Multiangle light scattering (MALS) is a well-established technique used to determine the size of macromolecules and particles. In this study, different extrapolation procedures used in MALS were investigated with regard to accuracy and robustness in the obtained molar mass and rms radius. Three different mathematical transformations of the light scattering function referred to as the Debye, Zimm, and Berry methods for constructing the Debye plot were investigated for two idealized polymer shapes, homogeneous spheres and random coils, with radii from 25 to 250 nm. The effect of the angular interval used for the extrapolation was investigated, as was the robustness of the different transformations toward errors in the measured light scattering intensity at low angles. For an rms radius less than 50 nm, the relative error in molar mass due to extrapolation was less than 1% independent of the method used. For larger radii, the error increased and the extrapolation procedure became more critical. For random coil polymers, the Berry method was superior in terms of accuracy and robustness. For spheres, the Debye method was superior. The Zimm method was inferior to the others. The different extrapolation methods were evaluated and compared on experimental data from a size exclusion chromatography-MALS analysis of an ultrahigh molar mass poly(ethylene oxide) (PEO). The PEO data qualitatively verified the calculations and stressed the importance of optimizing the extrapolation procedure after careful evaluation of the experimental data. A discussion of how to detect erroneous data in an experimental Debye plot is given.

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A new trapezoidal geometry of the asymmetrical flow fieldflow fractionation (FFF) channel Introduces an additional means to regulate the longitudinal flow velocity. The trapezoidal geometry, where the breadth decreases on going toward the channel outlet, makes It possible to level out the steep linear velocity gradients that can appear In the rectangular channels. Equations for the channel flow velocity gradient and the void time are derived. In addition, an equation for the nonequilibrium plate height Is derived for asymmetrical channels. The equation suggests that the same efficiencies should be obtained In asymmetrical as In the original symmetrical channels under similar flow conditions. The relative peak dilution occurring In channels of different designs Is evaluated. The results favor the trapezoidal versus the rectangular channel and asymmetrical versus symmetrical channels. The plate numbers calculated from the derived equations were compared with those found experimentally. The latter were found to deviate from the calculated numbers at high retention levels. Approximately 2600 plates have been reached.

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