ION-TRANSPORT THROUGH A STRONG MAGNETIC-FIELD GRADIENT BY RF-ONLY OCTUPOLE ION GUIDES

An ion transport system consisting of two consecutive octupole ion gui des is proposed to inject externally generated ions into an ion cyclot ron resonance (ICR) ion trap in a high (9.4 T) magnetic field through an off-axis injection port. A first octupole brings continuously gener ated ions on-axis from the source at near-zero magnetic field through the magnetic field gradient into the homogeneous high-field region of the solenoid. A second octupole tilted by a 1.14 degrees from the magn etic field direction transports ions to a point 1 cm off-axis at the f ront end cap of the ICR ion trap. Simulated ion trajectories show that the proposed ion transport system should be capable of guiding ions o f a wide range of mass-to-charge ratios (up to m/z 5000) and a wide ra nge of initial azimuthal kinetic energies (up to 4 eV) and initial (ch i, gamma) positions (up to +/- 1 mm or more), at relatively low r.f. a mplitude (approx. 100 V) and r.f. frequency (approx. 2 MHz), over a di stance of about 1 m from an external ion source to an ICR ion trap in less than 1 ms. For example, externally generated ions of m/z 500 with 2 eV of initial azimuthal kinetic energy are transported with greater than or equal to 99% efficiency. The relative senses of cyclotron and magnetron rotation are reversed for an r.f.-only octupole in a magnet ic field, compared to ion rotations in a magnetic field alone. Compari son of trajectories computed from instantaneous electric potential and pseudopotential shows that the pseudopotential approximation can be m ade semi-quantitatively accurate by reducing the r.f. voltage by a con stant scale factor (approx. 0.4 for the present configuration). The pr esent simulations improve our understanding of the mechanism of ion in jection mechanism, so that we may now proceed to design and optimize a n experimental r.f.-only octupole ion guide for off-axis injection of externally generated ions into a high-field FT-ICR mass spectrometer.