STRUCTURAL CHARACTERIZATION OF LIGNIN POLYMERS BY TEMPERATURE-RESOLVED IN-SOURCE PYROLYSIS MASS-SPECTROMETRY AND CURIE-POINT PYROLYSIS-GAS CHROMATOGRAPHY MASS-SPECTROMETRY

Temperature-resolved in-source pyrolysis-mass spectrometry (Py-MS) and complementary Curie-point pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) were used to characterize a set of lignin polymers. No sample pretreatment was required and over 100 thermal degradation products were identified in the pyrolysates. Results were compared with whole biomass reference materials. Coniferyl alcohol and sinapyl alcohol, which were formed upon pyrolysis of lignin, could be selectively ionized by ammonia chemical ionization (CI). Curie-point Py-GC with flame ionization detection was applied to classify the lignin polymers according to syringyl/guaiacyl ratios, the number of preserved phenylpropane structural units and functional group content. Multivariate analysis of the in-source Py-MS data was employed to classify the lignin polymers according to the origin and degree of modification. Combined results showed that enzyme-treated cottonwood-milled wood lignin is a relatively homogeneous polymer with a large fraction of preserved alkyl-aryl ether linkages. Technical lignins, isolated by the steam-explosion, organosolv and Kraft pulping processes, were found to be modified depending on the severity of the isolation procedure employed, and contained large amounts of beta-1-linked stilbene and beta-beta-linked resinol types of structures. Bagasse lignin shows the most condensed polymer structure with a large proportion of ether bonded p-coumaric acid and ferulic acid.