The P1+71 and P2+98 fragment ions in the MALDI-MS/MS acquired without collision gas (Figure ) were considerably more intense than other linker fragment ions detected at m/z 984.46 and m/z 1018.44 and tentatively assigned to P1+169 and P1+203. These linker fragment ions are originated by the fragmentation of the thioether bond in the hydrolyzed thiosuccinimide linker (Figure and 2g) and their tentative structures are shown in Fig. .