**2. Materials and Methods**

#### *2.1. Materials*

Compound K and 20(S)-protopanaxadiol (PPD) was purchased from the Ambo Institute (Daejeon, Korea). To be used as internal standards (IS), 13C-caffeine was purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA). All other chemicals and solvents were of reagent or analytical grade.
