POLYDISPERSITY STUDY OF NARROW POLYSTYRENE STANDARDS USING LIGHT-SCATTERING

Manufacturers of narrow polystyrene standards for use as column calibr ants with size exclusion chromatography (SEC) often label the correspo nding polydispersity (M-w/M-n) as < similar to 1.05. These values appe ar to reflect more the errors associated with the measurement than the true narrow polydispersity. ideally, we would measure polydispersity by a light scattering measurement of M-w and a membrane osmometry meas urement of M-w. The ratio of such measurements would be expected to ha ve an error on the order of 5%- a number comparable to the difference of the measured polydispersity and unit! Multi-angle light scattering (MALS) allows the absolute determination of the rms radius independent of the separation technique and any other detector. Using this inform ation and following the method of Shortt,(1) column band broadening ef fects may be corrected and a far more precise determination of polydis persity made. A variety of standards from several manufacturers were m easured, and the polydispersity of each was determined to be far small er than stated, further confirming their suitability for column calibr ation purposes.