VERFAHREN UND VORRICHTUNG ZUR ERZEUGUNG EINES BELIEBIGEN ERREGUNGSSPEKTRUMS FÜR FOURIER-TRANSFORMATIONSMASSSPEKTROMETRIE

A desired mass domain excitation profile is selected and converted to a frequency domain excitation spectrum in which the frequency of excitation is generally proportional to the inverse of the mass-to-charge ratio. In the direct method of the invention, the specified frequency domain spectrum is converted by inverse Fourier transformation to a time domain waveform and multiplied by an expanded window function. The time domain waveform is forward Fourier transformed to produce a second discrete frequency spectrum each frequency of which is assigned a phase scrambled such that maximum reduction of peak excitation voltage is achieved with no distortion of the excitation amplitude spectrum. The phase-scrambled frequency spectrum is inverse Fourier transformed to produce the final time domain waveform which is used to generate the electric field which excites the ions in an ion cyclotron resonance cell. In the iterative method of the invention, the desired frequency spectrum is phase scrambled such that all frequencies are not in phase in any point in time, an inverse Fourier transform is performed on the phase scrambled frequency spectrum, and the result multipled by a window function. The time domain waveform is forward Fourier transformed to produce an output spectrum which is compared to a reference spectrum to provide correction factors which are used to predistort the magnitude of the final frequency spectrum, and the steps are repeated until the output frequency spectrum is sufficiently close to the reference spectrum, whereafter the time domain waveform corresponding to that output frequency spectrum is applied as the excitation signal.