To do the biological study of how particular (good) chemicals like antioxidants affect people, there needs to be a quick, reliable, and consistent way to measure those chemicals in humans. Furthermore, a good method would test for multiple chemicals, because the health benefits from some chemicals only exist in the presence of others (phenolic acids and flavonoids in this case). The authors of this paper demonstrate a gas-chromatography mass-spectrometry (GC-MS) method to investigate how some of the chemicals present in cranberries (from cranberry juice) go into your bloodstream after you ingest them.
