Pyrolysis-mass spectrometry analysis of dehydrogenation lignin polymers with various syringyl/guaiacyl ratios

Abstract

The present paper describes the application of pyrolysis-mass spectrometry (PYMS) to the analyses of lignocellulosic materials. Dehydrogenation lignin polymers (DHPs) with various syringyl/guaiacyl (S/G) ratios, and a standard, milled wood lignin from Aesculus turbinata Blume, were pyrolyzed at 500 °C for 4 s and the volatile products were ionized by low-voltage (20 eV) electron impact. The PYMS spectra of the lignins are shown with identification of most mass peaks. Of the observed mass peaks, the intensitities of the monomeric peaks corresponding to monomethoxyphenols (m/z 124, 137, 138 and 180) and dimethoxyphenols (m/z 154, 167, 182 and 210) were sensitive to the S/G ratio shifts of the DHPs. Changes in the monomeric mass peak intensities reflected those in the yields of the corresponding products determined by pyrolysis-gas chromatography (PYGC). The S/G pryolytic ratios, calculated from the summed mass peak intensities of syringyl and guaiacyl monomeric products, showed a linear correlation with syringaldehyde/vanillin (S/V) ratios determined by nitrobenzene oxidation. This finding suggested that the S/V ratios of lignocellulosic materials are predicted from the S/G pyrolytic ratios obtained by PYMS. Similar linear correlations were observed also between the S/G pyrolytic ratios determined by PYGC and the S/V ratios, for the DHPs and for Japanese hardwoods. Unlike the PYGC method, however, the PYMS method does not need time-consuming determinations of the response factor of each product to a flame ionization detector. This advantage may outweigh the inherent problems of PYMS, such as the ambiguous structural assignment of the mass peaks in the quantitative PYMS analyses of lignocellulosic materials. The dimeric mass peaks, which were hardly detected in an analytical system with a gas chromatograph, also contributed to the mass spectra. © 1997 John Wiley & Sons, Ltd.