FT-Raman spectroscopy for direct measurement of lignin concentrations in kraft pulps

This study used near-infrared (IR) Fourier transform (FT) Raman spectroscopy was used to quantify residual lignin in unbleached and partially bleached pine kraft pulps. The pulps used in this study were bleached using CEDED and polyoxometalate (POM) bleaching sequences. The intensity of lignin's 1600 cm ^-1 Raman band, which represents one of the phenyl ring vibrations, was used as a measure of the total amount of lignin in pulp samples. When the 1600 cm ^-1 band intensities were compared with pulp kappa numbers, the band intensities were found to be linearly related to the kappa numbers. This linear relationship holds for both CEDED- and POM-bleached pulps, indicating that the 1600 cm ^-1 band intensity is mostly independent of bleaching chemistry and is dependent upon the number of phenyl groups in pulps. Consequently, FT-Raman spectroscopy has the potential for determining lignin concentrations in unbleached and partially bleached kraft pulps. The Raman technique is attractive because it is rapid, is not labor intensive, and does not result in the destruction of pulp samples. Application: A new method for measuring kappa numbers, near-IR FT-Raman spectroscopy is capable of determining lignin concentration in pulps.