A 2-ELECTRODE ION-TRAP FOR FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY

A two-electrode ion trap consisting of concentric single-sheet ''core' ' and ''ring'' hyperboloids has been constructed and experimentally te sted 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'' ele ctrode. The trap is operated in ''parametric'' mode (i.e., both d.c. a nd r.f. voltages superimposed on the core and ring electrodes). Ion cy clotron motion is excited (or detected) by applying (or measuring) the voltage difference between the core and ring electrodes at the ''para metric'' resonant frequency, omega(p) = (omega(+) - omega(-)), in whic h omega(+) and omega(-) are the reduced cyclotron and magnetron freque ncies. The equations defining each electrode surface are presented, al ong with the electrostatic potential generated inside the trap. Ion mo tion in the two-electrode trap is discussed and expressions for the io n cyclotron and magnetron radii as a function of time during excitatio n are presented. The ion parametric signal may be maximized by appropr iate choice of the initial magnetron radius. We also provide an expres sion for estimating the number of ions contributing to the observed IC R signal in the two-electrode trap. The two-electrode trap is evaluate d experimentally: observed cyclotron frequency shift vs. trapping volt age; tests of exactness of the quadrupolar electrostatic potential; sp ace 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 th e dynamic range. Compared to dipolar-mode Penning and cubic geometries , the parametric-mode two-electrode trap exhibits a more exact quadrup olar d.c. potential but suffers from severe mass-dependent axial eject ion. However, the use of off-axis ionization in the two-electrode trap serves to minimize space charge effects by an order of magnitude; fur thermore, the two-electrode geometry provides excellent mass accuracy, with mass errors less than 10 ppm for several analyte species.