Isotopic labeling isotopically-substituted radiolabeling English isotope tracers English English English Isotopic labeling radiolabeling isotopically-substituted radiolabeling English radiolabeling isotopically-substituted Isotopic labeling English radioactive tracer isotopically-substituted isotope tracers Isotopic labeling isotope-labeled English isotopically-substituted English English radiolabeling isotope tracers English Isotopic labeling radiolabeling English radiolabeling English English radiolabeling isotopically-substituted English radiolabeled