The second stability region of the quadrupole mass filter. I. Ion optical properties

Abstract The quadrupole mass filter can be operated in a stability region at a much higher q value than that normally used. This requires a high r.f. voltage or a lower r.f. frequency and only a very small d.c. voltage. Details of the higher q stability region were calculated. Typical ion trajectories in the phase space were used to estimate ion acceptances both with and without fringing fields. The presence of any fringing field is not beneficial and transmissivity is almost an order of magnitude less than in normal operation. Good resolution is achievable with quadrupole fields in which the ions spend very few r.f. cycles and the effects of field imperfections which normally limit the ultimate attainable resolution might be minimised.