derives from iTRAQ8plex-121 reporter+balance reagent acylated residue iTRAQ8plex reporter fragment iTRAQ8plex-121 reporter fragment (13)C 6 H 13 (15)N 2 1+ 121.12000274658203125 121.12152099609375 4-methyl-1-methylidenepiperazin-1-ium MOD:01601 PSI-MOD The protein modification reporter fragment produced by an Applied Biosystems iTRAQ8plex-121 reagent derivatized residue. X artifact