A solid powder of caffeine from Sigma-Aldrich (Tokyo, Japan) and methamphetamine hydrochloride from Sumitomo Dainippon Pharma Co. (Osaka, Japan) were used as the standard products. Diazepam-d5 (Sigma-Aldrich, Tokyo, Japan) was used as an internal standard (IS). Deionized pre water was generated using the Milli-Q Purification System (Millipore, Bedford, MA, USA) and was used as distilled water.
As the solvent, 0.1 M phosphate buffer was prepared by dissolving 6.8 g KH2PO4 (Wako, Osaka, Japan) in distilled water at pH 6.0 with potassium hydroxide solution.
Then, 1 M acetic acid was prepared by dissolving 30 mL acetic acid (Wako) in 470 mL distilled water and used as a washing solvent. As one of the eluting solvents, 50 mL dichloromethane (Wako, Osaka, Japan), 20 mL 2-propanol (Wako), and 5 mL ammonia aqueous solution (Wako) were mixed to produce dichloromethane:2-propanol:ammonia (10:4:1).