Ultrafiltration as alternative purification procedure for the characterization of low and high molecular-mass phenolics from almond skins Original Research Article

A combination of sample preparation (ultrafiltration) and analysis techniques is proposed for the characterization of complex phenolic mixtures such as extracts from almond (Prunus dulcis (Mill.) D.A. Webb) skins. LC/ESI-MS analysis of the permeates obtained after ultrafiltration on semipermeable membranes (low molecular-mass phenolic fractions) allowed the identification of several benzoic acids and aldehydes, flavan-3-ol monomers and oligomers, and flavonol and flavanone glycosides in almond skins. MALDI-TOF and ESI-MS/MS analysis of the diafiltered concentrates (high molecular-mass phenolic fractions) demonstrated the presence of proanthocyanidin oligomers up to decamers, composed of (epi)afzelechin, (epi)catechin and (epi)gallocatechin units linked by Csingle bondC bonds (type B) and by both Csingle bondC and Csingle bondO bonds (type A). This analytical protocol can be of utility in the study of low and high molecular-mass phenolic compounds in natural products.