Structural micro-analysis of residual lignin in pulp by pyrolysis-GC mass-spectrometry

Abstract

It is difficult to analyze residual lignin in pulp, especially in bleached pulp because of its low lignin contents. However, it would be possible to estimate it by using pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) with an appropriate method. In this study, the ratios of syringyl units to guaiacyl units (S/G ratios) of wood meals, unbleached kraft pulps, oxygen bleached kraft pulps and total chlorine free (TCF) bleached kraft pulps were measured by Py-GC/MS. All samples were prepared from four kinds of eucalyptus woods, Eucalyptus nitens, E. grandis, E. macarthurii and E. smithii. The S/G ratios of wood samples were measured at intervals of 10 mm from pith to bark sides. E. nitens gave the highest S/G ratios in four woods. Results from all wood samples showed that the S/G ratios were slightly lower in bark side than pith side. It is important to know total ion peak areas of lignin degradation products from pulps for measurement of the S/G ratios. However, these peak areas were too small to measure directly. In order to estimate these peak areas, we calculated correction factors of lignin pyrolysis products obtained from wood samples, which were equal to the total ion peak areas divided by the main fragment ion peak areas. Then, the total ion peak areas of lignin pyrolysis products from pulps were estimated by multiplying the correction factor and the main fragment ion peak areas from pulps together, and the S/G ratios were measured using guaiacol, 4-methylguaiacol and 4-vinylguaiacol as guaiacyl type compounds, and syryngol, 4-methylsyringol and 4-vinylsyringol as syringyl type compounds. The result showed that the S/G ratios were decreased during pulping and bleaching. This result was correspondent to earlier studies on residual lignin with a different method. It was shown that this method was applicable for the calculation of the S/G ratios of bleached kraft pulps.