Centrifugation

Type of physical science: Chemistry

Field of study: Chemical methods

One mechanical technique useful for separating immiscible liquids or solids from liquids is centrifugation. By applying centrifugal force, these separations can be used for either analytical or preparative purposes.

Overview

Centrifugation is a mechanical technique useful for separating solids from liquids, liquids from gases, gases from other gases, and so on. A centrifuge is a device that relies on the power of centrifugal force to accomplish these types of separations.

Centrifugal force can be a confusing concept. Many texts define it as a "pseudoforce" that is "nonexistent" or "fictitious." Centrifugal force does exist, but only under certain conditions. To understand how this force is applied in centrifugation, it is necessary to consider another force, centripetal force.

Centripetal force is the inward force required to keep an object moving in a circular path. Imagine a turntable or flat disc, spinning about a central point. If an object is placed on the outer edge of this disc, gravity is the force that keeps it on the disc, but centripetal force is the force that allows it to move in a circle. If the object is too light, centripetal force may overcome the force of gravity, and the object will fly off the spinning disc.

If the spinning disc just described were large enough so that you could stand on the disc, the force you would feel pulling you toward the edge is centrifugal force. In other words, centripetal and centrifugal forces are mirror images; viewed from the outside, the force is termed "centripetal." Viewed from the inside, as the object upon which the force is acting, the force is called "centrifugal." One final example: The earth can be viewed as a large centrifuge. People are held on the face of the earth by the force of gravity. Without centrifugal forces acting to balance gravity, one would gradually be pulled in toward the center of the earth. Without gravity, one would be flung off into space. Most centrifuges seek to create forces many times the strength of gravity.

The amount of centrifugal (or centripetal) force a centrifuge can generate is defined by the following equation: F_c = Mw²R where F sub c equals the centrifugal force, M is the amount of mass, w is angular velocity, and R is the distance of an object to be spun from the center of the centrifuge. This equation has many implications for the construction of a centrifugal device. For example, the amount of centrifugal force can be greatly increased by increasing the angular velocity. To go from an initial velocity of 1 rotation per minute to 10 rotations per minute actually increases the centrifugal force by a factor of 100. Some of the fastest centrifuges can make more than 1 million revolutions per minute, generating forces that are five million times as great as gravity. The limiting factor is the device itself. The force generated by a centrifuge affects the structure of the centrifuge itself; the fastest centrifuges tend to be very small and made of materials that can withstand great tensile stress.

A generic centrifuge will contain a rotor to which is attached a container or bowl that holds the material to be separated. Many different types of centrifuges exist, many of them highly specialized devices constructed for limited applications. Most centrifuges, however, are of one of two basic types: sedimenters or filters.

A sedimenting centrifuge uses a bowl with a solid wall that rotates on a horizontal or vertical axis. These devices separate material on the basis of weight or density. Most of the centrifuges used in chemical, biological, or medical laboratories are of this general type. In a standard "bench-top" centrifuge, multiple test-tube-shaped containers are mounted symmetrically around a vertical shaft, driven by an electric motor. These containers may be of the "swinging bucket" type, which are attached by joints that allow them to spin at right angles to the shaft while centrifugation is in progress. Other centrifuges will have containers that are fixed in place, often at an angle of about 37 degrees to the shaft. This reduces the amount of distance that the material must settle.

Another type of sedimenting centrifuge is the ultracentrifuge. In an ultracentrifuge, samples spin within a chamber in which the air has been replaced by a vacuum, a feature that reduces both air resistance and potential problems caused by the buildup of heat. Ultracentrifuges can generate angular velocities of more than 20,000 revolutions per minute.

Disk-type centrifuges are also used for sedimentation, but lack the tube or bottle-shaped containers found in the more common devices. This centrifuge contains multiple thin, cone-shaped disks that form a stack. The material to be separated moves between the disks, with the angle of the cones helping to separate the heavier material from light phases.

Filtering centrifuges feature a bowl with a perforated wall. The size of the holes in such a wall depends on the material to be filtered. This type of device is primarily used to separate liquids from solids very quickly. One common example of a filtering centrifuge is the drum of a household washing machine. During the spin cycle, the drum rotates rapidly, throwing off water drops that pass through the holes in the drum, while retaining the solid articles of clothing.

Applications

The major application of centrifugation in the physical sciences is the centrifuge. This device, in its many different forms, can perform many tasks useful for both preparation of materials and for analysis of the physical nature of some materials.

