A method of mass spectrometry is disclosed comprising mass selectively transmitting precursor ions from a mass analyzer into a fragmentation or reaction device, wherein the mass to charge ratios of the ions transmitted varies with time; fragmenting the precursor ions in the fragmentation or reaction device so as to produce fragment or product ions; mass analyzing the fragment or product ions; determining the start and end times at which a first fragment or product ion is detected; using said start and end times to determine the start and end times at which a precursor ion of said first fragment or product ion is transmitted by said mass analyzer; and using the start and end times at which the precursor ion is transmitted by said mass analyzer to determine a mass to charge ratio of said precursor ion. The present invention enables precursor ion peaks to be resolved from the fragment data even when a low resolution mass analyzer is used to analyze the precursor ions.