[转帖]实验室离心机背景知识--英文

郑振寰 · 发表于 2010-3-4 11:53

Laboratory centrifuge
A tabletop laboratory centrifuge Uses Separation Related items Gas centrifuge
Ultracentrifuge A laboratory centrifuge is a piece of laboratory equipment, driven by a motor, which spins liquid samples at high speed. There are two main sizes for laboratory centrifuges. The larger ones are known simply as centrifuges; samples are contained in centrifuge tubes or centrifuge tips. The smaller centrifuges are known as microcentrifuges or microfuges, and microcentrifuge tubes or microfuge tubes are used with them.
Like all other centrifuges, laboratory centrifuges work by the sedimentation principle, where the centripetal acceleration is used to separate substances of greater and lesser density.
Contents 1 Operation 2 Types 3 Design 4 Centrifuge tubes 4.1 Microcentrifuge tubes 5 Safety 6 Theory 7 See also 8 References 9 External links //
Operation A 19th century hand cranked laboratory centrifuge. Increasing the effective gravitational force will more rapidly and completely cause the precipitate ("pellet") to gather on the bottom of the tube. The remaining solution is called the "supernate" or "supernatant".
The supernatant liquid is then either quickly decanted from the tube without disturbing the precipitate, or withdrawn with a Pasteur pipette. The rate of centrifugation is specified by the acceleration applied to the sample, typically measured in revolutions per minute (RPM) or g. The particles' settling velocity in centrifugation is a function of their size and shape, centrifugal acceleration, the volume fraction of solids present, the density difference between the particle and the liquid, and the viscosity.
The use of a centrifuge is known as centrifugation.

Types There are various types of centrifugation:
Differential centrifugation, often used to separate certain organelles from whole cells for further analysis of specific parts of cells Isopycnic centrifugation, often used to isolate nucleic acids such as DNA Sucrose gradient centrifugation, often used to purify enveloped viruses and ribosomes, and also to separate cell organelles from crude cellular extracts
Design A large laboratory centrifuge. Laboratory centrifuges are used in chemistry, biology, and biochemistry for isolating and separating solids from liquids in a suspension. The solids can be insoluble compounds, biomolecules, cell organelles, or whole cells. They vary widely in speed and capacity. They usually comprise a rotor containing two, four, six, or many more numbered wells within which centrifuge tubes may be placed.
When a suspension in a centrifuge tube is centrifuged, the solids settle at the bottom of the centrifuge tube; having a tapered wall helps to concentrate the solids, making it easier to decant the supernatant solution, leaving the solids.
The rotor is covered by a plastic cover. The cover is usually interlocked to prevent the motor from turning the rotor when it is open, and from allowing the cover to be opened before the rotor stops for several minutes. The cover protects the user from being injured by touching a rapidly spinning rotor. It also protects the user from fragments in case the rotor fails catastrophically.
The rotor must be balanced by placing samples or blanks of equal mass opposite each other. Since most of the mass is derived from the solvent, it is usually sufficient to place blanks or other samples of equal volume. As a safety feature, some centrifuges may stop turning when wobbling is detected.

Centrifuge tubes Centrifuge tubes or centrifuge tips are tapered tubes of various sizes made of glass or plastic. They may vary in capacity from tens of millilitres, to much smaller capacities used in microcentrifuges used extensively in molecular biology laboratories. The most commonly encountered tubes are of about the size and shape of a normal test tube (~ 10 cm long). Microcentrifuges typically accommodate microcentrifuge tubes with capacities from 250 μl to 2.0 ml. These are exclusively made of plastic.
Glass centrifuge tubes can be used with most solvents, but tend to be more expensive. They can be cleaned like other laboratory glassware, and can be sterilized by autoclaving. Plastic centrifuge tubes, especially microcentrifuge tubes tend to be less expensive. Water is preferred when plastic centrifuge tubes are used. They are more difficult to clean thoroughly, and are usually inexpensive enough to be considered disposable.

