Malonic acid

Malonic acid (99%; Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and choline-chloride (98%; Aladdin Reagent Co., Ltd., Los Angeles, CA, USA) were dried at 333 K and 353 K, respectively, for 24 h in a vacuum drying oven. ChCl and MA were mixed at a molar ratio of 1:1, and heated at 333 K until a colorless homogeneous liquid formed. Bismuth chloride (98%; Aladdin Reagent Co.) was then added to ChCl-MA DES and heated to dissolve to form the electrolyte.

OTC (CAS No. 79-57-2, purity ≥ 97.0%) and TC (CAS No. 60-54-8, purity ≥ 98.0%) standards were purchased from Sigma-Aldrich LLC (St. Louis, MO, USA). A DOX (CAS No. 564-25, purity ≥ 98.0%) standard was acquired from Merck Drugs & Biotechnology Co., Inc. (Fairfield, OH, USA). CIP (CAS No. 85721-33-1, purity ≥ 98.0%) and ENR (CAS No. 93106-60-6, purity ≥ 98.0%) standards were obtained from Macklin Inc. (Shanghai, China).
LC-grade methanol and acetonitrile were provided by Tedia Company Inc. (Fairfield, OH, USA). Sodium hydroxide, citric acid, magnesium chloride, malonic acid and other reagents were of analytical grade and were supplied by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Sodium dodecyl sulfate was obtained from Solarbio Life Science Co., Ltd. (Beijing, China). Ultrapure water (18.2 MΩ* cm, 25 °C), which met the standard of ‘water for analytical laboratory use-specification and test methods’ [12 ], was prepared with a Nanopure water purifier produced by Thermo Fisher Scientific Inc. (Waltham, MA, USA). All solutions used in the LC system were filtered through a 0.45 μm pore size polytetrafluoroethylene microporous membrane filter and then degassed.