OPTIMAL CYCLOTRON RADIUS FOR HIGH-RESOLUTION FT-ICR SPECTROMETRY

Many factors shift the cyclotron frequency of ions trapped in a Fourier transform ion cyclotron resonance (FT-ICR) trap. The major factors are radial components of the trapping electric field, ion axial displacements from the center of the trap, space charge density, and relativistic mass increase. It is shown that the ion post-excitation radius is the most crucial factor that influences the cyclotron frequency. For an elongated cylindrical FT-ICR trap, the analytical expressions of the cyclotron frequency shifts for the various factors are derived. They are useful for precise mass measurements carried out on an FT-ICR mass spectrometer.