Theory of Fourier transform ion cyclotron resonance mass spectroscopy: Response to frequency-sweep excitation

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Abstract
Although all previous theoretical treatments of Fourier transform ion cyclotron resonance (FT–ICR) spectralline shape have been based on the ICR time‐domain response to fixed‐frequency pulsed excitation, experimental FT–ICR signals are produced by frequency‐sweep excitation. This paper presents the first analysis of the FT–ICR line shape resulting from frequency‐sweep excitation, including the effects of circularly polarized versus linearly polarized exciting radiation, continuous versus stepwise variation in swept frequency, variation in sweep parameters (e.g., frequency range, sweep rate, size of step, and number of steps), phase conventions, reliability of fast Fourier transformdata reduction, and time delay between excitation and detection. In addition, we show how the nonlinear amplitude and phase variation of an FT–ICR spectrum obtained by direct Fourier transformation of a time‐domain ICR response can be corrected by deconvoluting the response with the frequency‐sweep excitation. Implementation of these results promises to enhance the appearance, reliability of peak height information, and resolution of experimental FT–ICR spectra.