Protein sequencing by mass spectrometry has transformed the field of biopharmaceutical analysis, but a missing part in the analytical toolkit is the ability to distinguish between the isomeric residues isoleucine and leucine because it is a requisite for efficient analysis of the primary structure of proteins. To address this need, we have developed a novel mass spectrometric method that combines reductive dimethylation and MS3 fragmentation with LCMS peptide mapping. The dimethylation of peptide N-termini leads to intense a1-ions upon collision-induced fragmentation, and further fragmentation of the isoleucine/leucine a1-ion leads to informative spectra with fragments that can discriminate between the two isomers. The methodology of a1-directed MS3 was applied to two antibodies in combination with the proteases trypsin, thermolysin, chymotrypsin, and pepsin to generate peptides exposing N-terminal I/L residues.