All standards and reagents were LC-MS grade and were used without any further treatments. Authentic standards of 2,5-Dihydroxybenzoic acid (gentisic acid), 3,4-dihydroxybenzoic acid (protocatechuic acid), 4-hydroxybenzoic acid, cinnamic acid, epicatechin, eriodictyol, ferulic acid, gallic acid, myricetin, p-coumaric acid, pinoresinol, quercetin, resveratrol, rosmarinic acid, salicylic acid, syringic acid, taxifolin, and vanillic acid were purchased from Sigma-Aldrich (Stenheim, Germany). Apigenin, caffeic acid, catechin, ethyl vanillin (internal standard, IS), galangin, genistein, naringenin, tyrosol, and vanillin were acquired from Alfa Aesar (Karlsruche, Germany), whereas hydroxytyrosol and luteolin were obtained from Santa Cruz Biotechnology (Santa Cruz, CA, USA).
2-Propanol (LC–MS grade) and methanol (MeOH) (LC–MS grade) were purchased from Fisher Scientific (Geel, Belgium) and Merck (Darmstadt, Germany), respectively. Formic acid 99%, ammonium acetate and sodium hydroxide monohydrate for trace analysis ≥ 99.9995% were all obtained from Fluka (Buchs, Switzerland). Ultrapure water (18.2 MΩ resistivity) was provided by a Millipore Direct-Q UV purification System (Millipore, Bedford, MA, USA). The wine samples were filtered using regenerated cellulose syringe filters (RC filters, pore size 0.2 μm, diameter 15 mm) purchased from Phenomenex (Torrance, CA, USA).
2-Propanol (LC–MS grade) and methanol (MeOH) (LC–MS grade) were purchased from Fisher Scientific (Geel, Belgium) and Merck (Darmstadt, Germany), respectively. Formic acid 99%, ammonium acetate and sodium hydroxide monohydrate for trace analysis ≥ 99.9995% were all obtained from Fluka (Buchs, Switzerland). Ultrapure water (18.2 MΩ resistivity) was provided by a Millipore Direct-Q UV purification System (Millipore, Bedford, MA, USA). The wine samples were filtered using regenerated cellulose syringe filters (RC filters, pore size 0.2 μm, diameter 15 mm) purchased from Phenomenex (Torrance, CA, USA).