A study on Schlieren patterns derived with the Beckman Optima XL-A UV-absorption optics

Abstract

This study describes how typical “Toepler” Schlieren patterns can be derived with the UV-absorption optics of the Beckman Optima XL-A ultracentrifuge without any modification of the optical system. Such a Schlieren effect can greatly enhance the application range of the XL-A ultracentrifuge which cannot yet be used to its possible full potential due to the present lack of a refractometric optical system and — not only for sedimentation velocity experiments — particularly the Schlieren optical system. It is shown that the Schlieren patterns detected with the UV-absorption optical system are caused by light refraction and not by light scattering or experimental artefacts. Two possible explanations for the generation of the Schlieren effect are given. Both explanations have in common that the refracted light cannot be detected by the photomultiplier system and is hence lost for detection. The intensity of the refracted light which can be registered with the photodetector of the optical system is dependent on the angle of incidence of the light, e.g., the local slope of the sedimenting boundary, and hence leads to a detection of the refractive index change with radial position: i.e., a Schlieren pattern. The advantages, disadvantages and differences of the XL-A Schlieren effect compared to conventional “Philpot-Svensson” Schlieren optics are discussed demonstrating that both optical systems deliver the same results for examples of sedimentation velocity experiments on several polysaccharides and bovine serum albumin. An estimation of the sensitivity of the Schlieren effect as a function of the wavelength selected is given as well as suggestions of how the Schlieren effect can be increased for suppressed. Considerations about the quantitative evaluation of XL-A Schlieren patterns show that the Schlieren effect is dynamic with respect to the concentration dependence. This creates some potential for new applications but is also a serious warning to those people who want to derive quantitative information from XL-A absorption traces as these can be superimposed by a Schlieren effect at certain wavelengths.