Mechanical Pulps, Ultraviolet/Visible Spectroscopy of Chromophores in
Pulp and Paper
Published Online: 15 SEP 2006
Copyright © 2000 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Analytical Chemistry
How to Cite
Schmidt, J. A. 2006. Mechanical Pulps, Ultraviolet/Visible Spectroscopy of Chromophores in. Encyclopedia of Analytical Chemistry. .
- Published Online: 15 SEP 2006
Mechanical pulps are those where the starting wood is rendered into a suspension of fibers by the application of mechanical energy. Most of the lignin component is retained in the fibers by this process. The yellowish coloration of wood fiber is a consequence of the presence of chromophores in the lignin that absorb both visible and ultraviolet (UV) light. In most cases, the fibers must be bleached before they are acceptable for their final use. Lignin-containing fibers are also susceptible to yellowing when exposed to UV light. To study the chemistry of these processes in pulp fibers, a technique capable of detecting chromophores in materials that both absorb and scatter radiation is needed. Diffuse reflectance ultraviolet/visible (UV/VIS) spectroscopy is such a technique. For nonscattering materials, the Beer–Lambert law provides a simple linear relationship between absorbance and the concentration of absorbers. This has been used for the analysis of soluble lignins and pulping liquors. For scattering materials, there is no similar simple linear relationship between reflectance and concentration of absorbers. A single reflectance measurement from an opaque sample can be related to the ratio of absorption to scattering by the Kubelka–Munk remission function, F(R∞) = k/s = (1 − R∞)2/2 R∞, where R∞ is the reflectance of an opaque sample, k is the specific absorption coefficient and s is the specific scattering coefficient. The specific absorption coefficient k is linearly related to absorber concentration. If the scattering coefficient remains constant, the remission function is proportional to chromophore concentration. However, for many samples of interest, such as pulps prepared by different mechanical processes, scattering is not constant. Here, it would be uncertain whether a difference in remission function should be attributed to a change in absorption or a change in scattering. Also, in samples where the chromophores are not homogeneously distributed throughout the sample thickness, such as sheets yellowed by light, the linearity of the remission function no longer holds. In such cases, k can be calculated explicitly by taking additional reflectance measurements from thin transmitting samples that are suspended over backgrounds of known reflectance. While spectra of k versus λ (wavelength) are usually broad and featureless, difference spectra can reveal a surprising amount of detail about the changes in chromophore content.