Lignin in plant cell walls is a complex, irregular polymer built from phenylpropanoid C6-C3 units that are connected via various C-C and C-O linkages. A recent study using time-of-flight secondary ion mass spectrometry (ToF-SIMS) with Ga primary ion bombardment showed that lignin polymers can be characterized by specific positive ions possessing a substituted aromatic ring (so-called guaiacyl or syringyl rings), which are the basic building units of lignin. To study the relationship between the characteristic ions of lignin and the common interunit linkages, various lignin dimer model compounds were investigated using ToF-SIMS. The resulting dimer spectra showed that the characteristic ions with a guaiacyl ring at m/z 137 and 151 result from rupture of most common interunit linkages, not only 8-O-4' linkages, which are the most abundant in lignin, but also 8-1', 8-5', and 8-8'. There was no evidence of rupture of 5-5' linkages. These results show that ToF-SIMS offers a new tool for the direct analysis of the depolymerized fragments of lignin polymers. The mechanisms for the fragmentation of lignin dimer models in ToF-SIMS were proposed that allow ToF-SIMS fragmentation rules to be deduced. Adduct ions such as [M + 13]+ ([M + CH]+) were also produced in fragmentation of the dimers and are thought to arise from the combination of the molecules with their stable fragments.

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In order to investigate the delignification mechanism during high-boiling solvent (HBS) pulping, softwood HBS lignin samples from todo fir (Abies sachalinensis) were analyzed by 1H–13C correlation NMR spectroscopy (HMQC). Nine lignin model compounds that may represent the substructures in HBS lignin were prepared. The 1H and 13C NMR spectral data were compared with HBS lignin samples. The HMQC spectra of HBS lignin samples showed the disappearance of β-O-4 structures. Introduction of a pulping solvent into lignin was plainly observed with new signals in aliphatic regions. Only the erythro β-1 structure was detected in softwood milled wood lignin from todo fir by HMQC spectra. The β-1 structure was very reactive and probably produced the stilbene structure. The β-β and β-5 structures remained in HBS lignin. The 4-O-5 structure was successfully detected by HMQC spectra, and it was found to be stable under the conditions used.

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