A mass spectrometry method for analyzing isobarically-labeled analyte compounds comprising (a) ionizing compounds including the isobarically-labeled analyte compounds to generate a plurality of precursor ion species comprising different respective m/z ratios, (b) isolating a precursor ion species, (c) fragmenting the precursor ion species to generate a plurality of first-generation fragment ion species comprising different respective m/z ratios, and (d) selecting and co-isolating two or more of the first-generation product-ion species, the method characterized by: (e) fragmenting all of the selected and isolated first-generation product ion species so as to generate a plurality of second-generation fragment ion species including released label ions; (f) generating a mass spectrum of the second-generation fragment ion species; and (g) generating quantitative information relating to at least one analyte compound based on peaks of the mass spectrum attributable to the released label ions.