A centrifuge is a piece of equipment that is used in a laboratory to separate substances or compounds with differing densities and sedimentation rates. The centrifuge is among the most important devices used in Chemistry, Biochemistry, Biology, and Clinical/Medical laboratories as well as in industrial and commercial facilities. Centrifuges are commonly used to separate suspensions and immiscible liquids in a typical Biochemistry lab.
To achieve the separation, materials are placed in a vessel (e.g., regular test tubes) that is suitable for the specific centrifuge. By spinning a vessel at high speed, relatively heavy materials (materials with higher density) are presented at the bottom of the vessel, while the materials that have lower density accumulate at the top of the tube. The force that impacts the migration of denser materials towards the bottom part of the tube is referred to as a centrifugal force.
Considering the fact that these devices are used for a wide array of applications, there is a need for different types of centrifuges, suiting specific industries and facilities. Fortunately, there are dozens of centrifuges with diverse functions and uses available on the market.
Therefore, this article will cover the most abundant and preferred types of centrifuges that currently exist.
Before actually providing specific types of centrifuges, it is essential to identify several factors that are considered for the categorization of the devices. Some of the criteria are as follows:
- The size of a centrifuge;
- The volume of a sample that can be placed in a centrifuge tube;
- The capacity of centrifugation tubes (the maximum number of tubes that can be placed in a centrifuge);
- Centrifuge rotor speed, which is also called an RPM (Revolutions Per Minute). The term is used to denote how many full rotations a centrifuge rotor completes in one minute.
- The design and type of a centrifuge rotor;
- The presence or absence of a vacuum system;
- The presence or absence of temperature control refrigeration system;
- Intended uses of a centrifuge.
The following is the categorization of centrifuges according to the factors listed above.
Categorization by Size
The size of a centrifuge is one of the several features that is reflected on the type of the device. If we categorize centrifuges by size, there are two general types – benchtop and floor centrifuges. Further information about these types of centrifuges is provided below.
- Benchtop Centrifuges
Benchtop centrifuges, also known as tabletop centrifuges, are small and portable space-saving devices that are used for the separation of small amounts of materials. It is a very broad category of centrifuges that can incorporate various features depending on the laboratory needs. Such centrifuges are especially useful for facilities that are limited in space. The relatively small size of the device makes it lighter and easier to relocate.
- Floor-Standing Centrifuges
If you have enough space in your laboratory for a bigger centrifuge, you can simply purchase a floor-standing option, which is ideal for larger labs and the ones that are limited in bench space. The bigger size of floor centrifuges makes them suitable for the separation of higher sample volumes. Besides, such centrifuges comprise much more advanced features, including but not limited to, the following: the place for different sized and shaped tubes, higher rotor speed, and temperature control. One of the most critical aspects of floor-standing centrifuges is that they are suitable for a wider variety of samples, such as cells, nucleus, organelles, membrane components, and macromolecules (in the case of a typical Biochemistry lab).
As mentioned earlier, centrifuges are commonly categorized according to the volume of samples that can be placed in centrifuge tubes, as well as the number of tubes the centrifuge can hold at a time. Obviously, these characteristics are also influenced by the size of the centrifuge itself. Bigger centrifuges can accommodate more vessels that are larger in size, which makes it easier to process a larger number of samples at a time.
Categorization by Centrifuge Rotor Speed
The most common factor by which centrifuges are categorized is the rotor speed. The speed of a rotor (also referred to as an RPM) is the measurement of many spins the rotor makes in a minute. Generally, there are three types of centrifuges if we consider the speed of a rotor.
- Low-Speed Centrifuges
Low-speed centrifuges are typically benchtop centrifuges with a maximum speed of 5,000-6,000 RPM. Commonly, such centrifuges are not equipped with temperature control or vacuum systems. Therefore, they are relatively cheap options ideal for smaller laboratories that are limited in space and budget.
Besides, low-speed centrifuges are commonly used in educational facilities since they are small, quiet, and compact devices that can be easily relocated from bench to bench.
Due to the fact that the speed of a rotor is relatively low, low-speed centrifuges are suitable for the separation of materials that can be easily precipitated on the bottom of the centrifuge vessel. Since the lower speed of the rotor can slightly increase the temperature of the materials being separated, there is no need for a refrigeration system. Instead, some of the centrifuges use a simple forced ventilation system to prevent materials from extra hear and maintain the defined temperature.
