Determination of molecular mass distribution of κ-carrageenan and xanthan using asymmetrical flow field-flow fractionation

Flow field-flow fractionation combined with multi-angle laser light scattering was used to determine the molar mass distribution of kappa-carrageenan and xanthan gum. K-Carrageenan was studied above and below the order-disorder transition temperature. The molar mass of the helical form was approximately double that of the coil form in agreement with previously observed data obtained by GPC/MALS. The hydrodynamic radius was found to be lower for the ordered state suggesting that FFFF theory is not correct for rod-like polymers. For xanthan gum it was found that measurements had to be performed at a very low polymer concentrations. Above 0.4 mg/ml chromatograms showed a number of peaks, both in the RI and the LS spectra. Below 0.4 mg/ml, however, chromatograms with one well defined peak were obtained. No significant difference in the calculated weight average molar mass was observed between the ordered state (25 degrees C) or if the analysis was performed at the midpoint of the order-disorder transition (50 degrees C), but a shoulder in the differential weight fraction at the high molar mass part was observed for the ordered state. This might indicate the presence of aggregated xanthan gum molecules. (C) 2000 Elsevier Science Ltd. All rights reserved.