1-Bromo-2,4-dinitro-benzene-1,2,3,4,5,6-13C6 is an intermediate in the synthesis of a novel isotope-labelled dinitrophenylhydrazines (DNPHs) and methods for their use in detecting and/or quantifying carbonyl groups in proteins and other analytes., 2,4-Dinitrophenylhydrazine-13C6