Measurements of the Refractive Index Increment on Hydroxyethyl Starch as a Basis for Absolute Molecular Weight Determinations

Solutions of hydroxyethyl starch are used as a blood plasma substitute. If their physiological efficiency is to be optimized, they need to be accurately characterized in terms of their molecular weight and its distribution. The absolute determination of molecular weight and molecular weight distribution by means of light scattering require a knowledge of the refractive index increment. Although numerous investigations of the refractive index increment of hydroxyethyl starch have already been published, the results vary significantly due to the use of different samples and the choice of different measuring parameters. There was therefore an urgent need to examine the extent to which the refractive index increment depends on molecular parameters, the experimental method used and the type of processing. Here it was found that different sample preparations result in different contents of solid matter, so that an exact determination of the quantity is required. Hydroxyethyl starches in the molecular weight range of about Mw = 200.000 g/mol and varying degrees of substitution between DS 0.38 and 0.50 which are regarded as optimal for clinical use give a refractive index increment of dn/dc^633nm = 0.146 = 0.005 cm³/g (solvent: H₂O/0.02% NaN₃; T = 25°C).