A two-electrode ion trap for Fourier transform ion cyclotron resonance mass spectrometry

Abstract

A two-electrode ion trap consisting of concentric single-sheet “core” and “ring” hyperboloids has been constructed and experimentally tested for Fourier transform ion cyclotron resonance mass spectrometry. The trap may be visualized as a modified Penning trap in which the end caps are brought together until they merge into a single “core” electrode. The trap is operated in “parametric” mode (i.e., both d.c. and r.f. voltages superimposed on the core and ring electrodes). Ion cyclotron motion is excited (or detected) by applying (or measuring) the voltage difference between the core and ring electrodes at the “parametric” resonant frequency, ω p = (ω + - ω −), in which ω + and ω − are the reduced cyclotron and magnetron frequencies. The equations defining each electrode surface are presented, along with the electrostatic potential generated inside the trap. Ion motion in the two-electrode trap is discussed and expressions for the ion cyclotron and magnetron radii as a function of time during excitation are presented. The ion parametric signal may be maximized by appropriate choice of the initial magnetron radius. We also provide an expression for estimating the number of ions contributing to the observed ICR signal in the two-electrode trap. The two-electrode trap is evaluated experimentally: observed cyclotron frequency shift vs. trapping voltage; tests of exactness of the quadrupolar electrostatic potential; space charge effects; and radial and axial ejection characteristics. We also evaluate mass discrimination and mass accuracy, and determine the maximum and minimum number of ions trapped (and detected) and thus the dynamic range. Compared to dipolar-mode. Penning and cubic geometries, the parametric-mode two-electrode trap exhibits a more exact quadrupolar d.c. potential but suffers from severe mass-dependent axial ejection. However, the use of off-axis ionization in the two-electrode trap serves to minimize space charge effects by an order of magnitude; furthermore, the two-electrode geometry provides excellent mass accuracy, with mass errors less than 10 ppm for several analyte species.