A method of mass spectrometry comprising: mass selectively transmitting precursor ions from a mass analyser 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 analysing and detecting the fragment or product ions; determining a time period over which a first fragment or product ion is detected; using said time period to determine a time period over which a precursor ion of said first fragment or product ion was transmitted by said mass analyser; and using the time period over which the precursor ion was transmitted by said mass analyser to determine a mass to charge ratio of said precursor ion; determining a time period over which a second fragment or product ion is detected; using this time period to determine a time period over which a precursor ion of said second fragment or product ion is transmitted by said mass analyser; and using the time period over which this precursor ion is transmitted by said mass analyser to determine a mass to charge ratio of this precursor ion; wherein the time period over which the first fragment or product ion is detected only partially overlaps with the time period over which the second fragment or product ion is detected; wherein the mass analyser that transmits the precursor ions to the fragmentation or reaction device is a mass filter; and wherein the mass transmission window of the mass filter is scanned with a step size smaller than the size of the mass transmission window. mass selectively transmitting precursor ions from a mass analyser 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 analysing and detecting the fragment or product ions; determining a time period over which a first fragment or product ion is detected; using said time period to determine a time period over which a precursor ion of said first fragment or product ion was transmitted by said mass analyser; and using the time period over which the precursor ion was transmitted by said mass analyser to determine a mass to charge ratio of said precursor ion; determining a time period over which a second fragment or product ion is detected; using this time period to determine a time period over which a precursor ion of said second fragment or product ion is transmitted by said mass analyser; and using the time period over which this precursor ion is transmitted by said mass analyser to determine a mass to charge ratio of this precursor ion; wherein the time period over which the first fragment or product ion is detected only partially overlaps with the time period over which the second fragment or product ion is detected; wherein the mass analyser that transmits the precursor ions to the fragmentation or reaction device is a mass filter; and wherein the mass transmission window of the mass filter is scanned with a step size smaller than the size of the mass transmission window. <EOS>