The installation of an eight-cell Schlieren optics multiplexer in a Beckman Optima XLI/XL analytical ultracentrifuge used to measure steep refractive index gradients

Because the Beckman Company was unable to deliver any further drives for our Model E analytical ultracentrifuge (AUC), we were forced to install a modified Model E Schlieren optics in a Beckman Optima XL in order to transform it from a preparative ultracentrifuge into an analytical one. We need to have Schlieren optics available, because Schlieren optics are more universal as interference optics (especially for complex samples). It is only possible to resolve very steep refractive index gradients in the AUC cell and to see the different colours of samples of mixed coloured with white-light Schlieren optics. The configuration of the Schlieren optics is described in detail in the first part of this paper. We used an analytical 8-cell XLI-Ti rotor, a flash lamp as the light source (which can be moved by remote control on a sliding carriage) and a 70 mm reflex film camera instead of the Model E photographic plate. To trigger the flash lamp, we installed a photoelectric cell under the rotor and a small line mirror at the base of the rotor. An electrical circuit (completely independent of the Beckman XL circuits) controlled by a 10 MHz clock allows us to flash any of the eight cells independently and to superimpose any combinations of cells. The modification of the XL itself was only a minor one. We merely drilled holes for the optical path into the heat sink and installed the condensing lens in the rotor chamber. The light source, collimating lens, phase plate, camera lens, cylindrical lens and photographic camera are situated outside the chamber. This modified XL is completely equivalent to the old Model E with Schlieren optics, a flash lamp and an 8-cell multiplexer. Some practical examples of measurements made with this system are described in the second part of this paper. These include steep density gradients in a chemically heterogeneous 11-latex mixture, a rapid steep dynamical H2O/D2O density gradient run, synthetic boundary runs of complex coloured core/shell nano-particles and flash superimposition of different cells during a sedimentation run.