Disclosed is a method of quantification of one or more ion species, in a sample of ions, using a mass spectrometer, the method including the steps of: obtaining a time domain data set corresponding to a signal induced by motion of the ions in the mass spectrometer; adjusting the data set by applying an asymmetric window function thereto; generating an absorption mode mass spectrum in the frequency domain including the step of applying a Fourier transform to the adjusted data set; determining peak ranges for one or more peaks in the mass spectrum associated with the one or more ion species; integrating, for each determined peak range, the spectral data within the respective peak range to generate a respective peak intensity value; and quantifying each of the one or more ion species on the basis of the respective peak intensity values.