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Tools of Biologist

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Introduction Centrifugation is a process used to separate or concentrate materials suspended in a liquid medium. The theoretical basis of this technique is the effect of gravity on particles (including macromolecules) in suspension. Two particles of different masses will settle in a tube at different rates in response to gravity. Centrifugal force (measured as xg, times gravity) is used to increase this settling rate in an instrument called a centrifuge. Centrifuges are devices used in a variety of applications which spin carrier vessels (centrifuge tubes) at high centrifugal force. The centrifugal force generated is proportional to the rotation rate of the rotor (in revolutions per minute, rpm) and the distance between the rotor centre and the centrifuge tube. Therefore, a given centrifuge may use multiple rotor sizes to give flexibility in choosing centrifugation conditions. Each centrifuge has a special graph, a nomograph, or a table which relates rotation rate (rpm) to centrifugal force (xg) for each size of rotor it accepts. Centrifuges come in three general classes: low speed, up to about 5000 rpm, high speed machines of up to about 25,000 rpm and ultracentrifuges which will turn at up to 100,000 rpm. Another form of centrifuge very common in laboratories is the micro centrifuge. These are simple machines used with 0.5 or 1.5 mL disposable plastic tubes. Most of these machines generate between 10,000 and 13,000 rpm. Certain procedures necessitate precise centrifugation conditions, which must be specified in terms of relative centrifugal force (rcf) expressed in units of gravity (times gravity or × g). Many micro centrifuges only have settings for speed (rpm), not relative centrifugal force. Consequently, a formula for conversion is required to ensure that the appropriate setting is used in an experiment.

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