Someone ran an SDS-PAGE separation of a crude protein mixture product, and found a protein band at around 18,000 Dalton. It is suspected that this protein might be a mutant of a known protein. The mutant protein differs from the known protein by only the last two amino acids at the N-terminus of the peptide chain. Propose a most suitable method that can be used to determine whether this protein band is the known protein or the mutant protein. Must explain why you make this choice.