Guidelines for Identifying the Structure of Heavy Phenolics in Lignin Depolymerization by Using High‐Resolution Tandem Mass Spectrometry

The efficient conversion of lignin contributes to reducing human reliance on fossil energy. As a complicated biopolymer, studies on the mechanism of lignin depolymerization is limited by inadequate structural identification of high molecular weight (MW) products like heavy phenolics. Up to now, no individual method can generate both MW and structural information in operando conditions. As a promising approach, tandem mass spectrometry (MS/MS) techniques can provide structural information via the dissociation of target ions. In this study, MS/MS technique was performed both in offline and in-situ mode during lignin depolymerization. The fundamental guidelines based on MS/MS dissociation principles for typical inter-unit linkages like β-O-4, 5-5, β-β, β-5, and β-1 were well established. Based on that, major phenolic dimers are successfully identified, including chemical formula and types of inter-unit linkages. More significantly, real-time monitoring of structural evolution was achieved by applying in-situ MS/MS analysis during lignin depolymerization. The results show the different evolution pathways of isomers with same chemical formula, confirming that structural changes during lignin depolymerization are common and obvious. Overall, this study develops an advanced strategy for the full-view analysis of lignin depolymerization, achieving the static analysis of composition and structure, both monitoring the dynamic evolution of structures.