Stored waveform inverse Fourier transform, SWIFT

Figure 2. Time-domain excitation waveforms (left) and corresponding frequency-domain magnitude-mode spectra (right) of four excitation waveforms used in FT/ICR. A time-domain rectangular rf pulse gives a "sine" excitation spectrum in the frequency-domain. A time-domain frequency-sweep gives a complex profile described by Fresnel integrals. Single-scan time-domain noise gives noise in the frequency-domain. Finally, Stored Waveform Inverse Fourier Transform (SWIFT) excitation can provide an optimally flat excitation spectrum (see Figure 3 for details).

Stored Waveform Inverse Fourier Transform (SWIFT) Excitation... 

Stored Waveform inverse Fourier Transform (SWIFT) excitation for FT/ICR is a newly implemented technique which includes all other excitation waveforms as subsets. Compared to prior excitation waveforms (e.g., frequency-sweep), SWIFT offers flatter power with greater mass resolution and the possibility of magnitude steps (without additional delays or switching transients) in the excitation spectrum. Briefly, SWIFT increases the mass resolution for FT/ICR excitation to the ultrahigh mass resolution already demonstrated for FT/ICR detect ion. 

Stored waveform inverse Fourier transform (SWIFT) pulses [17] have been applied as a means of broadband ejection of matrix ions generated by Cs+ desorption [18]. These pulses are generated by taking the inverse Fourier transform of the desired frequency domain spectrum and applying the stored time domain waveform to the endcap electrodes via an arbitrary waveform generator. The magnitude of the SWIFT pulse determines the degree of excitation for ions of specific secular frequencies. 

R.K. JULIAN and R.G. COOKS develop broadband excitation of ions using the stored-waveform inverse Fourier transform (SWIFT) [67],... 

In an ICR cell, two-dimensional spectrometry begins with application of the stored waveform inverse Fourier transform (SWIFT) excitation. This technique removes all but a single chosen ion from the trap (Marshall et al., 1985) and is performed by first determining which ions are to be ejected and their cyclotron frequency. Inverse Fourier transform then produces an excitation waveform that excites selected ions radially until they come into contact... 

Fig. 3.12. Series of electrospray ionization Fourier-transform ion cyclotron resonance mass spectra obtained in a two-dimensional mass spectrometry experiment. Proceeding from top to bottom (a) full mass spectrum of a fulvic acid mixture (b) stored waveform inverse Fourier transform (SWIFT) waveform ejection from the ion cyclotron resonance cell of ions of all but a narrow m/z range (c) the resulting isolated parent ion mass spectrum and (d) the product ion mass spectra produced by collision-induced dissociation. Reprinted from Fievre etal. (1997) with permission from the American Chemical Society.

Fig. 3.21. (a) Spectrum of a stored waveform inverse Fourier-transform (SWIFT) isolated ion at nominal mass 453 m/z. This molecule is present in dissolved organic matter at the Experimental Nutrient Removal wetland outflow, (b) Spectrum of this ion and resulting products after fragmentation by sustained off-resonance irradiation collision-induced dissociation. Formulae in parentheses represent compositions of lost fragments. From these fragments, unambiguous determination of the elemental composition of the precursor ion at 453 m/z is possible. 

One method for temporal precnrsor isolation is stored waveform inverse Fourier transform (SWIFT) [40]. In this method, the desired freqnency domain profile (all frequencies except that of the ion of interest) is inversely Fonrier transformed to a time domain waveform. This waveform is then applied to the excite electrodes in the ICR cell and, thns, the precursor ions are isolated in the cell. An alternative techniqne for in-cell isolation is correlated sweep excitation (COSE) [41], also known as correlated harmonic excitation fields (CHEF) [42]. This method involves application of radiofrequency pulses to the excite electrodes. The technique correlates the duration and frequency of the RF-pulses with those appropriate to the ions to be isolated. Both SWIFT and COSE are capable of isolating single isotopomers in peptide and protein ions [43-45]. 

Guan, S. Marshall, A.G. Stored waveform inverse Fourier transform (SWIFT) ion excitation in trapped-ion mass spectrometry theory and applications. Int. J. Mass Spectrom. Ion Processes 1996, 157/158, 5—37 Marshall, A.G. Wang, T-C.L. Ricca, T.L. Tailored excitation for Fourier transform ion cyclotron resonance mass spectrometry. / Am. Chem. Soc. 1985,107, 7893-7897. 

FTICR-MS instruments operate on the principle of ion cyclotron resonance. As ions have resonant frequencies, these frequencies can be used to isolate the ions prior to further fragmentation or manipulation. For example, a resonant frequency pulse on the excite plates (E+/— in Figure 2.8b) will eject the ions at, or near, that frequency. Furthermore, frequency sweeps - carefully defined to not excite the ion of interest - can be used to eject unwanted ions. However, the most elegant method for ion isolation is that of Stored Waveform Inverse Fourier Transform (SWIFT) [86] in which an ion-exdtation pattern of interest is chosen, inverse Fourier-transformed, and the resulting time domain signal stored in memory. This stored signal is then clocked-out, amplified, and sent to the excite plates when needed. The typical isolation waveform in SWIFT uses a simple excitation box with a notch at the frequencies of the ion of interest, a few kHz. 

The best results are to be expected from stored waveform inverse Fourier transform (SWIFT) excitation [194]. First, the ideal excitation waveform is tailored to the needs of the intended experiment and then produced by an RF generator. SWIFT excitation also allows to remove ions of predefined m/z ranges from the ICR cell. This results in storage of a small m/z range, or after repeated SWIFT pulsing of a single nominal mass out of a broad band spectrum. Those ions are then accessible for ultrahigh resolution measurements or as precursors for tandem MS. 

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