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 quadrupole 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 subjected to resonance excitation broadens with the amplitude of ion oscillation and shifts to higher frequencies, an observation consistent with behavior predicted for ion motion in a quadrupole electric field with relatively 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 scan direction relative to the direction of the ion frequency shift with oscillatory amplitude.