Discovering the Ellagitannin Landscape of Dried Walnut Shells by Liquid Chromatography with Tandem Mass Spectrometry

Walnut shells, often discarded as waste, hold hidden potential as a source of ellagitannins (ETs), compounds known for their promising antioxidant properties and health benefits. This study employed reversed-phase liquid chromatography (RPLC) coupled with Orbitrap-based high-resolution mass spectrometry (HRMS) via electrospray ionization (ESI) in negative polarity to investigate the ET profile in extracts of dried powdered walnut shells. Several compounds belonging to various ET families were successfully identified as deprotonated molecules ([M - H]^-) and characterized, including mono-, di-, tri-, tetra-, and pentagalloyl glucopyranoses, as well as ETs containing the hexahydroxydiphenoyl (HHDP) group. Characteristic product ions were identified in HR tandem MS spectra and employed to recognize the ET landscape. Analysis revealed a complex picture with more than 10 isomers identified in some cases. However, the structural similarity and limitations in MS/MS data hindered the definitive identification of all isomers. Characterization of ETs featuring HHDP groups also remained challenging. Despite these restraints, the estimated total content of ETs suggests potential application in the food, pharmaceutical, and cosmetic industries of those extracts. These findings indicate that walnut shells can be considered a sustainable source of health-promoting compounds, contributing to a greener economy.