Standard stock solutions of STS and DHEAS-D5 (IS) were prepared by dissolving appropriate amounts in 10 mL methanol, respectively. Working standard solutions of STS were diluted with 50% methanol solution. The IS working solution was prepared by diluting its stock solution with 50% methanol solution to 2 g/mL. Working solutions were prepared daily and stock solutions were stored at 4 ◦C.