Resonance Raman spectroscopy in the analysis of residual lignin and other unsaturated structures in chemical pulps

Ultraviolet resonance Raman spectroscopy (UVRRS) was applied to determine the characteristic vibrations of several monomeric and dimeric lignin model compounds. The characteristic vibrations of p-hydroxyphenyl, guaiacyl, and syringyl lignin structures were then detected in the UVRR spectra of extracted wood samples. Further, a partial least squares (PLS) model was developed to assist the determination of the characteristic UVRR bands of different lignin structures. The UVRRS-PLS method was further applied to interpret the more complex lignin spectra. The results correlated well with the results obtained by Py-GC/FID for the quantification of guaiacyl and syringyl lignin structures and with results obtained by NMR spectroscopy for the quantification of condensed lignin structures in guaiacyl-type lignin. Chromophoric lignin structures were studied by exciting pulp samples in the visible range. The overwhelming fluorescence emission was suppressed by applying a carbon disulfide (CS2) based Kerr gate technique. The fluorescence rejection ratio was estimated to be as high as 250. Chromophores in chlorine dioxide and peroxide bleached pulps were of different nature, as indicated by the different correlations with brightness and kappa number. Residual lignin and hexenuronic acids in the unbleached and bleached chemical pulps were quantified with PLS calibration and with direct band height ratios. PLS calibration gave relatively good predictions, while the direct band height ratio was more a straightforward method. The band height ratio method is not, however, applicable to unbleached pulps with lignin contents of about 5% because in these pulps the cellulose bands are indiscernible and cannot be used as a reference band. Carbonyl groups in cellulosic pulps were studied by the UVRRS and by the CCOA (carbazole-9-carbonyloxyamine) method, which showed that part of the carbonyls exist in hydrate, hemiacetal and/or hemiketal forms. It was also shown that these structures interconvert upon drying and wetting of the pulps. UVRRS was efficient in detecting extremely low contents of carbonyls, unlike IR spectroscopy, which did not detect any differences in the carbonyl band of these pulps.reviewe