Control of Aberration and Space-Charge Effects in the Orbitrap Mass Analyzer

Abstract

The idea of orbital ion confinement dates back to 1923 when the orbital ion trap was proposed and implemented by Kingdon [1]. In the following several decades this principle was often used in ion spectroscopy but it was not until 1981 that Knight proposed a mass-selective orbital ion trap [2]. Similar to Kingdon trap, the ions were trapped revolving around a negatively biased wire. The ions generated by laser ionization were captured in the trap where rudimentary mass analysis was performed by means of resonance excitation of axial oscillations between two conical guarding electrodes that formed a quasiharmonic potential well. This first attempt demonstrated that the quality of mass analysis mostly depends on the trapping field accuracy, and particularly, on the isochronism of the axial oscillatory motion.The simplest axisymmetric electrostatic potential distribution with ideal isochronism in the axial direction is given by the formulawhere and k 0 are geometrical parameters and is the applied voltage. A detailed description of the ion dynamics in the quadro-logarithmic potential Eq. (1) can be found in the paper by Gall et al. [3]. The most important property of this unique potential distribution is that the motion equations are separable for the axial coordinate and the other coordinates (radius and the rotational angle ), and the axial motion is harmonic. The axial oscillation frequency in the ideal field isand thus depends on the ion's mass-to-charge ratio / but not on the oscillation amplitude and the revolving radius. It means that a bunch of identical ions, once injected into the trap, keeps oscillating as a single packet preserving the phase coherency. Having detected the oscillation frequency, one can determine the mass-to-charge ratio of the ions and construct a mass spectrum of the analyte. The other important property of the quadro-logarithmic potential is the confinement in the radial pseudopotential 176

Ion traps in modern mass spectrometry

“…Significant collaborative research was also directed toward understanding fundamentals of ion motion inside the analyzer itself, demonstrating the need for careful balancing of electrode tolerances.…”

Crowd control of ions in the Astral analyzer

“…Experiments were performed using a prototype version of the Thermo Scientific™ Orbitrap Astral mass spectrometer 20 . The mass analyzer 19 is a tabletop‐sized assembly shown in Figure 1A, through which ions follow a zigzag flight path. Electrosprayed analyte ions were isolated by a quadrupole mass filter, transferred via an ion guide, and accumulated in the ion processor, which has been previously described 21 and incorporates a quadrupole collision cell (not shown) conjoined via an apertureless interface to a pulsed extraction ion trap.…”

“…It follows that a mass analyzer that allows the ion packets to widely disperse should have an inherent advantage in space charge tolerance versus those that keep the ion beam transversally compressed. One such multi‐reflection device, the Astral™ analyzer, has been described whereby ions oscillate between a pair of converging elongated ion mirrors while drifting down the elongate channel 19 . The ions spread out widely as they drift, but the mirror convergence deflects the drifting ions back with every oscillation, eventually reversing the drift velocity and focusing them to a detector.…”

Phase-Constrained Spectrum Deconvolution for Fourier Transform Mass Spectrometry

“…The ion–ion scattering is expected to produce a signal loss which is proportional to the square of the ion count; this effect is relatively small, however. Field imperfections, , including Columbic field of the ion pool, are the major contributors to ion dephasing. Its nature consists in spreading out of same m / z ion packets because of slightly different oscillation frequencies, while the number of trapped ions is constant.…”