The peptide mapping method examines RM 8671 primary structure by monitoring its trypsin digested peptides, which are resolved using reverse-phase ultrahigh pressure liquid chromatography instrumentation coupled to an ultraviolet wavelength detector and a high-resolution mass spectrometer with electrospray ionization source (together “LC-UV-MS/MS”). A chromatographic trace, or peptide map, results from the signal generated as peptides eluting from the LC column pass through the UV and MS detectors producing “peaks”. Differing amino acid sequences give each peptide unique chromatographic properties and the presence of online MS/MS detection provides confident assignment of primary structure based on mass and fragmentation consistent with the predicted amino acid sequence.