Identification of residual lignin markers in eucalypt kraft pulps by Py–GC/MS

Abstract

Pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) has been used to characterize the residual lignin in eucalypt ( Eucalypt globulus) kraft pulp during the pulping and bleaching processes. The composition and structure of eucalypt wood lignin and kraft lignin have also been studied by Py–GC/MS and 13C-nuclear magnetic resonance (NMR). Pulp samples were collected at different stages of the pulping process (after cooking, oxygen delignification, and bleaching with chlorine dioxide or hydrogen peroxide). By comparison with lignin pyrolysis products obtained from Py–GC/MS of eucalypt wood and kraft lignin, characteristic features of lignin in the different pulps have been established. The main lignin-derived markers detected in pulps were guaiacol, 4-methylguaiacol, 4-ethylguaiacol, 4-vinylguaiacol, syringol, 4-methylsyringol, 4-ethylsyringol, 4-vinylsyringol and trans-4-propenylsyringol, all of them in very low amounts. Analytical pyrolysis was also used to characterize the changes in the residual lignin after kraft pulping of eucalypt wood pretreated with two lignin-degrading fungi ( Bjerkandera adusta and Poria subvermispora) in wood ‘biopulping’ experiments. In general, Py–GC/MS has shown to be valuable in the characterization of residual lignin markers in kraft pulps due to the sensitivity of the technique when individual-ion chromatographic profiles for selected lignin markers are used.