Anthocyanin-type pigments cause great interference in the chromatographic separation and identification of non-anthocyanin phenolic compounds and especially flavonols. The removal of anthocyanin pigments was carried out according to a method
described by Nixdorf and Hermosín-Gutiérrez (2010) by means of solid-phase extraction using a combination of reversed-phase and cationic-exchanger material, Oasis MCX cartridges (Waters,Milford, Massachusetts, 6 cm3 capacity filled with 500 mg of absorbent).