Characterizing Phenolic Compounds in Apples

Commercially available standards of phenolics mostly present in different apple cultivars, including gallic acid (97.5–102.5%), chlorogenic acid (≥95%), (-)-epicatechin (≥90%), catechin (≥98%), caffeic acid (≥98%), rutin hydrate (≥94%), quercetin (≥95%), guaiaverin (≥99.3%), quercitrin (≥97%), hyperoside (≥98.35%), hirsutrin (≥90.1%), reynoutrin (≥97%), phloridzin (≥99%), and phloretin (≥99%), were purchased from Sigma–Aldrich (Steinheim am Albuch, Germany). The LC/MS grade solvents acetonitrile (ACN), methanol (MeOH), and absolute ethanol (EtOH, ≥99.7%) were supplied by Sigma–Aldrich (Prague, Czech Republic). Formic acid (≥98%) was purchased from Honeywell (Seelze, Germany). Ultrapure CO₂ (>99.99%) in cylinders with a dip tube was provided by Messer (Prague, Czech Republic). Ultrapure water was acquired using a Milli-Q purification system (Millipore, Billerica, MA, USA).