Biochemical Techniques and Instrumentation

4.2.1.1. Preparative ultracentrifuge

Preparative ultracentrifugation is operated at relative centrifugal fields of up to 9,00,000g. They are available with a wide variety of rotors such as swinging bucket rotor, fixed angle rotor and zonal rotor. Zonal rotors are designed to contain a large volume of sample in a single central cavity rather than in tubes. Some zonal rotors are capable of dynamic loading and unloading of samples while the rotor is spinning at high speed.

The rotor chamber is sealed, evacuated and refrigerated in order to minimize the excessive rotor temperatures generated by frictional resistance between the spinning rotor and the air. Heavy armour plating encapsulates the ultracentrifuge during uncontrolled rotor movements or dangerous vibrations. The modern ultracentrifuge  contains temperature regulation system, flexible drive shaft and over-speed control device to prevent unfavourable conditions. A slight rotor imbalance will cause the centrifuge to switch off automatically.

Preparative rotors are used for pelleting of cellular organelles such as mitochondria, microsomes, ribosomes and viruses. They are also be used for sucrose gradient separation of cellular organelles. and caesium salt gradient separation of nucleic acids. After the centrifugation, the rotor is allowed to stop and the gradient is gently pumped out of each tube to isolate the separated components. There is no optical read-out to collect fractions and analyze after each run. The organelles and molecules are separated on the basis of their sedimentation velocities or buoyant densities .