Lignin is nature's second most abundant polymer after cellulose. However, many uncertainties still remain about the structure, formation, and reactions of lignin as well as its distribution in plant cells. The analysis of the lignin structure is challenging, and the isolation of lignin from biomass introduces changes, regardless of the isolation method applied. In order to be able to utilize lignin efficiently and sustainably, potent analytical techniques are needed. Raman spectroscopy is a nondestructive technique that is applicable to characterize wood biomass in situ. The objective of this research was to utilize Raman spectroscopy in wood biomass research, especially in the analysis of the structure and reactions of lignin. To obtain a more detailed understanding of applicability of Raman spectroscopy in bleaching studies, unbleached and bleached kraft pulps were analyzed. Furthermore, chemical changes in thermomechanical pulp (TMP) during enzymatic treatment were monitored. UV resonance Raman (UVRR) spectroscopy was shown to be a valuable technique in lignin analysis in situ, especially for pulp samples with a low lignin content. Monitoring small changes in TMP samples with a high lignin content using Raman spectroscopy proved to be challenging, as the resonance enhancement of the aromatic band became very strong. To develop the quantitative Raman spectroscopic analysis method for phenolic hydroxyl group determination, the effect of pH on lignin analysis was investigated using lignin model compounds and wood pulps. UVRR spectroscopy was shown to be equally applicable to samples with a high lignin content and those with a very low lignin content. Raman spectroscopy of lignin-containing samples may produce laser induced fluorescence which overlaps with Raman bands. The fluorescence properties of the lignin model compounds were studied. Lignin model compounds containing a biphenyl structure exhibited strong laser induced fluorescence with visible excitation in Raman spectroscopy. Wood did not emit laser induced fluorescence, but when it was chemically treated under alkaline conditions, considerable amount of fluorescence was detected in the Raman spectrum. The effect of excitation wavelength on the Raman spectra of wood-based samples was investigated. Several lasers were used to obtain excitation ranging from ultraviolet to near infrared radiation to see how the different laser frequencies affected the Raman spectra of wood biomass samples. The intensities of the characteristic vibration bands of different structural features in the Raman spectra of wood-based samples were shown to depend on the excitation wavelength.
|Translated title of the contribution||Puubiomassan karakterisointi ramanspektroskopialla|
|Publication status||Published - 2013|
|MoE publication type||G5 Doctoral dissertation (article)|
- wood biomass
- Raman spectroscopy