Working with multidetection in the analytical ultracentrifuge: the benefits of the combination of a refractive index detector and an absorption detector for the analysis of colloidal systems

Abstract

The XL-I is an analytical ultracentrifuge equipped with two detection systems: an absorption detector and an interference optical system. For the analysis of colloidal systems the absorption detector is normally denoted as a special detector, since, if Mie scattering is not taken into account, most polymers are transparent to UV and visible light. The interference optics is a detector for measuring the refractive index (RI) difference and is denoted in this context as a universal detector. Most systems in colloidal science of synthetic colloids can only be analyzed with the refractive index detector. If a colloidal (polymeric) system contains components detectable with an absorption detector, the combination of the two detectors of the XL-I will provide additional information about the sample. This paper will show that the specific combination of the detectors described above can not only give a deep insight into the colloidal structure of the sample, but can also help to understand the mechanism of the polymerization reaction applied to generate the dispersion under investigation. This can be achieved by thorough sedimentation velocity analysis in both an aqueous and an organic medium. Spectra before and after the runs yield information about the general whereabouts and the spatial distribution of the absorbing component. Density gradient analysis with both detectors can determine the chemical differences between particles containing or not containing the absorbing component.