Types of Centrifuge and Rotors with Principles and Uses

A centrifuge is a device that uses the principle of centrifugal force to separate the components of a mixture based on their density, size, shape, or viscosity. Centrifugal force is the apparent force that acts on an object moving in a circular path and pushes it away from the center of rotation. By spinning a mixture at high speed, a centrifuge can create a force that is much greater than gravity and cause the heavier or denser components to move towards the outer edge of the container, while the lighter or less dense components remain near the center. This process is called centrifugation and it can be used for various purposes in different fields of science and technology.

Centrifuges are widely used in laboratories for the analysis and purification of biological samples, such as blood, urine, DNA, proteins, viruses, bacteria, and cell organelles. Centrifugation can help to isolate specific molecules or cells from a complex mixture or to separate them based on their properties, such as size, shape, charge, or affinity. For example, centrifugation can be used to separate red blood cells from plasma, to extract DNA from cells, to purify proteins from a solution, or to isolate viruses from a culture.

Centrifuges are also used in industrial applications for the processing and separation of materials, such as food, beverages, chemicals, pharmaceuticals, metals, minerals, and fuels. Centrifugation can help to remove impurities or contaminants from a product, to concentrate or dry a substance, to separate solid-liquid or liquid-liquid mixtures, or to enrich or deplete certain components. For example, centrifugation can be used to separate cream from milk, to extract oil from seeds or nuts, to remove water from ethanol, to separate uranium isotopes for nuclear fuel, or to recover precious metals from ores.

Centrifuges vary in their design and functionality depending on their intended use and the type of mixture they are meant to separate. Some common features of centrifuges are:

• A rotor: This is the part that holds the containers with the samples and spins them at high speed. The rotor can have different shapes and sizes depending on the type and number of containers it can accommodate.
• A motor: This is the part that provides power to spin the rotor. The motor can have different speeds and modes of operation depending on the desired level of centrifugal force.
• A temperature control unit: This is the part that regulates the temperature inside the centrifuge. The temperature control unit can have different settings and functions depending on the sensitivity and stability of the samples.
• A control panel: This is the part that allows the user to set and monitor the parameters of the centrifugation process. The control panel can have different features and options depending on the complexity and automation of the centrifuge.

In this article, we will discuss some of the most common types of centrifuges and rotors used in laboratories and industries and their principles and uses. We will also explain how to calculate the relative centrifugal force (RCF), which is an important factor for determining the optimal conditions for centrifugation.

Relative centrifugal force (RCF) is a measure of the strength of the centrifugal force applied to the samples in a centrifuge. It depends on the speed of rotation (RPM) and the radius of the rotor (r). The higher the RPM and r, the higher the RCF.

RCF is expressed in units of gravity (g), which is the acceleration due to gravity on Earth. One g is equal to 9.81 m/s^2. RCF indicates how many times the gravitational force is acting on the samples in a centrifuge. For example, an RCF of 10,000 g means that the samples are experiencing a force 10,000 times greater than gravity.

The formula to calculate RCF is:

RCF = 1.118 × 10^-5 × r × (RPM)^2

where r is the radius of the rotor in centimeters, and RPM is the speed of rotation in revolutions per minute.

To use this formula, we need to know the radius of the rotor and the speed of rotation. The radius of the rotor can be measured from the center of the rotor to the bottom of the sample tube. The speed of rotation can be set by adjusting the control panel of the centrifuge.

Alternatively, we can use online calculators or nomograms to find RCF from RPM and r, or vice versa. A nomogram is a graphical tool that shows the relationship between three variables. By drawing a straight line that connects two known variables, we can find the third variable from the scale.

Here is an example of a nomogram for calculating RCF from RPM and r:

[Insert image of the nomogram]

To use this nomogram, we need to locate the point on the RPM scale that corresponds to our speed of rotation, and then draw a straight line from that point to the point on the r scale that corresponds to our radius of the rotor. The point where this line intersects with the RCF scale gives us our relative centrifugal force.

