Multiparticle Simulations of Quadrupolar Ion Detection in an Ion Cyclotron Resonance Cell with Four Narrow Aperture Detection Electrodes

Abstract

The current paradigm in FT-ICR cell design is to approximate the ideal three-dimensional quadratic trapping potential as closely as possible to maintain ion cloud spatial coherence and achieve long transients, either with hyperbolically shaped electrodes, shimming electrodes, or by dynamic harmonization. In sharp contrast, the FT-ICR analyzer cell with four narrow aperture detection electrodes (NADEL) introduces significant anharmonic terms to the trapping potential. This analyzer cell is capable of quadrupolar detection by which one can measure a signal that is close to the unperturbed cyclotron frequency. This is far less sensitive to trapping potential and space charge shifts than the reduced cyclotron frequency measured in conventional ICR cells. The quadrupolar mode of ion detection in NADEL cells has been examined previously by SIMION simulations of ion clouds with up to 500 ions per simulation. Here, the behavior of the NADEL analyzer cell is examined through particle-in-cell (PIC) simulations, which allows us to examine the behavior of large populations (tens of thousands) of ions with space charge considerations, and to calculate the induced charge on the NADEL detection electrodes, and thus the transient signal. PIC simulations confirm a unique spatial distribution of the ions, with a coherent motion that results in long transient signals. Dependence of the ion cloud and image current signal on cell design, ion energy, and magnetron radius are examined. Coalescence effects are compared with those found in a dynamically harmonized cell. The insensitivity of the measured cyclotron frequency to space-charge is demonstrated both with simulations and experimentally. Graphical Abstract ᅟ.

Narrow Aperture Detection Electrodes ICR Cell with Quadrupolar Ion Detection for FT-ICR MS at the Cyclotron Frequency

“…Our previous work on computer simulations of ion motion suggests a co-existence of these ion distributions. 39 On the other hand, it also confirms a drastic difference in ion motion characteristics between the standard and described here frequency detection methods.…”

“…The latter is known to enhance the efficiency of ion signal detection at the (true) cyclotron frequency. [38][39][40] The original analytical theory of quadrupolar ion detection favoring ion signal measurement at the cyclotron frequency was formulated and experimentally verified by Schweikhard and co-workers. 40 Their approach to quadrupolar ion detection implied the use of linear trapping fields (quadratic potential distribution generated by electrodes of a hyperbolic shape) and the compact ion bunch principle of ion excitation and detection.…”

Fourier transform ion cyclotron resonance mass spectrometry at the true cyclotron frequency

“…The reported multiparticle ion motion simulations performed with SIMION and particle-in-cell approaches show that it is possible to establish not only disordered states of ion spatial distribution but also homogeneous, ordered ones (Figures 1C,D and 3B,C) (Driver et al, 2018;Nagornov et al, 2018). A particular example of true cyclotron frequency detection following formation of a well-defined ion structure, termed "an ion slab," is realized when ion post-excitation magnetron and cyclotron radii are equal (r m = r c ).…”

“…Only recently has FT-ICR MS operation at the (true) cyclotron frequency been implemented and demonstrated as a potentially useful analytical tool for applications beyond fundamental studies (Figure 1C,D) (Nagornov et al, 2017;Nagornov, Kozhinov, Nicol, et al, 2020). According to this novel concept, the collective behavior of many ions spatially dispersed under nonquadratic (in the transversal plane) trapping potential conditions is responsible for the generation of ion signals at the "true" cyclotron frequencies (Driver et al, 2018;Nagornov et al, 2018). This constitutes a drastic departure from the space-and phase-coherent ion bunching under the quadratic trapping potential conditions in conventional FT-ICR MS.…”

“…Nevertheless, the peak at the cyclotron frequency (i.e., the sideband) can become the base peak. Driver et al (2018) recently demonstrated insilico that, similar to the cylindrical ICR cells with pronounced DC offset potentials, the nonquadratic trapping potentials of the original cubic ICR cells enhance the generation of peaks at the true cyclotron frequency, but these will not become the only peaks in mass spectra. In fact, a certain resemblance of peak distortion can be made with the earlier reports on peak structure artifacts in the intrinsically nonquadratic potential of the cubic ICR cells (Rempel et al, 1990).…”