*2.1. Standards and Reagents*

A 99.5% pure Promethazine Hydrochloride standard was procured from the China National Institute for Food and Drug Control, China. A Promethazine-d6 Hydrochloride standard with 98% chemical purity and 99.5% isotopic purity, a Promethazine Sulfoxide (PMZSO) standard with 96% purity, and a Monodesmethyl-Promethazine Hydrochloride standard with 97% purity were all sourced from Toronto Research Chemicals, Canada.

HPLC-grade acetonitrile (ACN) and methanol (MeOH) were obtained from Thermo Fisher Scientific, Waltham, MA, USA. HPLC-grade formic acid was bought from Shanghai Macklin Biochemical Co., Ltd., Shanghai, China. Analytical grade n-hexane was purchased from Tianjin Damao Chemical Reagent Factory, Tianjin, China. Ultrapure water was acquired from a Milli-Q water purification system (Millipore, Billerica, MA, USA).
