F-19 NMR spectroscopy for the quantitative analysis of carbonyl groups in lignins

The amount of carbonyl groups in an organosolv lignin was determined by F-19 NMR spectroscopy after different derivatizations: derivatization with trifluoromethylphenylhydrazine, fluorobenzoylation of alcohol groups formed after NaBH4 reduction of lignin, fluorobenzoylation of phenolic hydroxyl groups after catalytic hydrogenation of lignin. Oximation was also used for comparison. Carbonyl group contents determined by F-19 NMR after trifluoromethylphenylhydrazine derivatization were in good correlation with results from other methods. Trifluoromethylphenylhydrazine derivatization is a fast and easy technique, which provides a distinct quantification of aldehyde, ketone and quinone groups. Some round robin lignins (Milne et al. 1992), using trifluoromethylphenylhydrazine derivatization and modified oximation (Zakis 1994) provided significantly higher carbonyl group contents than the round robin test data, which had been measured either by classical oximation or by reduction-UV methods. Other recently reported carbonyl group estimations on round robin lignins (Faix et nl. 1998; Ahvazi et al. 1999) were either in agreement or in disagreement with the herein-presented results. This indicates that carbonyl group analysis in lignins is difficult and that care must be taken when selecting an appropriate method.