Centrifugation

Centrifugation is a technique that uses centrifugal force to separate components of a mixture based on density and size. A sample in a liquid is spun at high speed inside a rotor, causing heavier or denser particles to move outward and sediment toward the bottom as a pellet, while lighter components remain in the supernatant. The outcome depends on particle properties, fluid viscosity, rotor geometry, and run time.

Common approaches include differential centrifugation, where increasing speeds progressively pellet larger particles first (for example, cell debris, then organelles, then smaller particles). Density gradient centrifugation uses a medium with varying density to separate components by buoyant density, producing sharper bands or distinct pellets. Ultracentrifugation achieves very high centrifugal forces and is used for subcellular fractionation, virus purification, and macromolecule separation.

Equipment and performance metrics are key. Centrifuges can employ fixed-angle or swinging-bucket rotors. Speed is expressed as relative centrifugal force (RCF) or revolutions per minute (rpm). RCF is approximately 1.118×10^-5 multiplied by the rotor radius in centimeters and by the square of the rpm. Temperature control is often important to preserve sample integrity and prevent heating artifacts. The process yields a pellet at the bottom of the tube and a clarified supernatant above it, though some protocols aim to retain material in suspension.

Applications span biology, medicine, and chemistry. Centrifugation is used to purify organelles, ribosomes, viruses, proteins, nucleic acids, and lipoproteins; to separate blood components in clinical settings; and to characterize sedimentation properties and particle sizes.

The technique was advanced in the early 20th century by Theodor Svedberg, whose work on analytical ultracentrifugation and density-gradient methods earned him the Nobel Prize in Chemistry in 1926.