UV-absorbance of oxidized xylan and monocarboxyl cellulose in alkaline solutions

The absorbance in the 190-500 nm range of oxidized lignin-free xylan isolated from unbleached and ozone-bleached hardwood kraft pulps as well as monocarboxyl cellulose in NaOH solution was examined by UV/VIS-spectroscopy and SEC employing multi-wave UV-detection. The first derivative of UV-spectra has been shown to be more useful to investigate the chromophore composition. The chromophore composition of oxypolysaccharides was as follows: carboxyl and carbonyl groups absorbing at 200-220 and 270-290 nm, respectively, heteroaromatics of furan- and pyron-type absorbing additionally at 230-250 and 290-320 nm as well as conjugated heteroaromatics absorbing above 300 nm with maximums at 350-370 and 430-450 nm due to the charge-transfer complexes. Problems regarding the attribution of absorbance to lignin- and heteroaromatics-derived chromophores are discussed. Conjugated heteroaromatic compounds, non-uniformly distributed over MWD, are supposed to be the source for the gel-fraction and contributors to the colour of oxidized polysaccharides. (C) 2003 Elsevier Ltd. All rights reserved.