Ferulic acid

Ferulic acid (purity >99%)
was purchased
from J&K Scientific Ltd. (Shanghai, China). β-sitosterol
(purity >95%) and phytosterols (54.5% sitosterol, 15.9% stigmasterol,
and 27.3% campesterol) were provided by Sigma-Aldrich Co., Ltd. (Shanghai,
China). All ILs (purity >99%) were obtained from Shanghai Chengjie
Chemical Co., Ltd. (Shanghai, China). n-Hexane, methanol,
and iso-propanol used for high-performance liquid
chromatography (HPLC) analysis were of chromatographic grade and purchased
from Tedia Co., Ltd. (Shanghai, China). Other reagents were of analytical
grade and were obtained from Sinopharm Chemical Reagent Co., Ltd.
(Shanghai, China).

Gallic (99%), protocatechuic (99%), p-hydroxybenzoic (99%), caffeic (98%), p-coumaric (98.0%) and ferulic acid (99.0%) were purchased from J & K Scientific (Beijing, China). Benzoic acid (99.5%) and flavonoids, i.e., rutin (95.0%), myricetin (96.0%), apigenin (95.0%), kaempferol (97.0%), quercetin (98.0%), morin (95.0%), pinocembrin (95.0%), naringenin (98.0%), chrysin (97.0%), galangin (95.0%) caffeic acid phenethyl ester (CAPE, 97.0%) and 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F, purity ≥ 98.0%) were supplied by Sigma–Aldrich (St. Louis, MO, USA).
l-Proline (Pro) was supplied by Aladdin (Shanghai, China). Methanol and acetonitrile (HPLC grade) were purchased from MREDA (MREDA Technology Inc., Columbia, TN, USA). Ultrapure water was produced by using a Milli-Q system (Millipore, MA, USA). Stock solutions of single phenolic compounds (1~10 mg/mL) were prepared by dissolving appropriate amounts of phenolic standards in methanol. Mixed intermediate solutions were prepared by mixing of appropriate stock solutions and dissolving with methanol. Series of working standard solutions were diluted directly to the required concentrations with pure water on the day of use, based on the sensitivity of detection and the linearity range of the study.