L ascorbic acid

Sodium hydroxide and L-ascorbic acid were from Carlo Erba (Milano, Italy). Ferulic acid was purchased from Sigma-Aldrich (Steinheim, Germany). 2,4,6-Tris(2-pyridyl)-s-triazine (TPTZ) was from Fluka (Steinheim, Germany). Sulfuric acid (96%) and sodium carbonate anhydrous were from Penta (Prague, Czech Republic). The stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) was from Alfa Aesar (Karlsruhe, Germany). Anhydrous citric acid and iron chloride hexahydrate (FeCl₃•6H₂O) were from Merck (Darmstadt, Germany). Folin-Ciocalteu reagent and absolute ethanol were from Panreac (Barcelona, Spain). All solvents used for chromatographic determinations were of appropriate (HPLC) grade.

Catechin hydrate (>98%), trans-resveratrol (>99%), quercetin 3-O-glucuronide (≥95%), cyanin chloride (≥90%), ferulic acid (99%), kaempferol, p-coumaric acid, kaempferol 3-O-rutinoside (98%), myricetin, isorhamnetin and rutin (quercetin 3-O-rutinoside) hydrate (>94%) were obtained from Sigma-Aldrich (Darmstadt, Germany). Sodium carbonate anhydrous (99%), Folin–Ciocalteu reagent, and glycerol anhydrous (99%) were purchased from Penta (Praha, Czechia). Formic acid (99%), L-ascorbic acid and citric acid (≥99.5%) were from Carlo Erba (Milan, Italy). All solvents used for chromatography were of appropriate (HPLC) grade. The radical probe 2,2-diphenyl-1-picrylhydrazyl (DPPH), quercetin hydrate (97%) and caffeic acid (99%) were from Alfa Aesar (Karlsruhe, Germany). Iron chloride hexahydrate (FeCl₃ • 6H₂O) was purchased from Merck (Darmstadt, Germany). 2,4,6-Tris(2-pyridyl)-s-triazine (TPTZ) was from Fluka (Steinheim, Germany).