Optimization of a Digital Mass Filter for the Isolation of Intact Protein Complexes in Stability Zone 1,1

Digital mass filters are advantageous for the analysis of large molecules due to the ability to perform ion isolation of high-m/z ions without the generation of very high radio frequency (RF) and DC voltages. Experimentally determined Mathieu stability diagrams of stability zone 1,1 for capacitively coupled digital waveforms show a voltage offset between the quadrupole rod pairs is introduced by the capacitors which is dependent on the voltage magnitude of the waveform and the duty cycle. This changes the ion's a value from a = 0 to a < 0. These effects are illustrated for isolation for single-charge states for various protein complexes up to 800 kDa (GroEL) for stability zone 1,1. Isolation resolving power (m/Δm) of approximately 280 was achieved for an ion of m/z 12,315 (z = 65+ for 800.5 kDa GroEL D398A), which corresponds to an m/z window of 44.