This disclosure relates to compositions and methods for selective labeling of aliphatic or aromatic secondary amines. In certain embodiments, the secondary amines are N-methyl lysine, N-terminal proline, N-alkyl lysine, N-alkyl adenosine, N-alkyl cytosine, or peptides and nucleic acids containing the same. In certain embodiments, this disclosure relates to methods of forming 2-indazol-3-carbonyl labeled compounds, benzotriazole labeled compounds, or carbamodithioate labeled compounds comprising contacting a compound containing a secondary amine group with agents disclosed herein.

A solvent medium for diazotization reaction includes a mixed solution of polyethylene glycol and water. The molecular weight of polyethylene glycol is 200-600, and the volume ratio of polyethylene glycol and water is 1:0.4-1.5. The mixed solution also includes hydrochloric acid or sulfuric acid, and the concentration of H.sup.+ in the mixed solution is 0.32-1.72 mol L.sup.1.

A solvent medium for diazotization reaction includes a mixed solution of polyethylene glycol and water. The molecular weight of polyethylene glycol is 200-600, and the volume ratio of polyethylene glycol and water is 1:0.4-1.5. The mixed solution also includes hydrochloric acid or sulfuric acid, and the concentration of H.sup.+ in the mixed solution is 0.32-1.72 mol L.sup.1.