2.6. GRP identification and quantification
GRP was identified at HPLC–DAD comparing the spectrum with that reported by Cheynier, Trousdale, Singleton, Salgues, and Wylde (1986) and it was quantified as caftaric acid. Its identification, however, was confirmed by injection to LC–MS with the same conditions of HPLC–DAD method. The presence of the pseudomolecular ion (m/z = 618) at the same retention time of GRP peak at HPLC–DAD gave a further certainty of its identity.