- High-Speed Centrifuges
High-speed centrifuges can make around 15,000-25,000 revolutions per minute. Hence, these devices can perform separation tasks much more efficiently in less time than low-speed ones. They are used to isolate a more extensive array of materials, including cells, nucleus, and organelles.
Since centrifugation is an exothermic process as a result of which the heat is produced, the temperature can dramatically increase. These temperature changes can have a significant influence on the centrifugation process and results themselves.
The majority of high-speed centrifuges are equipped with a powerful refrigeration system. Therefore, they are commonly referred to as high-speed refrigerated centrifuges. The temperature control unit ensures that the temperature is below the value specified by the user.
- Ultracentrifuges
An ultracentrifuge is a more advanced version, which is typically designed as a floor-standing centrifuge. Ultracentrifuges can spin the rotors at very high speeds, varying from 60,000 to 1,000,000 RPM. Such a high speed of the rotor is the main reason why ultracentrifuges are bigger in size and are not suitable for placing on a bench.
Ultracentrifuges are equipped with a temperature control system, which guarantees that the temperature is not significantly increased as the materials are separated. There is also a vacuum system implemented in the device through which the centrifuge uses additional forces (vacuum, centrifugal force, heat, or gas flow) to get rid of any excess liquid and isolate the material of interest (e.g., DNA, RNA, nucleotides, macromolecules, membrane components, etc.).
Ultracentrifuges can be further categorized into two primary types: preparative and analytical.
Preparative ultracentrifuges are used to separate various materials and are commonly involved in ligand kinetic studies.
On the other hand, analytical ultracentrifuges are used to monitor the process in real-time and determine the speed at which the particles settle to the bottom of the centrifuge vessel. Such devices comprise three types of optical systems (light absorption, Schlieren system, Rayleigh Interferometer) through which one can detect the changes in the samples.
Categorization by the Type of a Centrifuge Rotor
The central part of a centrifuge is its rotor, which is responsible for spinning the centrifuge tubes to separate the materials. The type of a centrifuge rotor is yet another factor by which centrifuges can be categorized.
The following are several rotor types that are commonly presented in various centrifuges.
- Fixed-Angle Centrifuge Rotors
In fixed-angle centrifuge rotors, vessels are held in at a stable angle varying from 14o to 45o. This type of positioning allows for faster sedimentation results. But, since the tube is somewhat bent in the rotor, the particles concentrate along the side of the vessel. This might make the collection process a bit difficult.
- Vertical Centrifuge Rotors
In the case of vertical centrifuge rotors, vessels are aligned vertically with respect to the device. Such rotors are often called 0o fixed-angle rotors. Compared to regular fixed-angle rotors, vertical alignment of tubes makes it relatively easy to collect the sedimented material.
- Horizontal Centrifuge Rotors
Horizontal centrifuge rotors are designed in the way that the test tube racks swing up into the horizontal plane while influenced by the centrifugal force. Sedimentation of particles occurs along the radial path away from the rotation center. The time required to complete the sedimentation process in horizontal rotors is less than in the case of fixed-angle centrifuge rotors.
- Swing-Bucket Centrifuge Rotors
Swing-bucket centrifuge rotors are one of the most common types of rotors in which tubes are in a vertical position in the beginning and they slightly swing into a horizontal position as the rotor accelerates. As the rotor decelerates, the tubes return to the original vertical position.
- Zonal Centrifuge Rotors
Zonal centrifuge rotors are ideal for the separation of larger samples. Such rotors are designed to minimize the wall effect and maximize the sample volume. In zonal rotors, separation of materials is influenced by one of the two factors: density and sedimentation rate.
Zonal centrifugation makes it much easier to fractionate larger quantities of samples without using huge tubes.
- Elutriation Centrifuge Rotors
Generally speaking, elutriation is a process in which particles are separated by a flow of gas or liquid in the direction opposite to the sedimentation. The method is especially effective in the case of small particles (smaller than 1μm).
A typical elutriation rotor uses two techniques to fractionate monodisperse suspensions of single particles: centrifugal force and fluid velocity. In the case of centrifugal force, the particles are driven away from the axis of rotation. This process is commonly referred to as sedimentation. On the other hand, fluid velocity drives the particles towards the axis of rotation in the process called separation by washing.
