Experimental ACTH18-39 and oxidized Insulin B chain peptide samples were separately dissolved in 0.1% trifluoroacetic acid aqueous with the concentration fixed at 10 pmol/μL. 1,5-diaminonaphtalene (DAN), which can provide good S/N for ISD fragment ions , was used as the MALDI matrix. The DAN matrix was dissolved to 0.1% trifluoroacetic acid aqueous/ 50% acetonitrile with the matrix concentration fixed at 10 mg/mL. Subsequently, the matrix and sample solutions were mixed 1/1 (v/v), and then 1 μL of each solution was deposited and dried on the MALDI target plate. Afterwards, each sample was analyzed in triplicate on the JEOL JMS-3000 SpiralTOF MALDI-MS system.