Two-plate vs. four-plate azimuthal quadrupolar excitation for FT-ICR mass spectrometry

Abstract

Azimuthal quadrupolar excitation has become one of the most useful techniques for ion axialization and translational cooling for FT-ICR mass spectrometry, leading to order(s)-of-magnitude improvement in mass selectivity, resolving power, mass accuracy, remeasurement efficiency, etc. Recently, Hendrickson et al. (J. Am. Soc. Mass Spectrom. 6 (1995) 448–452) showed that axialization may be achieved by 2-plate azimuthal quadrupolar excitation, in which an rf voltage of the same amplitude and phase is applied to a single pair of opposed side electrodes, while grounding the other orthogonal opposed pair (rather than applying an rf voltage of equal amplitude but shifted in phase by 180° to the second pair as in prior 4-plate experiments). Here, we analyze theoretically and test experimentally the performance of these two electrode geometries. As previously shown, either geometry can achieve axialization by resonant excitation at the unshifted ion cyclotron frequency, ( ω c = qB/ m). For either excitation configuration, excitation at twice the reduced cyclotron frequency, 2ω +, leads to unwanted exponential growth of the ion cyclotron radius. For 2-plate geometry, we show that excitation at twice the axial oscillation frequency, 2 ω z, leads to exponential growth in z-oscillation amplitude, whereas excitation at ω c = ω + + ω − and ω p = ω + − ω − (‘parametric’ frequency) leads to complex ion behavior. Finally, we discuss the effect of azimuthal quadrupolar excitation amplitude (as well as frequency) on axialization.