Analytical ultracentrifugation with the new Optima XL-A and its digital u.v./vis. detector: determination of molar mass distribution of polymers from sedimentation velocity

The determination of average molar masses and molar mass distributions (MMDs) of polymers and polyelectrolytes is demonstrated with the new analytical ultracentrifuge Optima XL-A and its very sensitive digital u.v./vis. detector (180–800 nm continuously) from Beckman Instruments, Inc. The machine was tested with the narrow distribution polystyrenes PS 106 000 and 300 000 (and a 50:50 mixture), sodium poly(styrene sulfonate) NaPSS 100 000 and the broad distribution polystyrene NBS 706. Full details of the calculation of average molar masses and MMDs of the polymers from the experimental values are given. The results show that the new absorption optics enable the above-mentioned quantities to be determined as the absorption optics are able to measure at very low concentrations, which allows either a precise extrapolation to zero concentration or averaging of the data. Owing to the fact that a sedimentation run is a real fractionation process (with a higher resolution than size exclusion chromatography) one measures the real MMD. There is no need to presume a special model MMD to analyse the measurement data. The data from the Optima XL-A are accessible as ASCII-files. As the original Beckman computer programs are not suitable for our purposes, we have developed own programs, which are available on request.