ION TRAJECTORIES IN AN ELECTROSTATIC ION GUIDE FOR EXTERNAL ION-SOURCE FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY

An electrostatic ion guide (EIG) that consists of concentric cylinder and central wire electrodes can transport ions efficiently from an ext ernal ion source to an ion cyclotron resonance (ICR) ion trap for mass analysis, with several advantages over current injection methods. Bec ause the electrostatic force of the EIG captures ions in a stable orbi t about the wire electrode, ions with initially divergent trajectories may be redirected toward the ICR ion trap for improved ion transmissi on efficiency. SIMION trajectory calculations (ion kinetic energy, 1-2 00 eV; elevation angle, 0-30 degrees; azimuthal angle, 0-360 degrees) predict that ions of m/z 1000 may be transmitted through a strong (0.0 1 --> 3.0-T) magnetic field gradient. Judicious choice of ion source p osition and EIG potential minimizes the spread in ion axial kinetic en ergy at the ICR ion trap. Advantages of the EIG include large acceptan ce angle, even for ions that have large initial kinetic energy and lar ge radial displacement with respect to the central z-axis, low ion ext raction voltage (5-20 V), and efficient trapping because ions need not be accelerated to high velocity to pass through the magnetic field gr adient.