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

A new method for transmitting externally formed ions into an ICR ion trap is demonstrated. In an electrostatic ion guide, a potential difference is applied between a conductive cylinder and a rigid wire suspended along the central axis of the cylinder. The cylinder is then positioned between an ion source located outside the bore of a superconducting 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 the fringe field of the magnet to reach the ICR ion trap. The theoretical predictions are borne out by experiments in which Na+ and K+ ions from a thermionic emitter are transmitted with high efficiency through the fringe field of the magnet to the ICR ion trap. Mass resolving power of 285 000 for K+ is shown. The electrostatic ion guide offers the advantages 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 magnetic field direction (and subsequent deceleration to slow the ions on entry into the ICR ion trap) is not required; (c) ions are focused along magnetic field lines once the ions have passed through the magnetic fringe field; and (d) ions formed initially off axis are efficiently captured and transmitted by the ion guide without additional focusing.