Syringic acid

Dehulled seeds of organic black quinoa (Chenopodium quinoa Willd.), labelled as originating from Bolivia (South America) and packed in a protective atmosphere, were purchased in a retailer specialized in organic and biodynamic products in Italy. Proximate composition per 100 g was, as reported in the nutrition label: 380 kcal energy; 6.2 g fat, of which 0.7 g saturates; 65 g carbohydrates, of which 4.0 g sugars; 9.0 g fibre; 12 g protein; and 0.01 g salt.
Folin–Ciocalteu’s Reagent, methanol, ethanol, acetone, acetic acid and acetonitrile were purchased from Carlo Erba Reagents (Milan, Italy).
Caffeic acid, (+)-catechin, o-coumaric acid, p-coumaric acid, (−)-epicatechin, 3,4-dihydroxycinnamic acid, t-ferulic acid, gallic acid, gentisic acid, 4-hydroxybenzoic acid, protocatechuic acid, rutin, sinapic acid, syringic acid, vanillic acid, and vanillin were purchased from Extrasynthèse (Geney, France) and Sigma-Aldrich (St. Louis, MO, USA).
HPLC grade solvents and water purified by a Milli-Q system (Millipore Corp., Billerica, MA, USA) were used in HPLC analysis.

Ultrapure water was obtained through a purification system (Millipore Direct-Q UV, Bedford, MA, USA). Formic acid, of LC-MS grade, and ammonium acetate ≥99% were obtained from Fluka (Buchs, Switzerland). Sodium hydroxide >99% and methanol, of LC-MS grade, were acquired from Merck (Darmstadt, Germany). 2-propanol was acquired from Fisher Scientific (Geel, Belgium).
All standards had an analytical grade ≥95%. Epicatechin, ferulic acid, gallic acid, homovanillic acid, oleuropein, pinoresinol, p-coumaric acid, and syringaldehyde were purchased from Sigma-Aldrich (Steinheim, Germany). Apigenin, caffeic acid, tyrosol, and vanillin were acquired from Alfa Aesar (Karlsruhe, Germany). Luteolin and hydroxytyrosol were purchased from Santa Cruz Biotechnologies (Heidelberg, Germany). Syringic acid was obtained from Extrasynthèse (Genay, France). Stock solutions for each analyte (1000 mg/L) were dissolved in methanol and stored at −20 °C. Mixed standard working solutions were prepared every laboratory day diluting the stock solutions with water:methanol (20:80, v/v).

Methanol (LC-MS grade) and sodium hydroxide (>99%) were purchased from Merck (Darmstadt, Germany). Ammonium acetate (≥99% for HPLC) and formic acid (LC-MS Ultra) were purchased from Fluka (Buchs, Switzerland). Isopropanol was obtained from Fisher Scientific (Geel, Belgium). Ultrapure water was provided by a Milli-Q purification apparatus (Millipore Direct-Q UV, Bedford, MA, USA). Regarding the standards that were used, syringic acid 95% was purchased from Extrasynthèse (Genay, France). Gallic acid 98%, ferulic acid 98%, epicatechin 97%, p-coumaric (4-hydroxycinnamic acid) 98%, homovanillic acid 97%, quercetin 98%, oleuropein 98%, pinoresinol 95%, caffeic acid 99%, taxifolin 98%, vanillic acid 97% and syringaldehyde 98% (internal standard) were obtained from Sigma-Aldrich (Steinheim, Germany). Hydroxytyrosol 98% and luteolin 98% were acquired from Santa Cruz Biotechnologies. Vanillin 99%, apigenin (4,5,7-trihydroxyflavone) 97% and tyrosol (2-(4-hydroxyphenyl) ethanol) 98% were purchased from Alfa Aesar (Karlsruhe, Germany). Stock standard solutions of individual compounds (1000 mg/L) were solubilized in methanol and stored at −20 °C in dark brown glass bottles. Intermediate standard working solutions containing the analytes were prepared by appropriate dilution of the stock solutions with methanol:water (80:20, v/v) over the concentration range 0.1–8 mg/L.