Comparison of daughter spectra obtained by tandem quadrupole mass spectrometry from ions selected from an isotopic cluster of the same elemental composition can be efficiently used to find out the elemental composition of a fragment. This even applies with ions of high mass and with low abundance isotopes such as "0. By this method, loss of CO and HCO' from the phenol molecular ion is confirmed and the ahsence of CO loss from the p-tertbutyl-phenol molecular ion is made obvious. Limitations of the method are discussed in the case of a bicyclic imidazolidinone.