Ion traps for Fourier transform ion cyclotron resonance mass spectrometry: principles and design of geometric and electric configurations
Virtually all Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry experiments are based on ion confinement in a Penning trap: namely, a spatially homogeneous static magnetic field B, and a three-dimensional axial (along B) quadrupolar electrostatic trapping potential. In addition, it is desirable to provide an alternating azimuthal (i.e. in a plane perpendicular to B) spatially uniform dipolar electric potential to generate or detect ion cyclotron coherence and an alternating azimuthal quadrupolar potential in the presence of collisional cooling to axialize ions. In this paper, we show how each of these three potentials may be generated by infinitely extended electrode arrangements. We then examine the potentials actually produced by various (finite size) ICR ion trap geometric configurations. Finally, we show that by segmenting the electrodes of an ICR ion trap, and applying an appropriate potential to each segment, near-perfect potentials of all three types may be generated. The necessary principles and tools (Laplace's equation: harmonic potential; reciprocity theorem; V-vector method) are presented, followed by derivation and/or description and rationale for all of the principal ICR ion trap designs. We close by proposing a new "universal" trap consisting of six planar grids arranged in a cube: such a configuration is the first to promise near-perfect potentials of all three types while maintaining the symmetry and compact shape of a cubic Penning trap.