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

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Doctoral thesis (article-based)
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54, [88]
Reports / Helsinki University of Technology, Laboratory of Forest Products Chemistry. Series A, 20
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.
carbonyls, hexenuronic acid, Kerr gate, lignin, model compounds, resonance Raman spectroscopy
Other note
  • Saariaho, A.-M., Jääskeläinen, A.-S., Nuopponen, M., and Vuorinen, T., 2003. Ultra violet resonance Raman spectroscopy in lignin analysis: determination of characteristic vibrations of p-hydroxyphenyl, guaiacyl, and syringyl lignin structures. Applied Spectroscopy, 57: 1, pages 58-66.
  • Saariaho, A.-M., Hortling, B., Jääskeläinen, A.-S., Tamminen, T., and Vuorinen, T., 2003. Simultaneous quantification of residual lignin and hexenuronic acid from chemical pulps with UV resonance Raman spectroscopy and multivariate calibration. Journal of Pulp and Paper Science, 29: 11, pages 363-370.
  • Saariaho, A.-M., Jääskeläinen, A.-S., Matousek, P., Towrie, M., Parker, A. W., and Vuorinen, T., 2004. Resonance Raman spectroscopy of highly fluorescing lignin containing chemical pulps: suppression of fluorescence with an optical Kerr gate. Holzforschung, 58: 1, pages 82-90.
  • Saariaho, A.-M., Argyropoulos, D. S., Jääskeläinen, A.-S., and Vuorinen, T., Development of the partial least squares models for the interpretation of the UV resonance Raman spectra of lignin model compounds. Vibrational Spectroscopy, accepted for publication.
  • Jääskeläinen, A.-S., Saariaho, A.-M., and Vuorinen, T., Quantification of lignin and hexenuronic acid in bleached hardwood kraft pulps: a new calibration method for UVRR spectroscopy and evaluation of the conventional methods. Journal of Wood Chemistry and Technology, accepted for publication.
  • Potthast, A., Rosenau, T., Kosma, P., Saariaho, A.-M., Vuorinen, T., and Sixta, H., On the nature of carbonyl groups in cellulosic pulps. Cellulose, accepted for publication.
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