AN ELECTROSTATIC ION GUIDE FOR EFFICIENT TRANSMISSION OF LOW-ENERGY EXTERNALLY FORMED IONS INTO A FOURIER-TRANSFORM ION-CYCLOTRON RESONANCEMASS-SPECTROMETER

A new method for transmitting externally formed ions into an ICR ion t rap is demonstrated. In an electrostatic ion guide, a potential differ ence is applied between a conductive cylinder and a rigid wire suspend ed along the central axis of the cylinder. The cylinder is then positi oned between an ion source located outside the bore of a superconducti ng solenoidal magnet and an ion trap located at or near the center of the solenoid. SIMION simulations predict that low-energy ions entering the ion guide will spiral around the central wire and pass through th e fringe field of the magnet to reach the ICR ion trap. The theoretica l predictions are borne out by experiments in which Na+ and K+ ions fr om a thermionic emitter are transmitted with high efficiency through t he fringe field of the magnet to the ICR ion trap. Mass resolving powe r of 285 000 for K+ is shown. The electrostatic ion guide offers the a dvantages that: (a) a wide range of low-energy external ions sources ( e.g., fast-atom or fast-ion bombardment, electrospray, glow discharge, etc.) may be used; (b) prior acceleration of the ions along the magne tic field direction (and subsequent deceleration to slow the ions on e ntry into the ICR ion trap) is not required; (c) ions are focused alon g magnetic field lines once the ions have passed through the magnetic fringe field; and (d) ions formed initially off axis are efficiently c aptured and transmitted by the ion guide without additional focusing.