A method of mass spectrometry comprising: providing measured data from a Fourier transform type mass analyser comprising signal and noise measured over time that has been obtained from species of ions performing multiple changes of direction in a periodic manner within the mass analyser, the species being within a range of mass-to-charge ratios; characterised by determining a quantity representative of the noise in the measured data, and determining a noise range based upon the quantity representative of noise, and determining a model data set of K -harmonic component signals c* n, with n between 0 and N-1, and N the number of data points in the measured data set, from the measured data; wherein the harmonic component signals and their number K are determined iteratively on the basis of: using an initial value of K to calculate a minimised non-negative measure of difference R between the measured data and model data comprising data sets of K -harmonic component signals, wherein R is calculated under the condition that the model data set, c n *, at each data point n between 0 and N-K-1 in the model data set satisfies the equation a 0 c n * + a 1 c n + 1 * +.. + a K c n + K * = 0 at measured data points with a set of complex values a 0.. a K where a 0.. a K are autocorrelation coefficients for the signal c* n, and if R does not lie within the noise range, changing the value of K and recalculating R as many times as necessary until R does lie within the noise range; deriving mass spectral information about the ion species from the model data set, the mass spectral information comprising one or more of: a measure of the number of different species of ions; a measure of the mass-to-charge ratio of the species of ions; a measure of the signal intensity of each species of ions; and outputting the mass spectral information. providing measured data from a Fourier transform type mass analyser comprising signal and noise measured over time that has been obtained from species of ions performing multiple changes of direction in a periodic manner within the mass analyser, the species being within a range of mass-to-charge ratios; characterised by determining a quantity representative of the noise in the measured data, and determining a noise range based upon the quantity representative of noise, and determining a model data set of K -harmonic component signals c* n, with n between 0 and N-1, and N the number of data points in the measured data set, from the measured data; wherein the harmonic component signals and their number K are determined iteratively on the basis of: using an initial value of K to calculate a minimised non-negative measure of difference R between the measured data and model data comprising data sets of K -harmonic component signals, wherein R is calculated under the condition that the model data set, c n *, at each data point n between 0 and N-K-1 in the model data set satisfies the equation a 0 c n * + a 1 c n + 1 * +.. + a K c n + K * = 0 at measured data points with a set of complex values a 0.. a K where a 0.. a K are autocorrelation coefficients for the signal c* n, and if R does not lie within the noise range, changing the value of K and recalculating R as many times as necessary until R does lie within the noise range; characterised by determining a quantity representative of the noise in the measured data, and determining a noise range based upon the quantity representative of noise, and determining a model data set of K -harmonic component signals c* n, with n between 0 and N-1, and N the number of data points in the measured data set, from the measured data; wherein the harmonic component signals and their number K are determined iteratively on the basis of: using an initial value of K to calculate a minimised non-negative measure of difference R between the measured data and model data comprising data sets of K -harmonic component signals, wherein R is calculated under the condition that the model data set, c n *, at each data point n between 0 and N-K-1 in the model data set satisfies the equation a 0 c n * + a 1 c n + 1 * +.. + a K c n + K * = 0 at measured data points with a set of complex values a 0.. a K where a 0.. a K are autocorrelation coefficients for the signal c* n, and if R does not lie within the noise range, changing the value of K and recalculating R as many times as necessary until R does lie within the noise range; using an initial value of K to calculate a minimised non-negative measure of difference R between the measured data and model data comprising data sets of K -harmonic component signals, wherein R is calculated under the condition that the model data set, c n *, at each data point n between 0 and N-K-1 in the model data set satisfies the equation a 0 c n * + a 1 c n + 1 * +.. + a K c n + K * = 0 at measured data points with a set of complex values a 0.. a K where a 0.. a K are autocorrelation coefficients for the signal c* n, and if R does not lie within the noise range, changing the value of K and recalculating R as many times as necessary until R does lie within the noise range; deriving mass spectral information about the ion species from the model data set, the mass spectral information comprising one or more of: a measure of the number of different species of ions; a measure of the mass-to-charge ratio of the species of ions; a measure of the signal intensity of each species of ions; and outputting the mass spectral information.