Studies on hydroxyethyl starch. Part I: Molecular characterization by size exclusion chromatography coupled with low-angle laser light scattering.

Two commercially available hydroxyethyl starch (HES) preparations (in clinical use as plasma expanders) specified with Mw = 450,000/MS = 0.7 and Mw = 200,000/MS = 0.5, respectively, and three experimental HES-samples (supposingly similar to the commercial product with the specification 450,000/0.7, except of one with MS = 0.5) were studied. The latter were prepared via acid or enzymatic hydrolysis of waxy-maize starch. Each of the samples was characterized by its intrinsic viscosity and molar substitution, and was studied with low-angle laser light scattering (LALLS) and with size exclusion chromatography (SEC) coupled with LALLS. The weight-average molecular weight Mw of the commercial samples was found to be 60-80% higher than the value given in the product declaration. This discrepancy can be explained by the argument that previous measurements were not carried out at sufficiently small scattering angles to enable reliable extrapolation to zero angle. The calibration functions Mw(v) of the individual HES-samples measured by SEC/LALLS-coupling are identical over a broad range of the elution volume v and are used for calibration of conventional SEC in a subsequent paper. The small, but detectable differences in the Mw(v)-functions indicate interesting differences between these HES-preparations with respect to the effective hydrodynamic density of the branched HES-molecules.