Determination of ethylenic residues in wood and TMP of spruce by FT-Raman spectroscopy

A method based on FT-Raman spectroscopy is proposed for determining in situ concentrations of ethylenic residues in softwood lignin. Raman contributions at 1133 and 1654 cm(-1), representing coniferaldehyde and coniferyl alcohol structures, respectively, were used in quantifying these units in spruce wood with subsequent conversion to concentrations in lignin. For coniferaldehyde units, the intensity of the 1133 cm(-1) peak was measured in the difference spectrum obtained by subtracting the bleached-wood spectrum from that of the unbleached. In the case of coniferyl alcohol residues, the intensity of the 1654 cm(-1) band was calculated from the spectrum of extensively bleached wood. The concentrations of coniferaldehyde and coniferyl alcohol units in spruce lignin were found to be 3.8% and 3.4%, respectively, and were in good agreement with values determined by conventional techniques. This quantification of the ethylenic residues was based on the Raman intensities of 1% coniferaldehyde and 1% coniferyl alcohol in bleached softwood kraft pulp. Initially, as background for this work, a number of suitable lignin model compounds and a softwood lignin model polymer (G-DHP) were used to calibrate the Raman method and demonstrate that the Raman technique was well suited for quantification of ethylenic structures. Experimental results demonstrated that thermomechanical pulping reduced the concentrations of coniferaldehyde and coniferyl alcohol residues in comparison to wood by 28% and 24%, respectively.