Estimation of S/G ratio in woods using 1064 nm FT-Raman spectroscopy

Abstract

Two simple methods based on the 370 cm^-1 Raman band intensity were developed for estimation of syringyl-to-guaiacyl (S/G) ratio in woods. The methods, in principle, are representative of the whole cell wall lignin and not just the portion of lignin that gets cleaved to release monomers, for example, during certain S/G chemical analyses. As such, it is like the whole-cell-wall NMR methods. The Raman analysis is quick, free of the use of harmful chemicals, carried out nondestructively, and is insensitive to the wet or dry state of the sample. The only limitation is that a wood sample should not be significantly fluorescent, although this can be rectified in some cases. The reliability of the Raman approach was first tested by the quantitative analysis of several syringyl lignin models by sampling them, separately, in dioxane and in avicel. Good linear correlations between 370 cm^-1 band intensity and model concentrations were obtained. Subsequently, based on 370 cm^-1 intensity, S contents of woods were determined. To test the accuracy of the Raman methods, the obtained syringyl contents in woods were calibrated against the S/G values generated by thioacidolysis, DFRC, and 2D-HSQC NMR methods. The former two are S/G methods that take into account only the monomers cleaved from ß-O-4linked lignin units whereas NMR reports S/G ratio on the whole cell wall lignin.

Citation

Agarwal, Umesh P.; Ralph, Sally A.; Padmakshan, Dharshana; Liu, Sarah; Karlen, Steven D.; Foster, Cliff; Ralph, John. 2015. Estimation of S/G ratio in woods using 1064 nm FT-Raman spectroscopy. Proceedings of the 18th ISWFPC (International Symposium on Wood, Fiber, and Pulping Chemistry) held in Vienna (Sept 9 -11, 2015). 2015; pp. 333-336.