The pressure transducer in the membrane osmometer (Knauer, Model 01.00) failed and the instrument was serviced twice. However, even after repair, obtaining a stable, repeatable baseline has proved impossible with this instrument. As a result, we recently obtained another membrane osmometer (Gonotec, Model 090B) for evaluation. A 104 cm calibration rod, marked in 1 cm divisions, was fabricated and is currently being used to calibrate this osmometer. Until an acceptable membrane osmometer is obtained, the molecular weight study of the colloidal plasma volume expanders can not be continued. Thus far, number average molecular weights have been measured for 14 lots of dextran 70 (4 from each manufacturer). We have started a systematic study of hetastarch (high molecular weight hydroxyethyl starch) and have carried out measurements on 5 lots from the single manufacturer. In addition, 2 lots of dextran 40 from 2 manufacturers have been investigated. We plan to continue all these studies in order to obtain the number average molecular weight for each lot. Light scattering experiments will be continued when the osmotic pressure measurements resume in order to obtain complementary weight average molecular weights. The ratio of these molecular weights provides a measure of the polydisperity of these products thereby allowing further characterization of these heterogenous materials.
