Excitation frequencies of ions confined in a quadrupole field with quadrupole excitation

Resonant quadrupole excitation of ions confined in a radio frequency quadru pole field with angular frequency Omega by an excitation signal with angula r frequency omega has been investigated theoretically. It is shown that the spectrum of excitation frequencies has considerable structure which corres ponds to different orders of excitation. The resonance condition for orders K = 1,2,3,... in the general case has been obtained as omega(n)((K)) = (Om ega/K) \n + beta\, -infinity < n < infinity, where K is the order of the re sonance and beta and n determine the unperturbed oscillation frequencies. R esonance curves for ion oscillations with different stability parameters be ta = 0.1, 0.5, and 0.9 have been constructed by means of direct numerical s olution of the equations of motion. The trajectories of ion motion under re sonant excitation of different orders have been investigated. For orders K of two and higher, the ion motion shows a beat character with an overall in crease of amplitude with time. The stability diagram for ion motion in a ma ss filter in the presence of quadrupole excitation has been constructed. CT Am Soc Mass Spectrom 2000, 11, 10-18) (C) 2000 American Society for Mass S pectrometry.