STORED WAVE-FORM INVERSE FOURIER-TRANSFORM (SWIFT) ION EXCITATION IN TRAPPED-ION MASS SPECTOMETRY - THEORY AND APPLICATIONS

Stored waveform excitation produced by inverse Fourier transformation of a specified magnitude/phase excitation spectrum offers the most gen eral and versatile means for broadband mass-selective excitation and e jection in Penning (FT-ICR) and Paul (quadrupole) ion trap mass spectr ometry. Since the last comprehensive review of SWIFT excitation in 198 7, the technique has been adopted, modified, and extended widely in bo th the ICR and quadrupole ion trap communities. Here, we review the pr inciples, variations, algorithms, hardware implementation, and some ap plications of SWIFT for both ICR and quadrupole ion trap mass spectrom etry. We show that the most desirable SWIFT waveform is that optimized to reduce both the time-domain SWIFT maximum amplitude and the amplit ude near the start and end of the SWIFT waveform. We examine the ''tru e'' magnitude excitation spectrum, obtained by zero-filling and forwar d Fourier transforming the SWIFT time-domain waveform, in order to eva luate the trade-off between spectral magnitude uniformity and frequenc y (mass) selectivity. Apodization of the SWIFT waveform is optimally c onducted by smoothing the excitation magnitude spectrum prior to gener ation of the SWIFT waveform by inverse FT. When (as for broadband ejec tion in a quadrupole ion trap) it is important that ions be excited ne ar-simultaneously over a wide mass range, the phase spectrum (before i nverse FT to generate the SWIFT waveform) may be overmodulated or rand omly modulated (''filtered noise field''), with the recognition that v ery substantial non-uniformity in the ''true'' excitation magnitude sp ectrum will result.