Lab & analytical techniques · Reference
What is centrifugation?
Centrifugation separates the components of a mixture by spinning it at high speed, so that denser particles are driven outward to form a pellet while lighter material stays in the supernatant above.
The physics of the separation
When a sample is spun in a centrifuge, each particle experiences a centrifugal effect that pushes it away from the axis of rotation. The magnitude of this effect, expressed as relative centrifugal force (a multiple of gravity, g), grows with the square of the rotation speed and with the radius. Denser particles, and larger ones, move outward faster than the surrounding fluid resists, so they accumulate at the bottom of the tube as a pellet. The clarified liquid above is the supernatant. By choosing the speed and time, researchers control which components sediment and which stay suspended.
Differential and density-gradient methods
In differential centrifugation, a sample is spun in steps of increasing speed; at each step the heaviest remaining particles pellet and are removed, progressively fractionating a mixture — a routine way to separate cellular components.
In density-gradient centrifugation, the tube contains a medium whose density increases with depth, and particles settle until they reach the layer matching their own density, giving fine separation. Ultracentrifugation reaches extremely high speeds, generating forces large enough to sediment proteins and other macromolecules.
Uses in research
Centrifugation is a workhorse of biochemistry, cell biology, and clinical laboratories, used to harvest cells, clarify samples, and isolate organelles, proteins, and nucleic acids. It is often a preparation step ahead of analytical methods such as gel electrophoresis or HPLC. Reproducible separations depend on reporting the relative centrifugal force, the rotor, and the time, rather than only the rotation speed, so that conditions can be matched between laboratories.
Key facts
At a glance
- Separates: components by density using centrifugal force
- Force expressed as: relative centrifugal force (multiples of g)
- Pellet: dense material collected at the tube bottom
- Supernatant: the lighter liquid remaining above
- Differential method: stepwise increasing speeds
- Ultracentrifugation: very high speeds for macromolecules
Common questions
FAQ
What is the difference between a pellet and a supernatant?+
The pellet is the dense material that collects at the bottom of the tube after spinning, while the supernatant is the lighter liquid left above it. Separating these two fractions is the basic outcome of a centrifugation step.
What is ultracentrifugation used for?+
Ultracentrifugation spins samples at extremely high speeds, generating forces strong enough to sediment very small particles such as proteins, ribosomes, and viruses. It is used to isolate macromolecules and to study their size and density.
The step most authors miss
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