TABLE OF CONTENTS AUC equipment is well known and does not need to be described here. For the design of the Optima XL-A AUC, as well as the UV Vis absorption optical system, the reader is referred to ref. [14]. The principal design of the online Rayleigh interferometer of the Optima XL-I AUC can be found in ref. [15]. Besides these optical systems in modern AUCs, there is also the Schlieren optical system, which detects the refractive index/concentration gradient and belongs to the classical AUC detection systems, but is not used commercially anymore. It delivers the first derivative of the radial concentration gradient and is thus well suited to observe sedimenting boundaries during sedimentation velocity experiments. Probably, the typical Schlieren peak is the most well-known experimental output of an analytical ultracentrifuge. All three optical systems have their special advantages. The UV Vis absorption optics combines sensitivity with selectivity owing to its variable detection wavelength, whereas the Rayleigh interference optics yields very accurate experimental data owing to the acquisition of a number of interference fringes, which are then evaluated via a fast Fourier transformation. However, the interference optics can only determine relative concentration changes with respect to a fixed point, which is usually the air/solution meniscus. The Schlieren optics is well suited for high-concentration or density gradient work and all kinds of experiments that require a derivative of the concentration gradient for evaluation ( e.g. the determination of the z-average molar mass from sedimentation equilibrium).
