Fragment ions derived from reacting precursor ions are mass analysed in a number of consecutive time intervals wherein the intensities of spectral data obtained in a first interval between T0 and T1 are summed to determine a first summed intensity for each fragment ion associated with the first time T1, further summed intensities are then obtained for the following time intervals T1 to T2 and T2 to T3. A peak intensity centroid is determined for each time interval and their values are then used to identify a precursor ion. The mass spectroscopy method is useful for systems where the rise/fall time of the spectral features is less than a time bin duration such as low resolution scanning quadrupoles.
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