Laboratory centrifuges spin liquid samples at high speed, in order to separate substances according to their mass. They typically consist of a vertical motorized axis about which a rotor can spin — and a top loading well with a cover over it. The rotor is typically designed to securely hold special centrifuge tubes that fit tightly into the rotor and use a high strength plastic or glass. Smaller centrifuges are bench-top units, while larger centrifuges are floor standing. Rotational speeds range from about 4,000 rpm for low speed clinical centrifuges to around 20,000 rpm for micro and highspeed centrifuges, while modern ultracentrifuges rotate at speeds above 40,000 rpm.
Typically, fixed angle rotors hold sample tubes at a fixed angle, with a simple block of material containing holes into which the tubes are inserted. Swinging bucket rotors contain hinges for each tube, so that the tubes are orientated along the line of acceleration — pointing downwards at rest where the only acceleration is due to gravity, and moving progressively towards the horizontal at centripetal acceleration becomes greater than gravity. Zonal rotors have a single central area to hold samples.
It is very important that centrifuges are correctly balanced to prevent excessive vibration and potentially catastrophic failure. The kinetic energy stored by a centrifuge when operating at high speed can be very high and sudden release of this energy can be equivalent to an explosive device. Balance is achieved by combining sample tubes with balance tubes. Corrosion and cracking in the rotor can also result in the rotor bursting under the massive centrifugal forces, so inspection and maintenance are important.
In biochemistry and cell biology, differential centrifugation is often used to separate organelles or viruses. The principle is that the rate at which suspended particles settle depends on gravitational force, the difference in density between the particle and the fluid, the viscosity of the fluid, and the size and shape of the particle. If the particle is more dense than the fluid, it will settle … and if it is less dense, it will float. Larger particles settle more quickly and at lower forces. Buoyant density centrifugation more precisely separates particles according to their density. It can be used to separate molecules, molecular isotopes and DNA sequences.
