A combined linear ion trap for mass spectrometry

We propose a novel ion cyclotron resonance ion trap capable of confining ions even at high pressure. The trap consists of three capacitively coupled axial sections, each composed of four circular cross-section rods parallel to the magnetic field axis. Ion confinement along the magnetic field direction is provided by applying the same static voltage to each set of “endcap” rods. As for a two-dimensional quadrupole mass filter, a sufficiently high rf frequency (several MHz) leads to an “effective” electrostatic “pseudopotential” well with a minimum on the trap central axis. Ions are confined radially by the combination of an applied axial static magnetic field and a radially inward-directed electric field resulting from differential rf voltages applied to each set of four rods. Ion confinement properties are revealed from a Paul traplike “stability diagram,” whereas ion trajectories are analyzed in terms of Penning-type ion cyclotron rotation, magnetron rotation, and axial oscillation motional modes. Ion cyclotron frequency increases with the strength of the rf trapping field. Ion magnetron motion becomes stable if the rf voltage is high enough. Therefore, ion trajectories can be stable even in the presence of ion-neutral collisions. Adding an ac potential to a Penning trap should dramatically increase the upper mass detection limit.