COMPARISON OF EQUILIBRIUM ION DENSITY DISTRIBUTION AND TRAPPING FORCEIN PENNING, PAUL, AND COMBINED ION TRAPS

Starting from the classical Boltzmann distribution, we obtain the ion density distribution in the limit of either high temperature/low densi ty (Coulomb interaction energy much less than ion kinetic energy) or l ow temperature/high density (kinetic energy much less than Coulomb int eraction energy), and the trapping force for an ion cloud in Penning i on cyclotron resonance, Paul (quadrupole), and combined (Paul trap in a uniform axial static magnetic field) traps. At equilibrium (total an gular momentum conserved), the ion cloud rotates at a constant frequen cy in Penning and combined traps. Ln a Penning trap, the maximum ion d ensity is proportional to B-2/m (B is magnetic field and m is the mass of ions), whereas the maximum ion density in a Paul trap is proportio nal to (V-rf(2)/m Omega(2)r(0)(4)), with Mathieu equation axial q valu e < 0.4 to satisfy the pseudopotential approximation. Ion maximum dens ities in both Penning and Paul ion traps depend on the trapping field (magnetic or electric) and ion mass, but not on ion charge. Ln a Penni ng trap at maximum ion density (zero pressure), the radial (but not th e axial) trapping potential is mass dependent, whereas both radial and axial potentials in a Paul trap at maximum ion density are mass depen dent. (C) 1998 American Society for Mass Spectrometry.