RESONANCE EJECTION ION-TRAP MASS-SPECTROMETRY AND NONLINEAR FIELD CONTRIBUTIONS - THE EFFECT OF SCAN DIRECTION ON MASS RESOLUTION

Mass resolution obtained with the commonly employed stretched quadrupo le ion trap geometry is shown to be dependent upon scan direction when resonance ejection is used as the mass analysis method. Data are also presented showing that the energy absorption profile for ions subject ed to resonance excitation broadens with the amplitude of ion oscillat ion and shifts to higher frequencies, an observation consistent with b ehavior predicted for ion motion in a quadrupole electric field with r elatively small higher-order field components. The observed effect of scan direction on mass resolution is therefore reasoned to arise from an effect analogous to Doppler focusing or defocusing, depending on sc an direction relative to the direction of the ion frequency shift with oscillatory amplitude.