Excitation Modes in FT-ICR-MS

In the ICR cell, there is a stringent correlation of cyclotron frequency/c and m/z value. For simplicity, the very first FT-ICR experiment was therefore performed with an excitation pulse of a fixed/c tailored to fit the model analyte, methane molecular ion. [185] However, useful measurements require the simultaneous excitation of all ions in the cell, and this in turn demands for a large RF bandwidth. 

Example Rewriting Eq. 4.31 to obtain the cyclotron frequency/c delivers  

Note Although resolution and mass accuracy are already high in broadband spectra (R = 3 x 10 -10, Am 10 u), the performance of FT-ICR instruments improves by at least one order of magnitude by storing only narrow m/z ranges, because fewer ions in the cell mean less distortion by coulombic interactions. 

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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