Rapid screening of hardwood and softwood lignin using low temperature thermochemolysis with a GC injection port

Abstract

A low temperature thermochemolysis method was developed to rapidly differentiate softwood and hardwood lignin. The technique uses heat at 350 °C in the presence of trimethylsulfonium hydroxide (TMSH), whereby the sample is introduced via a fabricated solids injector into the injection port of a gas chromatography–mass spectrometry (GC–MS) instrument. No major modification or external devices are required for instrument conversion and the switchover to normal use is complete within minutes. Three different softwood lignin and two hardwood lignin samples were examined. All the thermochemolysis results revealed the presence of either yangambin, a hardwood dimeric marker containing syringyl groups, or sylvatesmin, a softwood dimeric marker containing guaiacyl groups. The results were confirmed by way of thermochemolysis using a traditional micro-furnace pyrolyzer. The study also revealed the presence of 3,3′,4,4′-tetramethoxystilbene, a marker for both types of lignin. It appears that the thermochemolysis-TMSH technique used with a 350 °C injection port produces larger methylated units of the lignin substructures than those obtained using thermochemolysis in the presence of the more common methylating agent, tetramethylammonium hydroxide (TMAH), which is often used at much higher temperature. In addition to lignin markers, other compounds such as resin acid methyl esters were liberated, but only as a result of thermochemolysis. These extract-like materials were not observed using solvent extraction–methylation, which indicates that these resin acid units may be covalently bound to the softwood lignin macrostructure.