Structural Analysis of Aromatic Rings in Residual Lignin from Hardwood Kraft Pulp by Analytical Pyrolysis

The syringyl to guaiacyl (S/G) ratio of aromatic structure in lignin has been used as an index of changes in lignin structures during pulping and bleaching. However, It is difficult to clarify lignin structure of oxygen bleached hardwood kraft pulp (LOKP) by using wet chemistry because the lignin content in LOKP Is very small. On the other hand, there is a possibility that pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) can detect a quite small amount of lignin without pre-treatments. Therefore, the aim of this study is to examine whether knowledge on the S/G ratios could be obtained from the pulps with very small amounts of residual lignin by using Py-GC/MS. The samples were prepared from wood chips, unbleached and bleached eucalyptus kraft pulps. After Py-GC/MS of the pulps, the total ion peak area of a pyrolysis product was estimated from the molecular ion and some fragment ions because the peak area from the residual lignin in each pulp was very small. The S/G ratios were calculated from the amounts of syringyl and guaiacyl compounds based on the total ion peak areas of the pyrolysis products. The results showed that the S/G ratios in the residual lignin decreased during the pulping and bleaching processes. It was clearly shown that Py-GC/MS was available for analysis of residual lignin even in bleached pulps.