The subject invention provides novel isotope-labeled dinitrophenylhydrazines (DNPHs) and methods for their use in detecting and/or quantifying carbonyl groups in proteins and other analytes. In particular, the present invention provides novel methods for identifying biomarkers of oxidative stress, which can be used to either forecast or detect diseases and/or conditions associated with oxidative stress. In one embodiment of the invention, isotope-labeled DNPHs are derived from [C]chlorobenzene.