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LEARNING AND DEVELOPMENT


‘The relative centrifugal field generated by a rotor depends on the speed of the rotor in revolutions per minute and the radius of rotation’


refrigeration systems to overcome the heat generated by the friction of the spinning rotor, and the higher-speed machines must incorporate vacuum systems. High-speed centrifuges are used in the separation of a number of cell constituents and in the isolation and purification of viruses.


Ultracentrifuges Ultracentrifuges are capable of speeds in excess of 30,000 rpm and RCFs of over 600,000 xg. They can be used in the isolation and purification of membrane components such as the endoplasmic reticulum and Golgi membrane, endosomes, ribosomes, DNA and RNA. Once again, refrigeration and vacuum systems are necessary.


Instrument components Rotor The design of most centrifuges allows the drive system to accept rotors of different sizes and capacities, although most instrument rotors are now capable of accepting a large range of tube sizes through the use of adaptors. Rotors have three basic designs: horizontal, in which the tubes are carried in buckets that can swing outwards to a horizontal position and can operate at speeds to about 3000 rpm; fixed angle, in which the sample tubes are held at a fixed angle to the vertical position and can attain much higher speeds (approximately 7000 rpm) because of the aerodynamic construction of the rotor; and vertical, in which the tubes are fixed in the vertical position. In general, the horizontal rotor offers advantages to the clinical laboratory because sedimentation of large particles (eg red blood cells) is efficient at low force and because a flat sediment is produced. The load on the rotor should always be


balanced before operating the centrifuge, particularly when using high-speed instruments in which the buckets and caps are often numbered so that they can be matched on opposite sides of the rotor. The load must be balanced both by


equal mass and by centres of gravity across the centre of rotation. Thus, it is important not to run the centrifuge with buckets, carriers or shields missing from the unit, and not to exceed the maximum rated speed of the rotor in use. Most modern rotors have microprocessor-controlled automatic rotor identification so that it is impossible to set the speed beyond the safety limit for that rotor.


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The centrifuge is ubiquitous in biomedical laboratories and a basic knowledge of the theory of centrifugation is very useful when separating serum or plasma, precipitated solids or liquids of varying density.


Motor In general, centrifuge motors are high- torque, series-wound DC motors, the rotation of which increases as the voltage is increased. The rotor shaft is driven directly or through a gyro, although occasionally a pulley system is used. Electrical contact to the commutator is provided by graphite brushes, which gradually wear down as they press against the commutator turning at high speed, and thus should be replaced at specified intervals. Modern centrifuges have induction drive motors that have no brushes to change. The shaft of the motor turns through sleeve bearings located at the top and bottom of the motor. Most instruments contain sealed bearings that are permanently lubricated, while others require periodic application of oil or grease. The speed of the centrifuge is controlled


by a potentiometer that raises and lowers the voltage supplied to the motor. The calibrations on the speed control are often only relative voltage increments and should never be taken as accurate indicators of speed. Therefore, periodic recalibration is required.


Imbalance detector Some instruments have an internal imbalance detector that monitors the rotor during operation, causing automatic


shutdown if rotor loads are severely out of balance.


Tachometer A tachometer indicates the speed in rpm. Most modern centrifuges use electronic tachometers, in which a magnet rotates around a coil to produce a current that can be measured.


Safety lid Modern centrifuges must have a door- locking mechanism to prevent the lid from being opened while the instrument is running. If there is a power failure or the safety latch fails for some reason it may be necessary to trip the door-locking mechanism manually to retrieve the samples. Manufacturers’ instructions should be checked for the exact procedure required.


Refrigerator A centrifuge generates heat as it rotates and if samples are temperature labile then a refrigerated centrifuge should be used. Some centrifuges enable the rotor and chamber to be precooled before a run.


Braking system Braking devices are incorporated to provide rapid rotor deceleration. Modern


‘Centrifugation is a technique designed to utilise centrifugal forces, which are greater than the force of gravity, to speed up the sedimentation rate of particles’


MAY 2016 THE BIOMEDICAL SCIENTIST


National Cancer Institute/Diane A Reid


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