A mass spectrometer is disclosed comprising an ion source 4, a field free or drift region 5 and an ion mirror 7 comprising a reflectron. Metastable parent ions which spontaneously fragment by Post Source Decay whilst passing through the field free or drift region 5 are arranged to enter the ion mirror 7 and be reflected by the reflectron towards an ion detector 8 when the reflectron is maintained at a certain voltage. The process is then repeated with the reflectron being maintained at a slightly lower voltage. Two related sets of time of flight or mass spectral data are obtained for the two different voltage settings of the reflectron. From the two data sets the different times of flight for the same species of fragment ion can be determined. The mass to charge ratio of the parent ion which fragmented to produce the particular species of fragment ion can then be determined from the times of flight of the fragment ions.