Essentially, centrifuges used in laboratory settings or in the industry are devices for separating particles from a heterogeneous mixture. Fundamental to separation is the increase in the effective gravitational force by spinning the sample in the single cavity or the tubes of a rotor as the ‘mobile part’ of a centrifuge. Subjected to the respective centrifugal force thus applied, the individual constituents of a sample will differentially sediment according to their physical properties as well as the density and viscosity of the sample solution.

Ultracentrifuges are optimised for spinning a rotor at very high speeds generating centrifugal forces as high as 1 000 000g. There are two kinds of ultracentrifuges, preparative and analytical ultracentrifuges. Preparative ultracentrifuges are used in biology for the fractionation of fine particulate samples such as tissue homogenates aiming to isolate subcellular organelles, macromolecules, bacteria or viruses. By means of analytical centrifuges, the physico-chemical properties of a sedimenting particle or molecular interactions of macromolecules and their possible subunits, respectively, can be unravelled.

Key Concepts:

• Centrifugation, in particular high-speed (ultra)centrifugation, is a widely used technique to elucidate fundamental processes such as adenosine triphosphate (ATP) synthesis in mitochondria, the synthesis of proteins by ribosomes or the interactions of intracellular multiprotein complexes.
  
• Ultracentrifugation is basically carried out in two ways: preparative and analytical centrifugation.
  
• The former aims to isolate and purify, for example, subcellular organelles or multiprotein complexes; the latter allows to analyse interactions between macromolecules and to unravel physico-chemical properties like mass and size of such particles.
  
• Preparative centrifugation performed as batch-type (conventional) centrifugation is mostly used to separate and enrich organelles out of complex biological mixtures.
  
• The alternative, continuous-flow centrifugation, is particularly useful for the large-scale collection of particles out of a diluted solution as it combines high centrifugal force with high throughput.
  
• Theoretically, two kinds of preparative centrifugation have to be distinguished: differential centrifugation and density gradient centrifugation.
  
• Differential centrifugation fractionates organelles of a tissue homogenate according to their size and shape, yet leads only to an enriched rather than a highly purified preparation of a particular organelle.
  
• To get a preparation genuinely purified, common contaminants have to be subsequently removed by density gradient centrifugation.
  
• Rate zonal and isopycnic density gradient centrifugations differ in their basic concepts and the types of density gradients used.