Mass spectral segments corresponding to each of five proteolytic peptides from the troponin complex, with all the subunits at natural abundance (top), isotopically depleted TnI (bottom left), and isotopically depleted TnC (bottom). In each case, the natural-abundance peptide could arise from isomeric peptides from either of two of the three Tn subunits, but isotopic depletion (and thus elimination of all but the monoisotopic peak) for either TnI or TnC clearly identifies the correct subunit (see text).