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

Application: A new method for measuring kappa numbers, near-IR FT-Raman spectroscopy is capable of determining lignin concentration in pulps.

In kraft pulps, lignin concentration is usually derived by determining the kappa number. Fourier transform (FT) Roman spectroscopy can directly measure phenyl groups in pulp lignin, serving as an alternative to the standard permanganate consumption measurement technique and may help to determine residual-lignin concentration in pulps.

For this study, we obtained Roman spectra of unbleached and bleached kraft pulps using near-infrared laser excitation (1064 nm). Pulps were bleached using (CEDED) or a second sequence (EVVVE) based on a specific polyoxometalate (V). Roman intensities of the lignin band provided a good fit with the kappa numbers of pulps. We analyzed multiple samples from each bleaching stage and found reliable measurements of lignin concentration using FT-Raman spectroscopy.

This method is nondestructive, rapid (15 min/sample), and easy to perform. Further work on analyzing additional kraft pulps bleached with different bleaching agents should show the versatility of the near-IR FT-Roman method.

Agarwal, Weinstock, and Atalla are scientists at the USDA Forest Service, Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI 53726-2398; email Agarwal at [e-mail].