Improved analysis of isomeric polyphenol dimers using the 4th dimension of trapped ion mobility spectrometry – mass spectrometry

Abstract

Dehydrodicatechins have recently received attention as oxidation markers especially in grapes and wine. Their analysis mainly uses LC-MS/MS which is able to differentiate them from their natural isomers (dimeric procyanidins), based on specific fragments. However, this technique does not distinguish coeluted compounds showing identical mass spectra. The objective of this work was to develop a method using ion mobility (UHPLC−ESI−TIMS−QTOF−MS/MS) to improve the detection and discrimination of dehydrodicatechins and procyanidins and apply it to grape seed extracts. Oxidation dimers of (+)-catechin and/or (−)-epicatechin were prepared from the reaction with a grape polyphenoloxidase (PPO) extract in aqueous medium (pH~5). A commercial grape seed extract was used for the application of the analytical method. Analyses were performed using the following conditions: an UHPLC C18 column, H2O/HCOOH (90/1) and C2H3N/H2O/HCOOH (80/19/1) as mobile phase, ESI in negative mode, TIMS analyser with the inverse reduced mobility (1/K0) range of 1–1.25, 150 ms ramp time , and a mass range of 150–1500 m/z, using collision-induced dissociation at 27 eV. The method was optimized for the detection and separation of dehydrodicatechins and procyanidinins in the ion mobility dimension using standards and mixtures of oxidation products. Approximately thirty dehydrodicatechins were produced in the reaction mixture with PPO. These compounds included B-type and A-type dehydrodicatechins derived from (+)-catechin and/or (−) epicatechin, containing interflavanic bonds of different natures (biphenyl and biphenyl ether) and positions. Our method allowed the separation by ion mobility of several pairs of isomeric dehydrodicatechins coeluted (or partially) in chromatography. Some of them had similar MS/MS fragmentation pattern and would hardly be distinguished by the use of LC-MS/MS alone. Application of the method on a sample of grape seeds revealed the presence of different B-type procyanidins and two dehydrodicatechins which were derivatives of (+)-catechin and (−)-epicatechin, respectively. It is noteworthy that among these compounds a good separation by ion mobility was obtained for a B-type dehydrodicatechin, procyanidin B1 and procyanidin B3 which were partially coeluted in chromatography.

To the best of our knowledge, this is the first time that ion mobility has been applied to the analysis of (+)-catechin and/or (−)-epicatechin-derived dehydrodicatechins. Mainly, the method proposed in this work provided the detection of several isomers of dehydrodicatechins and procyanidins in model solutions and grape seeds, thanks to the additional separation obtained by ion mobility. This method has the potential to be applied on several other natural complex matrices such as wine and by-products for the monitoring of dehydrodicatechins, considered as oxidation markers.