Fumaric acid

The chemical standards harpagide (1), harpagoside (2), ferulic acid (3), catalpol (4), aucubin (5), cinnamic acid (6) , and caffeic acid (7) were purchased from the Chinese National Institute of Control of Pharmaceutical and Biologiceal Products (Beijing, China); angoroside C (8) was purchased from Chengdu Chroma-Biotechnology Co., Ltd. (Chengdu, China); verbascoside (9) was purchased from Shanghai Yongye-Biotechnology Co., Ltd. (Shanghai, China); p-Coumaric acid (10) and p-methoxycinnamic acid (11) were purchased from Shanghai Yuanye-Biotechnology Co., Ltd. (Shanghai, China); fumaric acid (12) was purchased from Sinopharm chemical reagent Co., Ltd. (Nanjing, China). The purity of all chemical standards is greater than 99% (Figure 2). Methanol and acetonitrile of HPLC grade were purchased from Merck (Damstadt, Germany). Ultrapure water was prepared using a Milli-Q purifying system (Millipore, Bedford, MA, USA) under a resistivity of 18.2 MΩ/cm, other reagent solutions were analytical grade (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China).

The properties of the elegant spices are summarized in Table 1. L‐(+)‐Tartaric acid, formic acid, lactic acid, acetic acid, citric acid, fumaric acid, succinic acid, L‐(+)‐ascorbic acid, propionic acid, potassium dihydrogen phosphate dodecahydrate phosphate (all AR grade), and malic acid (BR grade) were obtained from Sinopharm Chemical Reagent Co. (Shanghai, China). Potassium dihydrogen phosphate (KH₂PO₄), phosphoric acid (H₃PO₄), hydrochloric acid (HCl), and disodium hydrogen phosphate dodecahydrate (Na₂HPO₄•12H₂O) (all AR grade) were purchased from Sinopharm Chemical Reagent Company (Shanghai, China). Inosine 5′‐monophosphate (5′‐IMP), adenosine 5′‐monophosphate (5′‐AMP), guanosine 5′‐monophosphate (5′‐GMP), and cytidine 5′‐monophosphate (5′‐CMP) were purchased from Sigma‐Aldrich (St. Louis). Durashell AA analytical reagents, including an internal standard solution, were purchased from Tianjin Bona Agel Technology Co., Ltd. Methanol, trifluoroacetic acid (TFA), and acetonitrile (ACN) (all HPLC grade) were purchased from Fisher Scientific (Shanghai, China). Ultrapure water was purchased from Hangzhou Wahaha Group Co., Ltd. Sulfosalicylic acid (AR grade) was obtained from Biochemical Technology Co., Ltd.

All chemicals were of analytical grade. Dimethylformamide (DMF), ethanol, fumaric acid (FA), ferric chloride hexahydrate (FeCl₃·6H₂O), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), urea, citric acid, and lead nitrate (Pb(NO₃)₂) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China).