The presently disclosed subject matter provides methods for evaluating and characterizing peptides, peptide mixtures, and polypeptide mixtures. More particularly, the presently disclosed subject matter provides methods for evaluating or characterizing complex peptide or polypeptide mixtures comprising glutamic acid, alanine, tyrosine, and lysine, e.g., Copolymer-1 or glatiramer acetate, including, but not limited to, methods of identifying, isolating, quantifying, and purifying amino acids, peptides, polypeptides, and combinations thereof having a diethylamide group instead of a carboxyl group present on the C-terminus. The presently disclosed methods can be used to determine the mole percent of polypeptides having a diethylamide group at a C-terminus thereof and can be used to evaluate one or more properties of a sample of one polypeptide mixture as compared to one or more properties of a different sample of a polypeptide mixture.