of m/z. Due to the unique performance characteristics of digitally operated quadrupoles, isolation with purely duty cycle enhanced waveforms was developed with three increasingly sophisticated isolation methods. First, the guide was used as a gas-filled transmission mass filter using the waveform duty cycle to generate a narrow mass window. The second method used broadband trapping to collect ions and translationally cool along the transmission axis before shifting the duty cycle to filter the trapped ions. A subsequent duty cycle change axially ejected the filtered population for measurement. The third method improved resolution by shifting the operating frequency during isolation. The resolving power was optimized with the shift frequency to yield a device limited resolving power of 400 (m/Δm). It is the temporal control of the duration of the isolation process that sets digital waveform based isolation apart from the current technology and that minimizes ion loss even when the mass is very large. Preconcentration by repeated trapping and isolation of an individual charge state was also demonstrated to saturate the ion guide with that charge state. These digital isolation and preconcentration techniques will permit the same isolation resolution (m/Δm) at any value of mass or m/z without significant ion loss as long as the secular frequencies do not significantly overlap while in the trapping mode. It is therefore ideal for the isolation and preconcentration of single charge states of large proteins and complexes.