Microcentrifuge tubes Microcentrifuge tube with Coomassie Blue solution Microcentrifuge tubes or microfuge tubes are small, cylindrical plastic containers with conical bottoms, typically with an integral snap cap. They are used in molecular biology and biochemistry to store and centrifuge small amounts of liquid. As they are inexpensive and considered disposable, they are used by many chemists and biologists as convenient sample vials in lieu of glass vials; this is particularly useful when there is only a small amount of liquid in the tube or when small amounts of other liquids are being added, because microcentrifugation can be used to collect the drops together at the bottom of the tube after pipetting or mixing.
Made of polypropylene,[1] they can be used in very low temperature (-80 °C to liquid nitrogen temperatures) or with organic solvents such as chloroform. They come in many different sizes, generally ranging from 250 μL to 2.0 mL. The most common size is 1.5 mL. Disinfection is possible (1 atm, 120 °C, 20 minutes) and is commoly performed in works related to DNA or microbes, where purity of the sample is of utmost importance. Due to their low cost and the difficulty in cleaning the plastic surface, they are usually discarded after each use.
Eppendorf tube has become a genericized trademark for microfuge tubes or microcentrifuge tubes. Eppendorf is a major manufacturer of this item, but is not the only one.
Three microcentrifuge tubes: 2 mL, 1.5 mL and 200 μL (for PCR). Four screw-top microcentrifuge tubes.
Safety The load in a laboratory centrifuge must be carefully balanced. Small differences in mass of the load can result in a large force imbalance when the rotor is at high speed. This force imbalance strains the spindle and may result in damage to centrifuge or personal injury.
Centrifuge rotors should never be touched while moving, because a spinning rotor can cause serious injury. Modern centrifuges generally have features that prevent accidental contact with a moving rotor.
Because of the kinetic energy stored in the rotor head during high speed rotation, those who have experienced the loss of a rotor inside of an ultracentrifuge compare the experience to having a bomb explode nearby.[citation needed]

Theory Protocols for centrifugation typically specify the amount of acceleration to be applied to the sample, rather than specifying a rotational speed such as revolutions per minute. The acceleration is often quoted in multiples of g, the acceleration due to gravity at the Earth's surface. This distinction is important because two rotors with different diameters running at the same rotational speed will subject samples to different accelerations.
The acceleration can be calculated as the product of the radius and the square of the angular velocity.
Relative centrifugal force is the measurement of the force applied to a sample within a centrifuge. This can be calculated from the speed (RPM) and the rotational radius (cm) using the following calculation.
g = RCF = 0.00001118 × r × N2 where:
g = Relative centrifuge force r = rotational radius (centimetre, cm) N = rotating speed (revolutions per minute, r/min) To avoid having to perform a mathematical calculation every time, one can find nomograms for converting RCF to rpm for a rotor of a given radius. A ruler or other straight edge lined up with the radius on one scale, and the desired RCF on another scale, will point at the correct rpm on the third scale. Example

See also Centrifuge Centrifugation Gas centrifuge Separation Ultracentrifuge
References ^ "Chemical Stability of Disposables" (pdf). Applications Note 05. Eppendorf. June 2005. https://www.eppendorfna.com/utilities/enewsletter.asp?ENLUID=e200606&REFUID=AP04.
External links Wikimedia Commons has media related to: Centrifuge Look up centrifuge in Wiktionary, the free dictionary. RCF Calculator and Nomograph Centrifugation Rotor Calculator Selection of historical centrifuges in the Virtual Laboratory of the Max Planck Institute for the History of Science

郑振寰 · 发表于 2010-3-4 11:56

Centrifuge

A laboratory tabletop centrifuge

A centrifuge is a piece of equipment, generally driven by an electric motor (but some older models are still spun with hand), that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis. The centrifuge works using the sedimentation principle, where the centripetal acceleration causes heavier particles to move out along the radial direction (the bottom of the tube). By the same token, lighter objects will tend to move to the top (of the tube; in the rotating picture, move to the centre).

In the picture shown, the rotating unit, called the rotor, has fixed holes drilled at an angle (to the vertical). Test tubes are placed in these slots and the rotor is spun. As the centrifugal force is in the horizontal plane and the tubes are fixed at an angle, the particles have to travel only a little distance before they hit the wall and drop down to the bottom. These angle rotors are very popular in the lab for routine use.

Theory

Protocols for centrifugation typically specify the amount of acceleration to be applied to the sample, rather than specifying a rotational speed such as revolutions per minute. The acceleration is often quoted in multiples of g, the standard acceleration due to gravity at the Earth's surface. This distinction is important because two rotors with different diameters running at the same rotational speed will subject samples to different accelerations.

Since the motion is circular the acceleration can be calculated as the product of the radius and the square of the angular velocity. Traditionally named "Relative centrifugal force" (RCF), it is the measurement of the acceleration applied to a sample within a centrifuge and it is measured in units of gravity (times gravity or × "g"). It is given by

where

is earth's gravitational acceleration,

is the rotational radius,

is the rotating speed, measured in revolutions per unit of time.

When the rotational speed is given in revolutions per minute (RPM) and the rotational radius is expressed in centimetres (cm) the above relationship becomes

where

is the rotational radius measured in centimetres (cm),

is rotating speed measured in revolutions per minute (RPM).

History and predecessors

A 19th century hand cranked laboratory centrifuge.

English military engineer Benjamin Robins (1707-1751) invented a whirling arm apparatus to determine drag. In 1864, Antonin Prandtl invented the first dairy centrifuge in order to separate cream from milk. In 1879, Gustaf de Laval demonstrated the first continuous centrifugal separator, making its commercial application feasible.

