Tailored waveform

Figures 37a and 37b show the wavefunction on the excited state potential surface at t = 200 a.u. and 400 a.u without the locking pulse. Figures 37c and lid show the wavefunction at the corresponding times with the locking pulse. The motion of the center of the wavepacket is greatly reduced. More important, with respect to selectivity, there is almost no wavepacket spreading. This example suggests that strong fields may be used in conjunction with the carefully tailored waveforms we have described above to achieve selectivity of reaction.

If several different masses are present, then one must apply an excitation pulse that contains components at all of the cyclotron frequencies. This is done by using a rapid frequency sweep ( chirp ), an impulse excitation, or a tailored waveform. The image currents induced in the receiver plates will contain frequency components from all of the mass-to-charge ratios. The various frequencies and their relative abundances can be extracted mathematically by using a Fourier transform which converts a time-domain signal (the image currents) to a frequency-domain spectrum (the mass spectrum). 

As ion trapping devices, FT-ICR instruments belong to the tandem-in-time category of instruments. The stage of precursor ion selection (MSI) is accomplished by selectively storing the ions of interest, whereas all others are ejected by means of a suitably tailored excitation pulse. For this purpose, the SWIFT technique [124] or correlated sweep excitation (CHEF) [125,126] are used (Chap. 4.7.7). Both methods generate tailored waveforms that cause excitation of all but the selected ions. Like QITs and LITs, FT-ICR analyzers are also capable of MS". 

Taylor,D. Schwartz,J. Zhou, J. James,M. Bier, M. Korsak, a. Staeford, G. Application of tailored waveform generation to the quadnipole ion trap. 

The individual harmonic distortion indicates the contribution of each harmonic frequency to the distorted waveform, and the total harmonic distortion describes the net deviation due to all the harmonics. These are both important parameters. In order to solve harmonic problems, we require information on the composition of the individual distortions so that any treatment may be tailored to suit the problem. The total harmonic distortion, while conveying no information on the harmonic makeup, is used to describe the degree of pollution of the power system as far as harmonics are concerned. Defining the individual and total harmonic distortions will be helpful as we look at some typical nonlinear waveforms and their harmonic frequency characteristics. 

FT/ICR experiments have conventionally been carried out with pulsed or frequency-sweep excitation. Because the cyclotron experiment connects mass to frequency, one can construct ("tailor") any desired frequency-domain excitation pattern by computing its inverse Fourier transform for use as a time-domain waveform. Even better results are obtained when phase-modulation and time-domain apodization are used. Applications include dynamic range extension via multiple-ion ejection, high-resolution MS/MS, multiple-ion simultaneous monitoring, and flatter excitation power (for isotope-ratio measurements). 

MS/MS. The capability of trapping ions for long periods of time is one of the most interesting features of FTMS, and it is this capability that has made FTMS (and its precursor, ion cyclotron resonance) the method of choice for ion-molecule reaction studies. It is this capability that has also lead to the development of MS/MS techniques for FTMS [11]. FTMS has demonstrated capabilities for high resolution daughter ion detection [42-44], and consecutive MS/MS reactions [45], that have shown it to be an intriguing alternative to the use of the instruments with multiple analysis stages. Initial concerns about limited resolution for parent ion selection have been allayed by the development of a stored waveform, inverse Fourier transform method of excitation by Marshall and coworkers [9,10] which allows the operator to tailor the excitation waveform to the desired experiment. 

In the early days of selective excitation, spectrometers were not equipped to generate amplitude modulated rf pulses and the DANTE method (Delays Alternating with Nutation for Tailored Excitation) was devised, [49] requiring only short, hard pulses. Although largely superseded by the amplitude modulated soft pulses, DANTE may still be the method of choice on older instramentation or on those newer instmments which lack waveform generators. 

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. 

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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