Identifying the origin of lignins and monitoring their structural changes by means of FTIR-PCA and -SIMCA

Lignins from sugarcane bagasse and eucalyptus, isolated respectively from soda and kraft liquors by precipitation with HCl or H2SO4, have been investigated together with samples of lignins isolated after organosolv pulping. Sugarcane bagasse lignin precipitated by HCl underwent oxidation by O-2 in aqueous NaOH. The reaction products, the oxidized lignin (reprecipitated with HCl) and the soluble products (isolated by evaporation) formed two other groups of samples. Fourier transform infra-red (FTIR) spectra of 49 of these lignins samples in KBr disks were recorded and analysed by principal component analysis (PCA) and statistical isolinear multicomponent analysis (SIMCA). After normalization of the spectra by dividing the absorbance value at a certain wavenumber by the absorbance value at 1510 cm(-1), PCA analysis using 597 data points collected between 1950 and 800 cm(-1) showed that the first two principal components describe most of the total spectral variance. The plot of these two principal components allowed the lignins to be grouped according to their isolation (kraft, alkaline pulping or organosolv), but no difference could be observed when the lignins were isolated from liquor by precipitation with HCl or H2SO4. Furthermore, the method allows a very good separation of the acetylated samples and can be used as a tool to rapidly identify the origin of lignin samples or to monitor their structural changes. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.