*2.3. Preparation of Solution*

Standard stock solution: Promethazine hydrochloride standard (calculated as PMZ, C17H20N2S), PMZSO standard (PMZSO, C17H20N2OS), and Monodesmethyl-Promethazine hydrochloride standard (calculated as Nor1PMZ, C16H18N2S) were accurately weighed and separately dissolved in HPLC-grade ACN in 50 mL volumetric flasks to achieve a concentration of 1000 μg/mL. These solutions were stored at −22 ◦C.

PMZ-d6 standard stock solution: A mass of 10 mg of promethazine-d6 hydrochloride standard (C17H15D6ClN2S) was transferred to a 10 mL volumetric flask, dissolved in HPLCgrade MeOH to achieve a concentration of 1000 μg/mL, sealed, and stored at −22 ◦C.

Mixed standard working solution: A volume of 1 mL of each of PMZ, PMZSO, and Nor1PMZ standard stock solutions were combined and diluted with HPLC-grade ACN to obtain series working solutions at concentrations of 2.5 μg/mL, 1.0 μg/mL, 0.5 μg/mL, 0.25 μg/mL, 0.05 μg/mL, 0.025 μg/mL, 0.005 μg/mL, and 0.0025 μg/mL. These solutions were stored at 4 ◦C.

PMZ-d6 working solution: An appropriate amount of PMZ-d6 standard stock solution was diluted with HPLC-grade ACN to obtain an internal standard solution to a final concentration of 1.0 μg/mL, sealed, and kept at 4 ◦C.

Acetonitrile saturated n-hexane: An appropriate amount of analytical grade n-hexane was added to an appropriate amount of ACN, mixed well, and allowed to stand until layered.

Formic Acid Solution (0.1%) in Water: A volume of 1.00 mL of HPLC-grade formic acid was transferred, diluted to 1 L volume with ultrapure water, and mixed well.

Formic Acid (0.1%) in Acetonitrile: A volume of 1.00 mL of HPLC-grade formic acid was transferred, and ACN was added to form a volume of 1 L.

Formic Acid Solution (0.1%) in Water–Acetonitrile (80:20, *v*/*v*): A volume of 200 mL of HPLC-grade ACN was transferred to a 1 L volumetric cylinder, approximately 800 mL of 0.1% formic acid solution in water was added, and the combination was mixed well.