For example, if we have a rotor with a radius of 10 cm and a speed of rotation of 10,000 RPM, we can draw a line from 10,000 on the RPM scale to 10 on the r scale. This line intersects with 11,180 on the RCF scale, which means that our samples are experiencing an RCF of 11,180 g.

Knowing RCF is important for choosing the appropriate centrifuge and rotor for our experiment. Different types of samples require different levels of RCF to achieve optimal separation. For example, blood cells can be separated at low RCF (around 1,000 g), while proteins and viruses need high RCF (above 100,000 g). By using RCF instead of RPM, we can compare rotors of different sizes and speeds and select the best one for our purpose.

Centrifuge rotors are the devices that hold the sample tubes and spin them at high speeds to create a centrifugal force. The design and shape of the rotors affect the performance and efficiency of the centrifugation process. There are three main types of centrifuge rotors: fixed angle rotors, swinging bucket rotors/horizontal rotors, and vertical rotors.

Fixed angle rotors

Fixed angle rotors are the most common type of rotors used in centrifuges. They hold the sample tubes at a fixed angle, usually 45°, relative to the axis of rotation. This means that the tubes do not change their orientation during the spinning process. The advantage of fixed angle rotors is that they have a shorter pathlength and a higher speed than other types of rotors, which results in faster and more efficient separation of particles. However, a drawback of fixed angle rotors is that some particles may remain stuck to the walls of the tubes due to the difference between the direction of the centrifugal force and the position of the tubes. This can affect the quality and yield of the separation.

Swinging bucket rotors/horizontal rotors

Swinging bucket rotors, also known as horizontal rotors, are another type of rotors used in centrifuges. They hold the sample tubes at a vertical angle when they are stationary, but as they start spinning, they swing out to a horizontal position. This means that the tubes change their orientation during the spinning process. The advantage of swinging bucket rotors is that they align the tubes with the direction of the centrifugal force, which ensures that all particles settle at the bottom of the tubes. This can improve the quality and yield of the separation, especially for low-density particles. However, a drawback of swinging bucket rotors is that they have a longer pathlength and a lower speed than fixed angle rotors, which results in slower and less efficient separation of particles.

Vertical rotors

Vertical rotors are a special type of rotors used in centrifuges. They hold the sample tubes at a vertical position throughout the spinning process. This means that the tubes do not change their orientation during the spinning process. The advantage of vertical rotors is that they have the shortest pathlength and the highest speed of all types of rotors, which results in very high resolution and very fast separation of particles. However, a drawback of vertical rotors is that they do not align the tubes with the direction of the centrifugal force, which causes some particles to spread along the walls of the tubes instead of settling at the bottom. This can affect the quality and yield of the separation.

Centrifuges are devices that use centrifugal force to separate components of a mixture based on their density, size, shape, and viscosity. There are different types of centrifuges available for various purposes and applications. Some of the common types of centrifuges are:

