modified L-cysteine residue IDBEST tag for quantitation 298.022735595703125 299.029998779296875 401.03192138671875 402.1400146484375 H 18 (12)C 13 (13)C 2 N 3 O 3 S 1 Br 1 Heavy IDBEST tag for quantitation artifact modification from Unimod Isotopic label MOD:00932 PSI-MOD none H 13 (12)C 10 (13)C 2 N 2 O 2 Br 1 IGBP:13C(2) C (13)C isotope tagged reagent