Characterization of residual lignins from chemical pulps of spruce (Picea abies L.) and beech (Fagus sylvatica L.) by analytical pyrolysis-gas chromatography/mass spectrometry

Residual lignins have been isolated by enzymatic hydrolysis from sulfite, kraft, ASAM and soda/ AQ/MeOH pulps obtained both of spruce and beech wood. The protein content of the samples were in the range of 2 to 61% and the carbohydrate content fluctuated roughly between 1 and 12%. Seven of the isolates have been submitted to analytical pyrolysis (Py-GC/MS) aiming at the determination of the lignin composition in terms of 4-hydroxy-phenylpropane, guaiacylpropane, and syringylpropane units, referred to in the following as H, G, and S units. Pyrolysis of proteins leads mainly to phenol, cresol, and indole. Phenol and cresol of protein origin may influence seriously the H content determination of lignins. However, the contribution of protein to this compounds can be corrected successfully based on the intensity of the indole peak in the pyrograms. Nevertheless, the pyrolysis results should not be over-interpreted concerning the H contents in the case of samples heavily contaminated with proteins. As analytical pyrolysis overestimates the S content in a similar manner as the chemical degradation techniques do, an empirical correction equation was used to approximate the "true" S content. Py-GC/MS gave systematic and reproducible differences between the residual lignins on the one hand, and the MWLs, which have been pyrolysed for comparison, on the other hand. The fingerprinting ability of analytical pyrolysis, the GIS ratios of belch residual lignins, and the unusual properties of ASAM residual lignins with their high S contents are discussed.