Development of a rapid method to determine phenolic and other polar compounds in walnut by capillary electrophoresis–electrospray ionization time-of-flight mass spectrometry

The aim of this work was to develop a capillary electrophoresis–mass spectrometry (CE–MS) method to identify and quantify phenolic and other related polar compounds in walnut samples. The extraction capacity of several solvent mixtures of phenolic compounds from walnut by conventional solid–liquid extractions was tested, and CE and electrospray ionization MS parameters were optimized. The finalized procedure is able to determine many well-known phenolic compounds present in walnuts and provide relevant information about the presence of minor polar compounds. A new compound in walnut ((2E,4E)-8-hydroxy-2,7-dimethyl-2,4-decadiene-1,10-dioic acid 6-O-β-d-glucopiranosyl ester, [M−H]− 403.161 m/z) with a structure similar to glansreginins was also identified. Phenolic compounds correspond to 14–28% of total polar compounds quantified. Aglycone and glycosylated ellagic acid represent the principal components and account for 64–75% of total phenols in walnuts. However, the sum of glansreginins A, B and (2E,4E)-8-hydroxy-2,7-dimethyl-2,4-decadiene-1,10-dioic acid 6′-O-β-d-glucopiranosyl ester was in the range of 72–86% of total quantified compounds. In addition, this is the first time that separation by CE with detection by electrospray ionization time-of-flight MS has been applied to the analysis of phenolic and other polar compounds in walnut samples, providing results in less than 15 min.