Analytical grade acids

All solutions were prepared using ultrapure water (18.2 MΩ, Milli-Q). Analytical-grade Na-acetate (NaCH₃COO; AppliChem, Germany), reagent-grade NH₄-acetate (NH₄CH₃COO; Fisher Chemical, USA), ReagentPlus®-grade hydroxylamine hydrochloride (NH₂OH∙HCl; Sigma-Aldrich, Germany), and trace metal-grade acetic acid (CH₃COOH; Fisher Chemical, USA) were used for the preparation of the extractants. Hydrochloric acid (HCl) and nitric acid (HNO₃) were purified by sub-boiling from analytical-grade acids (Merck, Germany). Adjustment of pH was done using sub-boiled HNO₃ and trace analysis supra-grade ammonia solution (NH₄OH; Bernd Kraft, Germany).

References used for the HPLC-MS analysis were purchased from Sigma Aldrich (St. Louis, MO, USA): Chlorogenic acid, p-coumaric acid, caffeic acid, rutin, apigenin, quercetin, isoquercitrin, quercitrin, hyperoside, kaempferol, myricetol, and fisetin, Roth (Karlsruhe, Germany): Ferulic acid, sinapic acid, gentisic acid, gallic acid, patuletin, luteolin or from Dalton (Toronto, ON, Canada): cichoric acid, caftaric acid. HPLC grade solvents, analytical grade acids used for mobile phases and Folin-Ciocâlteu reagent were purchased from Merck (Darmstadt, Germany), together with sodium carbonate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate and aluminium chloride used for antioxidant assays. ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt) ≥98% purity, potassium peroxodisulfate (≥99% purity), DPPH, and Trolox (6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid; ≥97% purity) also used in antioxidant tests were purchased from Sigma Aldrich (Schnelldorf, Germany). Gallic acid monohydrate (99.5%) was purchased from Serva, (Heidelberg, Germany).