Observation of higher order quadrupole excitation frequencies in a linear ion trap.

The resonant frequencies for quadrupole excitation of ions confined with a buffer gas in a linear quadrupole ion trap with Mathieu parameters a = 0 and q approximately 0.36 have been measured. The resonances are predicted to occur at angular frequencies omega(K)n given by omega(K)n = (omega/K)n + beta without the presence of a buffer gas where omega is the angular frequency of the trapping radio frequency, K = 1, 2, 3... is the order of the resonance calculated with perturbation theory, and n = 0, +/-1, +/-2, +/-3.... The resonances are measured through fragmentation of protonated reserpine. The observed frequencies agree closely with the theoretical values but there are small differences which vary from +0.6% at K = 2 to -2.7% at K = 6. This is believed to be the result of the dependence of the resonant frequencies upon the buffer gas density and/or the excitation amplitude. The resolution of the resonances (measured from the depletion of precursor and formation of fragment ions) increased by a factor of 2 as K increased from 1 to 6. This increase in resolution warrants further investigation into the use of higher order resonances for isolation and excitation of trapped ions.