Characterization and optimization of electron detachment dissociation Fourier transform ion cyclotron resonance mass spectrometry

We have demonstrated that electron detachment dissociation (EDD) can provide extensive oligonucleotide backbone fragmentation, complementary to that of other MS/MS techniques. In addition, we have shown that, for oligosaccharides, EDD provides additional cross-ring fragments compared to collision-activated dissociation and infrared multiphoton dissociation. In our EDD implementation, the potential difference between a hollow cathode electron source and an extraction lens located in between the cathode and the ion cyclotron resonance (ICR) cell was crucial for successful fragmentation with changes as small as 0.2 V drastically altering fragmentation efficiency, a behavior that was not fully understood. Here, we present a detailed characterization of the electron current passing through the ICR cell as a function of this potential difference, the cathode bias voltage, extraction lens voltage, and the cathode heating current under EDD conditions. Our results show that the extraction lens voltage serves to regulate the number of electrons passing through the ICR cell. Thus, similar electron numbers passing through the cell can be obtained at low (1.2 A) and high (1.8 A) heating current as well as at different cathode bias voltages by adjusting the extraction lens voltage. This characteristic allowed us to investigate the influence of electron energy at fixed electron number and we found that optimum EDD efficiency was obtained with 16-22 eV electrons. We also investigated the influence of charge state on oligonucleotide EDD efficiency and sequence coverage and found that higher charge states provided improved data for a DNA 10-mer, presumably due to a more extended gas-phase structure.