All reference standards were obtained as certified solutions in methanol from Cerilliant (Round Rock, TX, USA) at a concentration of 1 mg/mL. The studied illicit drugs and some of their studied metabolites were COT, MOR, MC, COD, AM, MA, MDA, BZE, NK, 6-MAM, MDMA, KET, and COC. Deuterated compounds COT-D3, MOR-D3, MC-D3, COD-D6, AM-D5, MA-D5, MDA-D5, BZE-D3, NK-D4, 6-MAM-D3, MDMA-D5, KET-D4, and COC-D3 used as isotope-labeled internal standards (IS) were also purchased from Cerilliant as 100 μg/mL solutions in methanol.
Mixed stock solutions were prepared with methanol at concentrations of 500 and 2500 ng/mL. Working standard solutions were prepared by appropriately diluting mixed stock solutions in methanol to different concentrations. All stock and working solutions were stored in the dark at −20 °C.
HPLC-grade methanol (MeOH), acetonitrile (ACN), and isopropyl alcohol (IPA) were purchased from Tedia (Fairfield, OH, USA). Formic acid (HCOOH) was obtained from Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). Sodium hydroxide (NaOH) and sodium dihydrogen phosphate dihydrate (NaH2PO4⋅2H2O) were acquired from Xilong Scientific Co., Ltd. (Shantou, China). Deionized water was prepared using a Milli-Q water purification system (Nanjing Miaozhiyi Electronic Technology, Nanjing, China).
Mixed stock solutions were prepared with methanol at concentrations of 500 and 2500 ng/mL. Working standard solutions were prepared by appropriately diluting mixed stock solutions in methanol to different concentrations. All stock and working solutions were stored in the dark at −20 °C.
HPLC-grade methanol (MeOH), acetonitrile (ACN), and isopropyl alcohol (IPA) were purchased from Tedia (Fairfield, OH, USA). Formic acid (HCOOH) was obtained from Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). Sodium hydroxide (NaOH) and sodium dihydrogen phosphate dihydrate (NaH2PO4⋅2H2O) were acquired from Xilong Scientific Co., Ltd. (Shantou, China). Deionized water was prepared using a Milli-Q water purification system (Nanjing Miaozhiyi Electronic Technology, Nanjing, China).