Types

There are at least five types of centrifuge:

preparative centrifuge
analytical centrifuge
angle fixed centrifuge
swing head centrifuge
haematocrit centrifuge

Industrial centrifuges may otherwise be classified according to the type of separation of the high density fraction from the low density one :

Screen centrifuges, where the centrifugal acceleration allows the liquid to pass through a screen of some sort, through which the solids cannot go (due to granulometry larger than the screen gap or due to agglomeration). Common types are :
- Pusher centrifuges
- Peeler centrifuges
Decanter centrifuges, in which there is no physical separation between the solid and liquid phase, rather an accelerated settling due to centrifugal acceleration. Common types are :
- Solid bowl centrifuges
- Conical plate centrifuges

Uses

Isolating suspensions

Main article: Laboratory centrifuge

Simple centrifuges are used in chemistry, biology, and biochemistry for isolating and separating suspensions. They vary widely in speed and capacity. They usually comprise a rotor containing two, four, six, or many more numbered wells within which the samples containing centrifuge tips may be placed.

Isotope separation

Other centrifuges, the first being the Zippe-type centrifuge, separate isotopes, and these kinds of centrifuges are in use in nuclear power and nuclear weapon programs.

Gas centrifuges are used in uranium enrichment. The heavier isotope of uranium (uranium-238) in the uranium hexafluoride gas tend to concentrate at the walls of the centrifuge as it spins, while the desired uranium-235 isotope is extracted and concentrated with a scoop selectively placed inside the centrifuge. It takes many thousands of centrifuges to enrich uranium enough for use in a nuclear reactor (around 3.5% enrichment), and many thousands more to enrich it to weapons-grade (around 90% enrichment) for use in nuclear weapons.

The 20 G centrifuge at the NASA Ames Research Center

Aeronautics and astronautics

Main article: high-G training

Human centrifuges are exceptionally large centrifuges that test the reactions and tolerance of pilots and astronauts to acceleration above those experienced in the Earth's gravity.

The US Air Force at Holloman Air Force Base, NM operates a human centrifuge. The centrifuge at Holloman AFB is operated by the aerospace physiology department for the purpose of training and evaluating prospective fighter pilots for high-g flight in Air Force fighter aircraft. It is important to note that the centrifuge at Holloman AFB is unrealistic in that it is far more difficult for a pilot to tolerate the high-g environment in the centrifuge than in a real fighter aircraft. This well-known fact is based on countless accounts from experienced operational fighter pilots.^{[citation needed]}

The use of large centrifuges to simulate a feeling of gravity has been proposed for future long-duration space missions. Exposure to this simulated gravity would prevent or reduce the bone decalcification and muscle atrophy that affect individuals exposed to long periods of freefall. An example of this can be seen in the film 2001: A Space Odyssey, while the James Bond film Moonraker shows an astronaut-training centrifuge in action on earth (albeit sabotaged with murderous intent).

Earthquake and blast simulation

The geotechnical centrifuge is used for simulating blasts and earthquake phenomena.^[1] For a discussion of their design, see Geotechnical Centrifuges by Philip Turner.

Commercial applications

Standalone centrifuges for drying (hand-washed) clothes - usually with a water outlet.
Centrifuges are used in the attraction Mission: SPACE, located at Epcot in Walt Disney World, which propels riders using a combination of a centrifuge and a motion simulator to simulate the feeling of going into space.
In soil mechanics, centrifuges utilize centrifugal acceleration to match soil stresses in a scale model to those found in reality.
Large industrial centrifuges are commonly used in water and wastewater treatment to dry sludges. The resulting dry product is often termed cake, and the water leaving a centrifuge after most of the solids have been removed is called centrate.
Large industrial centrifuges are also used in the oil industry to remove solids from the drilling fluid.
Disc-stack centrifuges used by some companies in Oil Sands industry to separate small amounts of water and solids from bitumen before it's sent to Upgrading.

References and notes

^ C. W. W. Ng, Y. H. Wang, L. M. Zhang (2006). Physical Modelling in Geotechnics: proceedings of the Sixth International Conference on Physical Modelling in Geotechnics. Taylor & Francis. p. 135. ISBN 0415415861. https://books.google.com/books?id=mzQlFBqJC1wC&pg=RA1-PA186&dq=centrifuge+earthquake&lr=&as_brr=0&sig=ACfU3U1SZImDanYOwrq15Jre19qtAqpyJw#PRA1-PA135,M1.

External links

RCF Calculator and Nomograph
RPI centrifuge video
Centrifugation Rotor Calculator
Selection of historical centrifuges in the Virtual Laboratory of the Max Planck Institute for the History of Science

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