Optimal phase modulation in stored wave form inverse Fourier transform excitation for Fourier transform mass spectrometry. I. Basic algorithm

Abstract

A new signal processing method has been proposed for generating optimal stored wave form inverse Fourier transform (SWIFT) excitation signals used in Fourier transform mass spectrometry (FTMS or FT-ICR). The excitation wave forms with desired flat excitation power can be obtained by using the data processing steps which include: (1) smoothing of the specified magnitude spectrum, (2) generation of the optimal phase function, and (3) inverse Fourier transformation. In contrast to previously used procedures, no time domain wave form apodization is necessary. The optimal phase functions can be expressed as an integration of the specified power spectral profiles. This allows one not only to calculate optimal phase functions in discrete data format, but also to obtain an analytical expression (in simple magnitude spectral cases) that is for theoretical studies. A comparison is made of the frequency sweeping or ‘‘chirp’’ excitation and stored wave form inverse Fourier transform (SWIFT) excitation. This shows that chirp excitation and SWIFT excitation with a square magnitude spectrum and a quadratic phase are counterparts of the Fourier transformation. Therefore, the results of theoretical work on chirp excitation can be used for the analysis of the time domain excitation wave forms in the SWIFT technique.