Reduction of ion magnetron motion and space charge using radial electric field modulation

Ions of the same m/z must remain in phase with each other during the detection time period used for FTICR-MS signal acquisition for optimal performance. The loss of coherence of the ion cloud during detection leads to faster rates of signal decay which results in a decrease in the achievable resolution and mass measurement accuracy with FTICR-MS technology. As the ions spin on their excited cyclotron orbit, many factors contribute to de-phasing of ion motion, such as the presence of radial electric fields and the Coulombic interaction of ion clouds of different mass-to-charge ratios. With the application of Electron Promoted Ion Coherence or EPIC, signal duration can be increased. Since FTICR-MS achieves high performance through measurement of frequency, the ability to observe ions over a longer time period increases the performance of FTICR-MS. Radial electric fields are an unavoidable consequence of applied trapping potentials to confine ion motion parallel to the magnetic field in the ICR cell; however, the presence of radial electric fields also induces magnetron motion. With EPIC it is possible to control the shape of the radial electric fields that the ions experience by changing the number of electrons that are sent through the center of the ICR cell. Furthermore, the electric field shape produced with EPIC can be altered to decrease space charge contributions by increasing the length of the ion oscillation path along the axis of the ICR cell and decrease radial electric field effects, which alter detected ion cyclotron frequencies. Here we report theoretical and experimental results to evaluate and describe the impact of EPIC on the performance of FTICR-MS.