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The ultracentrifuge is a centrifuge optimized for spinning a rotor at very high speeds, capable of generating acceleration as high as 1,000,000 G (9,800 km/s²) There are two kinds of ultracentrifuges, the preparative and the analytical ultracentrifuge. Both classes of instruments find important uses in molecular biology and polymer science. Theodor Svedberg invented the analytical ultracentrifuge in 1923, and won the Nobel Prize in Chemistry in 1926 for his research on colloids and proteins using the ultracentrifuge.
Analytical Ultracentrifuge
In an analytical ultracentrifuge, a sample being spun can be monitored in real time through an optical detection system, typically using ultraviolet light. This allows the operator to observe changes sample concentration versus the axis of rotation in the centrifugal field. With modern instrumentation, these observations are electronically digitized and stored for further analysis. Two kinds of experiments are commonly performed on these instruments: sedimentation velocity experiments and sedimentation equilibrium experiments. The first are sensitive to both the shape and molar mass of the sample being studied, whereas the second are insensitive to the shape, but are sensitive to the molar mass of the sample being studied.
The kinds of information that can be obtained from an analytical ultracentrifuge include the shape of macromolecules, the conformational changes in macromolecules, subunit stoichiometry of macromolecules, approximate molecular weights, and equilibrium constants for self associating systems.
Preparative Ultracentrifuge
Preparative ultracentrifuges are available with a wide variety of rotors suitable for a great range of experiments. Most rotors are designed to hold tubes that contain the samples. Swinging bucket rotors allow the tubes to hang on hinges so the tubes reorient to the horizontal as the rotor initially accelerates. Fixed angle rotors are made of a single block of metal and hold the tubes in cavities bored at a predetermined angle. 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.
Preparative rotors are used in biology for pelleting of fine particulate fractions, such as cellular organelles (mitochondria, microsomes, ribosomes) and viruses. They can also be used for gradient separations, in which the tubes are filled from top to bottom with an increasing concentration of a dense substance in solution. Sucrose gradients are typically used for separation of cellular organelles. Gradients of cesium salts are used for separation of nucleic acids. After the sample has spun at high speed for sufficient time to produce the separation, the rotor is allowed to coast to a smooth stop and the gradient is gently pumped out of each tube to isolate the separated components.
Hazards
The tremendous rotational kinetic energy of the rotor in an operating ultracentrifuge makes the catastrophic failure of a spinning rotor a serious concern. The stresses of routine use and harsh chemical solutions eventually cause rotors to deteriorate. Proper use of the instrument and rotors within recommended limits and careful maintenance of rotors to prevent corrosion and to detect deterioration are necessary to avoid this hazard.
See also
External links
"Ultracentrifuge"