A reagent suitable for the determination of N-terminal residue of a peptide is 

2, 4- Dinitroflurobenzene, also known as Sanger's reagent, reacts with the H₂N - group of the peptide to from 2, 4- dinitrophenyl (DNP) derivative of the peptide.  The DNP derivative of the peptide is hydrolysed to give DNP derivative of the single amino acid  

                                                                   2, 4, Dinitrofluoro benzene 

                                                                (DNP derivative of polypeptide)