There are many preparative uses of centrifuges. Sedimentation centrifuges can be used to purify many different types of materials including food products, vaccines, and oils. For example, in the process of rendering, oils from different sources including seeds, fruits, or animal fats are separated from liquid and solid protein components to produce a good yield of high-quality oil. Filtering centrifuges are used to produce sugar crystals from sugar cane syrup, and to wash and dry many other types of crystals or fibers. Disk-type centrifuges are used in the dairy industry. After whole milk is churned or homogenized, the fat-containing cream layer can be quickly drawn off through centrifugation, leaving behind the lower-fat skim milk.

In chemistry, gas centrifugation can be used to separate isotopes, which are forms of the same chemical element that differ slightly in their mass. This process works very much like the cream separator used by dairies. It was first successfully applied to isotopes of chlorine in 1936, and has also been used at times in the preparation of uranium. Natural uranium contains mostly uranium 238, but it is the lighter isotope, uranium 235, that undergoes fission. During the development of the atomic bomb, the United States government at one point considered investing millions of dollars in the construction of gas centrifuge plants devoted to the production of uranium 235. This idea was abandoned in favor of a different method, involving filtration. Still, a little less than 5 percent of the world's uranium is prepared in this manner.

One important analytical application of centrifugation is the determination of the molecular weights of different molecules. This technique is especially important for protein chemistry, and it is also widely used for determining the density of nucleic acids, such as DNA (deoxyribonucleic acid), and for separating DNA molecules of different densities. The ultracentrifuge is the device of choice for this type of analysis. Two different methods for the determination of molecular weight are possible. In one method, called equilibrium density centrifugation, the unknown material is added to a heavy salt solution, usually cesium chloride.

As the centrifugation progresses over many hours, the salt forms a gradient within the sample tube, with the heaviest concentration of salt found farthest from the center of rotation. At some point in time, any added material will find a level at which its forward motion in the tube is balanced by its tendency to diffuse back into areas of lesser concentration. The point at which this material reaches its equilibrium point in a gradient is a function of its density and molecular weight. By comparing the distance traveled of an unknown material with distances traveled by materials of known molecular weights, the weight of the unknown can be determined.

A second method of determining the molecular weight of a molecule is called the rate of sedimentation method. In this method, a sample of the molecule whose molecular weight is to be determined is added to a solvent, and the solution is centrifuged at very high speeds. This will produce a very sharp boundary between the pure solvent and the sedimenting molecules. Over time, this boundary will move out, away from the center of rotation. The rate at which the boundary moves (that is, how quickly sedimentation occurs) can be measured and used in the calculation of the molecular weight of the molecule. This method also requires an ultracentrifuge, which must run for many hours.

Centrifuges are also important for other types of analyses. The centrifugal fast-scan analyzer uses a low-speed (350 revolutions per minute) centrifuge and a spectrophotometer to analyze up to sixteen samples at once. A spectrophotometer measures the amount of light of a given wavelength that is transmitted through, or absorbed by, a given substance. Analyzers of this type are often used to determine the concentration of an enzyme in a sample. Samples and reagents are placed in adjacent chambers in the arm of a centrifuge, and are then mixed by the rotational action of the device. The mixture flows into a special chamber at the end of the centrifuge arm. This chamber has a quartz window through which the light from a spectrophotometer can pass. With each spin of the centrifuge, the relative amount of the reagent can be measured by the spectrophotometer and stored in computer memory. These data are then used to calculate the rate of the reaction.

Another application of centrifugation is found in the centrifuge microscope. In this device, a stationary objective lens is pointed down at prisms located at the center of rotation of a small centrifuge. The samples to be observed are located in wells on slides mounted at right angles to the center of rotation. As the centrifuge spins, light from the samples is reflected back toward the prisms and up to the objective lens, producing a two-dimensional image. This device can be used to observe the movement of grains or crystals in fluids of different viscosities. In the biological sciences, the centrifuge microscope is used to examine the effects of centrifugal forces on living cells.

A very different application of centrifugation is seen in astronaut and pilot training. The force exerted on a person at the end of a centrifuge arm can be varied by the speed of the centrifuge. This allows pilots and astronauts to feel the forces equivalent to "high-g" gravitational forces experienced during rapid acceleration. This allows them to practice many actions and maneuvers during simulated flights before experiencing the real event. This also allows scientists to test how well and how long the human body can endure strong gravitational forces.

Context

Like so many natural phenomena, the understanding of centrifugal and centripetal forces is based on the laws of Sir Isaac Newton. In the eighteenth century, Newton described three fundamental principles that form the basis of classical mechanics. These laws have proved to be valid for all examples that involve masses greater than the size of the atom, and for speeds less than the speed of light.

