Ion trapping and manipulation in novel open-ended trapped-ion cells for Fourier transform ion cyclotron resonance mass spectrometry.

摘要

Several novel open trapped-ion cells, based upon flexible electrode designation and function, are constructed and demonstrated to overcome limitations of closed trapped-ion cells in the Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR/MS) experiment. An optimized open cell is presented in which electrode dimensions are adjusted to increase the aspect ratio of the cell without increasing the physical dimensions of the cell. This cell geometry reduces the radial electric field that causes magnetron motion, ion loss, and cyclotron frequency shift without adversely affecting trapping-well depth and ion capacity. Another open cell is presented with a single annular trapping-electrode centered in the excitation/detection region, which is biased opposite the polarity of ion to be trapped. This cell geometry creates a trapping well within the homogeneous region of the excitation field, reducing axial ejection without capacitively-coupling the excitation electrodes to the trapping electrodes. In addition, ions are trapped without traversing a potential energy barrier. Another open cell design is demonstrated in which the addition of compensatory trapping electrodes in conjunction with the conventional trapping electrodes results in a reduction of the radial electric field by superposition of the two attendant electric fields. The consequence is virtual elimination of cyclotron frequency shift as a function of trapping potential. These same supplemental trapping electrodes can also be used to create "nested traps" by oppositely biasing each set of trapping electrodes, enabling the simultaneous detection of positive and negative ions. This circumvents a basic limitation of conventional FT-ICR cells, the detection of only one charge species at a time and provides an opportunity to monitor ion-ion interactions taking place in the cell.;Collective motion due to strong coupling between ions undergoing trapping motion is observed for the first time in FT-ICR. Cyclotron frequency is a function of axial position, with ions at larger axial displacement exhibiting higher cyclotron frequency because the radial electric field is inward directed in open cells at extended z-amplitude. This phenomenon is applied to the remeasurement experiment, and ions scattered to larger z-axis amplitude remain trapped for subsequent reexcitation.