Off-axis injection into an ICR ion trap: a means for efficient capture of a continuous beam of externally generated ions

Abstract

Highly efficient ion injection into an ICR ion trap of ions generated outside the magnetic field is crucial for successful coupling of a high pressure external ion source, such as electrospray ionization, to an FT-ICR mass spectrometer. Any of several ion transport systems, including r.f.-only quadrupole, r.f.-only octupole, electrostatic einzel lenses, electrostatic ion guide, or use of a supersonic jet expansion to ensure that ions remain virtually on-axis during injection, can overcome the magnetic mirror effect and transport ions at high pressure through a magnetic field gradient toward a low pressure ICR ion trap. However, ions thus transported may exhibit a wide distribution in kinetic energy, making it difficult to trap them all. Gated trapping and pulsed-gas methods are useful for trapping ions from a pulsed source (e.g. laser desorption/ionization), but are not efficient for trapping of continuously injected ions (e.g. electrospray). Here, we introduce a novel method for trapping a continuously generated beam of ions, by injecting ions through a port held at a potential lower than that of the axial front trapping electrode and displaced laterally from the symmetry axis of the trap. Ions reflected from the back trap plate undergo magnetron rotation which in general prevents the ions from exiting back through their injection point. Ion/neutral collisions damp axial motion, and further reduce the ion's chance of escape. Supercomputer simulations of ion trajectories in the three-dimensional electrostatic potential, with collisions modeled as a frictional resistive force, predict efficient trapping of continuously injected ions up to mass-to-charge ratio m/z 50 000 over a wide range (>7 eV) of ion kinetic energy.