Liquid chromatography-tandem mass spectrometric analysis of lignin oligomers and phenols

Abstract

The growing demands for the valorization of lignin into depolymerized aromatic chemicals have raised the need for the accurate characterization method of lignin oligomers (LOs) and phenolic compounds (PCs) in lignin degradation products. The complex molecular structure of LOs with multiple linkage types and the various types of chemical modifications on PCs make their characterization challenging. This thesis describes the development of selective and sensitive characterization methods of PCs and LOs. By utilizing UV absorption chromatogram and product ion chromatogram of phenylpropanoid units for selective detection of PCs and LOs, 7 PCs from organosolv rice straw lignin and 7 LOs from G-type lignin synthesized by enzymatic polymerization were identified. The combined use of different collision-induced dissociation methods provided additional linkage information on LOs. The use of acetic acid (Ac) mobile phase additive significantly enhanced the (−)-ESI-MS sensitivity of PCs and LOs on average by 3 and 2 times, respectively, compared with ammonium acetate used in previous studies. By comparing the (−)-ESI-MS sensitivity using acidic additives, their ammonium salts, and the acidified ammonium salts, the greater sensitivity for PCs and LOs using Ac against the other additives was rationalized by three factors: ion suppression effect of ammonium cation, gas-phase basicity of anion, and surface activity of PCs and LOs. The enhanced sensitivity for LOs using Ac resulted in a greater number of detectable linkage-specific fragment ions and LO identifications than using ammonium acetate. The developed characterization methods for PCs were applied to the enzymatic degradation products of various types of agricultural residue biomass lignin. The quantitative analysis of PCs in biomass lignin revealed the increase of p-hydroxybenzaldehyde and vanillin under a thermoalkaliphilic laccase treatment. Among the different biomasses, organosolv lignin from rice straw yielded the highest concentration of the benzaldehyde chemicals under the laccase treatment. The major advantage of the developed analytical method for PCs and LOs in lignin degradation products over the previous methods is a greater number of identifications owing to the enhanced (−)-ESI-MS/MS sensitivity. Through the identification and quantification of lignin degradation products with high selectivity and sensitivity, this work could provide a valuable tool for the development of pretreatment and degradation methods for various types of lignin biomasses.