Study of Residual Lignin in Pulp by UV Resonance Raman Spectroscopy

A major problem in the development of new bleaching technologies for pulp is the difficulty of analysing the changes induced by bleaching chemicals in the structures of residual lignin. Separation and isolation of the residual lignin before the analysis may modify the constituents of interest, whereas if the pulp is analysed directly, the responses from cellulose and hemicelluloses will overlap the signal from the residual lignin. In this study, UV resonance Raman spectroscopy, a powerful technique for detecting trace components in complex mixtures, was applied to detect the changes in the residual lignin content after bleaching stages. The resonance Raman technique was found to be highly sensitive and selective for lignin structures allowing the detection of trace amounts of lignin after the final bleaching stages. Furthermore, it enabled rapid and easy determination of hexenuronic acid content. UV resonance Raman spectroscopy would appear to be a technique of great potential for pulping and bleaching research.