sotopic labeling is a technique used to track the target molecule through a reaction, metabolic pathway, or signaling pathway. The target molecule is labeled with specific isotopes instead of its own atoms. In isotopic labeling assay, there are several ways for labeling isotopes determination, such as mass change, radioactive decay and vibrational mode. Depending on the labeling categories, isotopic trace can be subdivided into stable isotope labeling and radioactive isotope labeling.