Centrifugation is used for preparative and analytical separation of molecules and organelles (see ); centrifugation is one of the techniques used to determine the molecular mass from first principle, rather than by comparison with standard proteins. The other two are osmometry (see Chap. 26 on p. 245) and laser light scattering (see Sect. 29.1 on p. 255). For a recent review on modern centrifugation techniques see . Centrifuges can be categorised by their running speed: clinical centrifuges allow velocities of a few thousand rounds per minute, high speed centrifuges up to 25000rpm. Centrifuges which allow even higher speeds are called ultracentrifuges. An analytical ultracentrifuge has the facility to observe the sample during the run. In a preparative ultracentrifuge, analysis of the sample is possible only outside the centrifuge after the run is completed, but rotor capacities can be much larger. Observation of the sample during the run in an analytical ultracentrifuge can be done with a photometer (detection limit several μM), fluorimeter (detection limit for fluorescein 5nM) or by interferometry with a Schlieren-optics (see Fig. 8.2 on p. 62). Interferometry has the advantage that it can be used even in cases where the sample has no suitable absorption lines in the UVIS-range (e.g., polysaccharides). On the other hand it is possible to distinguish different sample species by their absorption spectra.