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 resi­ dues in softwood lignin. Raman contributions at 1133 and 1654 cm-1, representing coniferaldehyde and coni­ feryl alcohol structures, respectively, were used in quan­ tifying these units in spruce wood with subsequent conversion to concentrations in lignin. For coniferalde­ hyde units, the intensity of the 1133 cm-1 peak was measured in the difference spectrum obtained by sub­ tracting 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 concentra­ tions of coniferaldehyde and coniferyl alcohol units in spruce lignin were found to be 3.8% and 3.4%, respec­ tively, and were in good agreement with values deter­ mined by conventional techniques. This quantification of the ethylenic residues was based on the Raman inten­ sities 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 concentra­ tions of coniferaldehyde and coniferyl alcohol residues in comparison to wood by 28% and 24%, respectively.