The IS solution was added to the serum. (A) The mixture was immediately analyzed by GP-HPLC system. (B) The rest of the IS solution was stored at room temperature (22–24°C) for 10 days before addition to the same serum, and the serum was analyzed. In the same manner, chromatograms of sera including IS solution, which was added immediately (C) and after 10 days of storage at −80°C (D) before analysis were compared. The chromatograms are representative of three separate experiments.