Quadrupolar excitation and collisional cooling for axialization and high pressure trapping of ions in Fourier transform ion cyclotron resonance mass spectrometry

Abstract

A recently developed technique for axialization of ions in a Penning (i.e. hyperbolic) trap has been adapted for Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry in a dual cubic trap instrument. A resonant alternating electric azimuthal quadrupolar field at the combination frequency, ω c = ω + + ω −, converts ion magnetron motion into collision-damped cyclotron motion. The usual collision- induced drift of ions from the center of the trap under the influence of the radially outward-directed electric force of the trapping field is thereby effectively inverted, so that ions with off-axis ICR orbit centers may be driven back to on-axis low radius ion cyclotron orbits. This effect is frequency-dependent and therefore mass-selective. The experiment requires only minor wiring modifications to an existing cubic, tetragonal, or cylindrical ICR ion trap. With this technique, we have been able to store benzene molecular ions for several seconds at high pressure (> 1 x 10 −5 mbar) in one half of a dual trap, as demonstrated by subsequent transfer of these ions to the other half of the dual trap for ICR excitation and detection. We also report simultaneous axialization and extended trapping of ions of different mass-to-charge ratios by use of multiple-frequency stored-waveform inverse Fourier transform excitation. Proposed applications include axialization and cooling for improved detection of product ions in MSMS experiments, ion trapping at high pressure for extended periods for measurement of slow ion/molecule reaction rates, and increased sensitivity for detection of externally injected ions.