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Centrifuge Types, Uses and Other Factors to Consider Before Buying
Hopefully you’re here because you either have an application that requires the use of a centrifuge, or you are looking to purchase a centrifuge but you’re not quite sure which type will best meet your needs. If this sounds like you, then you have come to the right place. After reading this concise yet informative article you will gain an understanding into the different types of centrifuges available, why they are useful and how they work so you’ll have the information you need to choose the best design for your laboratory experiments.
How Do Centrifuges Work?
Centrifuges work by separating out two materials with different densities. They are best used to separate materials that have similar densities; or when insoluble particulates are present in a dissolved solution. A common misconception is that a centrifuge employs a centrifugal force. In fact, all centrifuges work by utilising the sedimentation principle. The sedimentation principle is where the acceleration of the rotor causes a centripetal force to act upon the rotor and centrifuge tubes. This causes the denser substances in the tubes to be forced outward in a radial direction. This also causes the lighter particles to be displaced and move towards the centre. Many particulates can become ‘stuck’ at the bottom of centrifuge tubes, especially when using a laboratory centrifuge. These particles are commonly known as pellets, and the clarified solution is coined the supernatant.
Although you will probably set a centrifuge to spin at revolutions per minute (rpm) or rotational speed, it is the acceleration that is the important factor. This is because two rotors could have different diameters but the same rotational speed. If this is the case the acceleration of the two rotors will not be the same. Because the acceleration is a product of the radius and the square of the angular momentum, the size of the rotor becomes a contributing factor. As a result, Relative Centrifugal Force (RCF) is used as a standard unit and is measured in terms of g.
Centrifuges can be used for a multitude of applications, because there are many different types available in today’s market. Some common applications for centrifuges are listed below:
- Separation of mixtures with close densities
- Separate immiscible liquids
- Sediment suspended solids
- Separation of blood
- Separation insoluble particles (e.g. insoluble proteins in a protein solution)
- Isotope Separation
- Gravity simulation environments for astronauts
- Separating creams
- Washing machine spin function
- Separation of wastewater sludge
- Material synthesis in a high gravity environment
There are of course many more. Any application that requires 2 components to be separate will find that a centrifuge can be a useful tool (unless you are separating an organic and an aqueous phase in organic chemistry, which you’ll find that a separating funnel is better suited for).
There are many types available on the market today, of which the two main factors are rotor speed and centrifuge size.
Let’s discuss some laboratory centrifuges. Regular laboratory centrifuges exhibit all the normal properties that have been mentioned and are the standard type of centrifuge available. Regular centrifuges are the larger centrifuges that you find in a laboratory and are known as floor model centrifuges. These types of centrifuges require more space than benchtop centrifuges, but they can hold a lot more material as the rotors and the centrifuge tubes are larger (centrifuge tubes can hold up to 500ml). Floor centrifuges come with a refrigeration function and temperatures can be set to between -20 °C and + 40 °C. The RCF can also reach up to 100,000 g. Regular centrifuges are particularly useful when using DNA, RNA, antibodies, viruses or proteins.
However, if space is a limitation then a benchtop microcentrifuge or a benchtop ultracentrifuge would be more beneficial. However, they only hold a small amount of material so they are best used for applications where the sample size is small. Benchtop centrifuges can be bought with a built-in refrigerator, but not all models contain one. In many cases, refrigeration protects the sample from thermal degradation caused by the spinning of the rotor so it would be advised to purchase a refrigerated centrifuge when sensitive samples are being used. In general, benchtop centrifuges can be used for a variety of applications including DNA, RNA and protein research, tissue culturing, and cell culturing.
Microcentrifuges provide an RCF of up to 30000 g. They generally work with small sample sizes of 0.2-2.0 ml, but some centrifuges such as the eppendorf centrifuge allow plates or larger centrifuge tubes to be substituted in place of the smaller ones, giving a greater flexibility.
Ultracentrifuges on the other hand are high speed centrifuges, with a RCF of up to 1,000,000 g (150,000 RPM). Ultracentrifuges come in two categories: preparative ultracentrifuge and analytical ultracentrifuge. Preparative ultracentrifuges sediment and separate biological/organic components such as DNA, RNA, lipoproteins, membranes, organelles and viruses. Analytical ultracentrifuges detect samples in real time. They can detect the equilibrium and velocity sedimentation, the shape of molecules and the mass of molecules. Ultracentrifuges also have applications in many areas of nanotechnology.
And of course there are other types of centrifuge for larger and industrial scale applications, space and human applications, but this article has focused around laboratory scale centrifuges. There are also continuous centrifuges known as bowl centrifuges, but these are less common than standard laboratory centrifuges.
What Types of Rotor Are Available?
In addition to the type of centrifuge, the type of rotor can have an influence on the centrifugation process as well. Rotors can be made from metal, plastic or a composite material. They come in three main categories: swing-bucket rotors, drum rotors and fixed-angle rotors.
In swing-bucket rotors, the samples are placed into centrifuge tubes and loaded into individual buckets. These buckets hang vertically when rested but swing out into a horizontal position when the centrifuge is operational. They can be used in smaller centrifuges up to 6,000 g. They allow for an easy separation of phases, separation of individual particles and for resolving density gradients. However, they are inefficient at pelleting.
Drum rotors are specialized rotors for high-centrifugal applications. The samples are held in a cassette and placed vertically into the rotor. The samples do not move from a vertical position. The sedimentation process is similar to swing-bucket rotors but at greater speeds. They have an excellent efficiency for density separation but this rotor is not suitable for pelleting applications.
Fixed-angle rotors are the most common type found in laboratories, and can withstand over 60,000 g. The centrifuge tubes are held at a fixed angle in the rotor and the solution re-orientates itself during the centrifugation process. They are particularly useful for pelleting applications and density separations. The amount of material that can be centrifuged with a fixed-angle rotor is more variable, and higher volumes can be used compared with any other type of rotor.