• Benchtop centrifuge: A benchtop centrifuge is a compact and versatile centrifuge that can be used in clinical and research laboratories for various tasks such as blood separation, cell culture, DNA extraction, and microfiltration. It can accommodate different sizes and types of tubes and rotors and can reach speeds up to 18,000 rpm. It has a lid that closes the working unit and prevents contamination and noise. Some benchtop centrifuges have features such as temperature control, timer, speed display, and safety lock.
• Continuous flow centrifuge: A continuous flow centrifuge is a high-throughput and efficient centrifuge that allows the processing of large volumes of samples without interrupting the centrifugal force. It has a feed pump that continuously introduces the sample into the rotor and a discharge pump that removes the separated fractions. It can achieve speeds up to 60,000 rpm and can separate particles as small as 0.5 microns. It is used for applications such as bioprocessing, wastewater treatment, oil refining, and blood banking.
• Gas centrifuge: A gas centrifuge is a specialized centrifuge that is used for separating isotopes of gases based on their molecular weight. It has a cylindrical rotor that spins at very high speeds (up to 100,000 rpm) and creates a strong centrifugal field. The gas mixture is introduced into the rotor through a nozzle and flows along the axis of rotation. The heavier isotopes are pushed towards the wall of the rotor while the lighter isotopes remain near the center. The gas fractions are then extracted through different outlets. Gas centrifuges are mainly used for enriching uranium for nuclear power and weapons.
• Hematocrit centrifuge: A hematocrit centrifuge is a specific type of centrifuge that is used for measuring the volume fraction of red blood cells (RBCs) in a blood sample. It has a rotor that holds capillary tubes filled with blood and spins them at high speeds (up to 15,000 rpm). The RBCs settle at the bottom of the tubes while the plasma remains at the top. The hematocrit value is calculated by dividing the length of the RBC column by the total length of the blood column. Hematocrit centrifuges are used for diagnosing conditions such as anemia, polycythemia, dehydration, and infection.
• High-speed centrifuge: A high-speed centrifuge is a powerful and sophisticated centrifuge that can reach speeds up to 30,000 rpm and generate forces up to 100,000 xg. It can accommodate different types of rotors such as fixed angle, swinging bucket, and vertical rotors and can process various types of samples such as cells, proteins, nucleic acids, and viruses. It has features such as temperature control, speed display, timer, safety lock, and imbalance detection. High-speed centrifuges are used for applications such as protein purification, DNA sequencing, virus isolation, and subcellular fractionation.
• Low-speed centrifuge: A low-speed centrifuge is a simple and basic type of centrifuge that can reach speeds up to 5,000 rpm and generate forces up to 4,000 xg. It can accommodate different sizes of tubes and rotors such as fixed angle and swinging bucket rotors. It operates at room temperature and does not have any special features or controls. Low-speed centrifuges are used for routine tasks such as blood separation, cell harvesting, urine analysis, and sedimentation.
• Microcentrifuge: A microcentrifuge is a small and convenient type of centrifuge that can process small volumes of samples ranging from 0.5 to 2 ml. It can reach speeds up to 13,000 rpm and generate forces up to 16,000 xg. It has a rotor that holds microtubes or PCR tubes and a lid that closes the working unit. Some microcentrifuges have features such as temperature control, timer, speed display, pulse mode, and safety lock. Microcentrifuges are used for applications such as DNA extraction, phenol extraction, PCR amplification, and enzyme digestion.
• Refrigerated centrifuge: A refrigerated centrifuge is a type of centrifuge that has a cooling system that maintains the temperature of the samples between -20°C to -30°C. It can reach speeds up to 60,000 rpm and generate forces up to 500,000 xg. It can accommodate different types of rotors such as fixed angle, swinging bucket, and vertical rotors and can process various types of samples such as cells, proteins, nucleic acids, and viruses. Refrigerated centrifuges are used for applications that require low temperatures such as protein precipitation, RNA isolation, lipid extraction, and organelle isolation.
• Ultracentrifuge: An ultracentrifuge is the most advanced and powerful type of centrifuge that can reach speeds up to 150,000 rpm and generate forces up to 1,000,000 xg. It can accommodate different types of rotors such as fixed angle, swinging bucket, and vertical rotors and can process various types of samples such as cells, proteins, nucleic acids, and viruses. It has features such as temperature control, speed display, timer, safety lock, vacuum system, and optical system. Ultracentrifuges are used for applications that require high resolution and precision such as density gradient centrifugation, isopycnic centrifugation, sedimentation equilibrium, and sedimentation velocity.
• Vacuum centrifuge/ Concentrators: A vacuum centrifuge or a concentrator is a type of centrifuge that uses a combination of centrifugal force, vacuum, and heat to speed up the evaporation of solvents from samples. It can process large numbers of samples (up to 148 samples at a time) and can remove solvents such as water, ethanol, methanol, and acetonitrile. It has a rotor that holds sample tubes or vials and a lid that seals the working unit. It also has a rotary evaporator that removes the evaporated solvents and prevents solvent bumping. Vacuum centrifuges or concentrators are used for applications such as sample concentration, sample drying, solvent recovery, and solvent exchange.

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