Investigation of eucalypt and pine wood acid-soluble lignin by Py-GC-MS

The Klason method is adequate for quantifying lignin in softwood (SWD) but is less so for hardwood (HWD). The latter contains a fraction of acid-soluble lignin (ASL) that is not measurable under the conditions prevailing in the Klason method and that must be quantified by other means. This study aimed at investigating the composition of ASL obtained from eucalypt and pine woods by using pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). About 2.7% and 0.9% ASL by dry wood weight were measured in the acid filtrate of eucalypt and pine, respectively, by the Technical Association of the Pulp and Paper Industry (TAPPI) UM 250 method. The acid filtrate was lyophilized and processed by Py-GC-MS, allowing identification of 26 primary pyrolysis products, with 2% and 52% relative molar abundance coming from lignin and carbohydrates, respectively, for eucalypt and 24 primary pyrolysis products, with 1% and 54% relative molar abundance from lignin and carbohydrates, respectively, for pine. The main products derived from lignin were phenol, 4-methylphenol, guaiacol, 4-methylcatechol, syringol, acetosyringone and vanillin. It was concluded that measurements of ASL remain a requirement for accurate eucalypt and pine wood lignin quantification because lignin derivatives remain in the acid filtrate from the Klason lignin quantification method. Corrections for carbohydrate-derived compounds are not necessary, despite their predominance in the filtrate, because these derivatives do not interfere with ultraviolet (UV) absorption.