A mass spectrometer comprising a collision, fragmentation or reaction cell (4) is disclosed. The collision, fragmentation or reaction cell (4) is repeatedly switched back and forth between a high fragmentation mode of operation and a low fragmentation mode of operation. Mass spectral data sets are obtained in both modes of operation. A decimal mass filter is applied to one or both sets of data. In particular, fragment ions or metabolites related to a parent or precursor ion of interest are identified on the basis of having a decimal mass which is similar to that of the parent or precursor ion of interest.