STORED WAVE-FORM INVERSE FOURIER-TRANSFORM AXIAL EXCITATION/EJECTION FOR QUADRUPOLE ION-TRAP MASS-SPECTROMETRY

A general method for high-resolution ion excitation, ejection, and isolation is developed from linear response theory for resonant dipolar excitation of the axial z-oscillatory motion of ions in a quadrupole (Paul) ion trap operated in rf-only mode. For a spatially uniform dipolar excitation field, the ion z-oscillation amplitude is directly proportional to the amplitude of the Fourier component of the excitation at the axial oscillation frequency of that ion. Thus, one may specify an arbitrary z-motion frequency-domain spectrum by applying a time-domain stored waveform obtained from the inverse Fourier transform of the corresponding frequency-domain excitation spectrum. The stored waveform inverse Fourier transform (SWIFT) waveform may be tailored for selective ejection or excitation of ions of arbitrary mass-to-charge ratio ranges. The method includes all other possible excitation/ejection waveforms (e.g., single frequency, frequency sweep) as special cases. The effect of collisional damping during excitation is included in the analytical solution of the ion response.