Centrifugation applies Newton's second and third laws of motion. According to the second law, the acceleration of a particle (for example, in a centrifuge tube) is directly proportional to the force acting on a particle (the speed of the centrifuge), and is inversely proportional to the mass of the particle (that is, heavier particles require more force to cause them to spin). Newton's third law states that interacting particles exert equal and opposite effects on one another. In centrifugation, centripetal and centrifugal forces represent such an equal and opposite pair.

Simple applications of centrifugation do not require an understanding of the theoretical basis behind the process. Long before Newton, simple centrifuges were used to separate cream from milk and to reduce the sediment in wines. However, more sophisticated machines and more analytical uses did not develop until the twentieth century.

As an analytical tool, centrifugation is valuable, but perhaps not as informative as other processes including chromatography, electrophoresis, and spectroscopy. Aside from molecular-weight calculations, which are useful in many fields of chemistry, centrifugation's biggest contribution has been in the biochemical area. Applications of centrifugation have allowed the identification of new classes of molecules. For example, lipoproteins are lipid-protein complexes held together by weak physical forces. The different classes of lipoproteins were first identified when human blood was spun in an ultracentrifuge. This allowed the identification of high-density (low-fat) lipoproteins (HDLs), low-density (high-fat) lipoproteins (LDLs), and very-low-density lipoproteins (VLDLs). The relative amounts of these molecules in the blood are an important indicator of susceptibility to diseases such as arteriosclerosis.

Centrifugation is also essential for the study of DNA, the genetic material of all living things. Centrifugation is an important step in isolating DNA from the cells in which it is found.

Centrifugation is also used in estimating the size and chemical makeup of DNA from different sources.

The principles of centrifugation may one day be applied to the construction of revolving space stations or space colonies. As one's distance from the earth increases, the pull of gravity is lessened, resulting in "weightlessness." Artificial gravity can be created by the forces exerted on people or objects standing inside the outer ring of a spinning circle. Artificial gravity will allow people to live and work normally, free from possible physiological problems resulting from long-term exposure to the lack of gravity.

Principal terms

ANGULAR VELOCITY: the rate of change of position of a body moving in a circle, commonly measured in rotations per minute

CENTRIFUGAL FORCE: the outward force exerted on an object moving in a circular path

CENTRIFUGE: a device that uses centrifugal force, usually employed to separate different types of materials

CENTRIPETAL FORCE: the inward force required to keep an object moving in a circular path

GRAVITATION: the mutual attraction between all bodies in the universe, of which gravity, the force the earth exerts on all bodies on or near it, is an example

ULTRACENTRIFUGE: a high-speed, low-turbulence centrifuge useful for both preparation and analysis

Bibliography

Fleisher, Paul. SECRETS OF THE UNIVERSE: DISCOVERING THE UNIVERSAL LAWS OF SCIENCE. New York: Atheneum, 1982. This book is intended for junior high or high school readers and contains many clear illustrations of important principles of physics. Chapter 7, "Conservation of Momentum," discusses angular momentum using common examples such as Frisbees and lawn sprinklers.

Haber, Ralph Norman. "Flight Simulation." SCIENTIFIC AMERICAN 235 (January, 1986): 96-103. This fascinating article discusses many aspects of flight simulation, including physical effects as well as computer imagery. Of special interest is a discussion of the g-forces felt by different types of airplanes and the difficulties in building an apparatus to produce similar sensations.

Hewitt, Paul G. CONCEPTUAL PHYSICS. 6th ed. Glenview, Ill.: Scott, Foresman, 1989. This is a truly outstanding text, written and illustrated by a teacher with a true flair for communication. Written at a level appropriate for high school or for a college course for nonscience majors, it is both instructional and entertaining. An extremely clear comparison of centrifugal and centripetal forces is presented in chapter 7, "Rotational Forces."

Rhodes, Richard. THE MAKING OF THE ATOMIC BOMB. New York: Simon & Schuster, 1986. Rhodes won many awards, including the Pulitzer Prize for this historical account of the people and events involved in the development of the atomic bomb. Of special interest is chapter 13, "The New World," which contains an account of the debate over building centrifuge plants to supply uranium for the project.

Skoog, Douglas A. PRINCIPLES OF INSTRUMENTAL ANALYSIS. 3d ed. Philadelphia: Saunders, 1985. This is a college text intended for upper-level chemistry students. It is clearly written and illustrated, however, and contains many useful descriptions of apparatuses used in the physical sciences, as well as explanations of how such pieces of equipment are used in the laboratory and industry.

Centrifugal/Centripetal and Coriolis Accelerations

Diffusion in Gases and Liquids