Centrifuges: how they work, rotor types and rpm vs RCF

A centrifuge spins samples at high speed to separate their contents. The spinning pushes denser or larger particles outwards, to the bottom of the tube, much faster than gravity alone would. Labs use centrifuges to pellet cells and precipitates, clear solutions of undissolved particles, and collect small amounts of liquid or powder at the bottom of a tube. This guide explains how centrifugation works, why speed is better given as RCF (× g) than rpm, the main types of centrifuge and rotor, and how to use one safely.

To convert between rpm and RCF for your rotor, use our rpm to RCF calculator. Terms are defined in the glossary.

How centrifugation works

Leave a cloudy mixture to stand and the particles slowly settle under gravity. A centrifuge speeds this up enormously. As the rotor spins, everything in the tube is pushed outwards. Particles that are denser than the liquid around them move towards the bottom of the tube and collect there as a pellet. The clear liquid left above is the supernatant.

How fast a particle settles depends on:

• Its size. Larger particles settle much faster. Under Stokes' law, the speed rises with the square of the particle's radius.
• The density difference between the particle and the liquid.
• The force applied, which is the RCF.
• The liquid's viscosity. Thick liquids slow settling down.

That is why a quick spin is enough to pellet cells or a precipitate, while separating large molecules from each other needs an ultracentrifuge running for hours.

rpm vs RCF

rpm (revolutions per minute) is how fast the rotor turns. RCF, written as × g, is the force the sample actually feels. RCF depends on both speed and distance from the centre:

RCF = 1.118 × 10⁻⁵ × r × rpm²

Because r changes from rotor to rotor, 10,000 rpm in a small rotor gives a much lower force than 10,000 rpm in a large one. A method that says "spin at 10,000 × g" can be followed on any suitable centrifuge. A method that says "spin at 10,000 rpm" only works on the same rotor. Our rpm to RCF calculator does the conversion and explains which radius to use.

Because RCF rises with the square of rpm, doubling the speed gives four times the force.

Types of centrifuge

Many centrifuges are refrigerated, which keeps samples cool during long or high-speed runs, when friction would otherwise warm them.

Rotor types

• Fixed-angle rotors hold tubes at a set angle, often 25–45°. Particles only travel a short distance before hitting the tube wall and sliding down, so pelleting is fast. The pellet forms partly up the side of the tube.
• Swing-out (swinging-bucket) rotors hold tubes in buckets that swing out to horizontal as the rotor spins. The pellet forms neatly at the bottom of the tube. They usually run at lower forces and take longer.

Rotor manuals sometimes give a k-factor, a measure of how quickly a rotor pellets particles at full speed. A lower k-factor means faster pelleting. It is mainly used to compare rotors and to plan ultracentrifuge runs.

Using a centrifuge safely

A rotor spinning at speed stores a lot of energy. An unbalanced load or a failed tube can damage the centrifuge and anything nearby. University safety procedures agree on the basics (University of Tennessee; UBC):

1. Balance the load. Place tubes of equal mass opposite each other. With an odd number of tubes, add a blank tube filled with water to the same mass. Matching volumes by eye isn't enough for high-speed runs; weigh them.
2. Check tubes and rotor for cracks, and only use tubes rated for the speed.
3. Close and lock the lid, and make sure the rotor lid or bucket caps are secured.
4. Don't exceed the rotor's maximum speed, which is marked on the rotor.
5. Stay until it reaches speed. If it shakes or sounds wrong, stop it.
6. Wait for the rotor to stop completely before opening the lid.
7. Use sealed buckets or an aerosol-tight rotor for hazardous material, and open them in a suitable cabinet (Santa Clara University).
8. Clean up spills promptly, following your lab's procedures and the substance's safety data sheet. See COSHH and safety data sheets.

When a quick spin helps

• Collecting freeze-dried powder. Powder in a vial often sticks to the walls and the stopper during shipping. A brief, gentle spin brings it to the bottom before the vial is opened, so less is lost. Use an adaptor that holds the vial securely. See why lyophilised vials look nearly empty.
• Collecting droplets. A short pulse in a microcentrifuge brings liquid on the cap and walls of a small tube down to the bottom.
• Clearing a solution. Spinning a solution and taking the supernatant removes undissolved particles, which would otherwise scatter light and give a falsely high reading on a spectrophotometer. See measuring peptide concentration and peptide solubility.

Record the RCF, time and temperature of each spin, so the step can be repeated exactly.

Related guides

For measuring liquids accurately, see pipettes and volumetric glassware. For working without contaminating samples, see aseptic technique and autoclaving. Every lot has its own COA, published on the product page and on that lot's own page. See Quality.