Techniques for differentiating isobaric species are described. An isobaric species may be substituted with a tagging species identified using mass spectrometry. The isobaric species may be a subunit of a first polymer having a defined sequence, e.g., the isobaric species may be an amino acid in a protein or a peptide sequence. A tagging species may be substituted for the isobaric species in a second polymer having an otherwise identical sequence as the first polymer. The second polymer may have the same number of sequences as the first polymer, and substantially the same sequence of subunits, with a few exceptions such as the tagging species for the isobaric species. The first polymer and the second polymer may be prepared in the same reaction vessel. A polymer/protein of defined subunit sequence containing an isobaric species or a tagging species may be analyzed by mass spectrometry to determine the sequence.