A new fragmentation process was proposed to interpret the characteristic product ion at m/z 130 of protonated arginine. The a-amino group was dissociated from protonated arginine and then combined with the (M + H-NH3) fragment to form an ion-neutral complex which further generated a hydroxyl-amino exchange intermediate compound through an ion-molecule reaction. This intermediate compound was synthesized from argininamide through a diazo reaction, and then the reaction mixture was analyzed using liquid chromatography combined with mass spectrometry (LC-MS). The collision-induced dissociation experiments under the same conditions revealed that this intermediate compound produced the characteristic product ion at m/z 130 as well as protonated arginine, and in addition, density functional theory calculations were performed to confirm simultaneous loss of NH3 and CO from this intermediate to give the m/z 130 ion.