Molecular-size determination of xanthan polysaccharide

Hydrodynamic chromatographic separations of xanthan polysaccharide of ultrahigh molecular weight have been obtained by using columns packed with 30-μm, non-porous spheres. From calibration curves of the elution volume versus particle size for spherical, polystyrene latexes, it was found that xanthan is eluted at the same volume as a 0.153-μm diameter sphere. Extremely dilute samples of xanthan (70 p.p.m.) were injected to preclude self-association and aggregate formation. Detection at these low concentrations was accomplished by tagging the xanthan with a fluorescein derivative and using a flow-through fluorometer detector. Flow rates of 1 mL/min yielded run times of ～7 min. Comparison of the accepted molecular conformation of xanthan—a rigid rod-like molecule—with the apparent molecular volume from the spherical-latex calibration indicates that the xanthan molecules are substantially oriented by the flow field in the chromatography column.