Ultracentrifuge

An ultracentrifuge is a highly specialized machine designed to spin a component called a rotor at extremely high speeds. These instruments are essential tools in several scientific fields. They are used extensively in molecular biology, biochemistry, and polymer science. By spinning samples very quickly, they generate massive amounts of acceleration. This force allows scientists to separate the tiniest components of a substance.

To understand how they work, we must look at the process of sedimentation. A rotor holds the samples, which are usually placed inside small tubes or central cavities. As the rotor spins, it creates a powerful force that acts on the particles within the sample. This force pulls different parts of the sample toward different locations based on their properties. Scientists often use a gradient to assist this separation. A gradient is a solution that increases in density from the top to the bottom of the tube. For example, sucrose gradients are used to separate cellular organelles. Scientists also use gradients of caesium salts to separate nucleic acids. Once the spinning is complete, the rotor stops smoothly. The separated components are then gently pumped out of the tubes for study.

There are two primary classes of ultracentrifuges used in laboratories today. The first is the preparative ultracentrifuge. These are often called "workhorses" in biomedical labs because they are used to collect specific parts of a cell. They are used for pelleting fine particulate fractions like ribosomes, microsomes, or viruses. The second class is the analytical ultracentrifuge. These are used to observe the properties of the samples while they are spinning.

Different types of rotors are used depending on the specific needs of an experiment. Swinging bucket rotors feature tubes that hang on hinges. As the rotor accelerates, these tubes reorient themselves to a horizontal position. Fixed angle rotors are different because they are carved from a single block of material. These rotors hold tubes in cavities that are bored at a set, predetermined angle. There are also zonal rotors, which are designed to hold a large volume of sample in one central cavity. Some of these special rotors allow for dynamic loading and unloading while the rotor is still spinning at high speeds.

The history of this technology is marked by several major breakthroughs. In 1924, Theodor Svedberg built a centrifuge that could reach 12,000 rpm. This machine generated 7,000 g of acceleration. Between 1925 and 1926, Svedberg built a new version that reached 42,000 rpm. This allowed for fields as high as 100,000 g. Because of his research on proteins and colloids using this tool, Svedberg won the Nobel Prize in Chemistry in 1926. In the early 1930s, Émile Henriot developed a design using jets of compressed air to spin a bearingless top. This principle was later adapted by Jesse Beams at the University of Virginia.

Early high-speed machines faced a major problem with overheating. Jesse Beams found that his rotors consistently grew too hot during use. In 1935, his student Edward Greydon Pickels solved this by vacuumizing the system. This vacuum reduced the friction that was generated at high speeds. The vacuum also helped maintain a constant temperature across the sample. This was important because it stopped convection currents from interfering with the results. In 1946, Pickels cofounded Spinco to market these machines. In 1949, Spinco released the Model L, the first preparative ultracentrifuge to reach 40,000 rpm. Beckman Instruments eventually purchased the company in 1954.

Operating an ultracentrifuge requires extreme caution due to the physics involved. The spinning rotor possesses tremendous rotational kinetic energy. If a rotor suffers a catastrophic failure, it can explode spectacularly. To prevent this, rotors are traditionally made from high-strength metals like titanium or aluminum. However, chemicals and routine use can cause these metals to deteriorate or corrode over time. Modern science has introduced carbon fiber composite rotors to address these risks. These new rotors are up to 60% lighter than metal versions. This lighter weight allows for faster rates of acceleration and deceleration. They are also resistant to corrosion, which helps prevent rotor failure.