The present invention is a method for performing a mass spectrometric analysis of analytes in a complex mixture. In particular, samples containing unknown analytes are analyzed by MS/MS to identify portions of a molecule of interest that has been labeled with a selected isotope. Ionization and detection identify a characteristic isotope shift in real time based on a selective precursor ion scan that in turn identifies precursor masses that also contain the isotopic shift. From the precursor ion scan, precursor masses are identified for further mass spectrometric analyses. The method of the invention is preferably performed in real time such that the precursor ion scan simultaneously identifies target precursor ions and identifies precursor masses for further analyses.