Rotors used in a centrifuge are categorized mainly into fixed angle rotors, swinging bucket rotors, and vertical rotors depending upon the type of centrifugation, speed of centrifuge, and volume of sample.
Other commonly used rotors are continuous flow rotors, elutriator rotors, and zonal rotors. Among these rotors, the fixed-angle and the swinging-bucket rotors are most commonly used for tabletop, low-speed, and high-speed floor-model centrifuge. The vertical rotors are used for ultracentrifugation. Rotors used for slow speed centrifugation are made up of steel, brass or perspex. Rotors are fabricated from range of materials, such as carbon fiber, aluminum, and titanium for high-speed centrifugation to withstand larger stress.
Laboratory centrifuges are routinely used for advance research and general laboratory work. Proper selection of type of centrifuge and rotor system for specific application can be a challenge. Furthermore, since rotors are significant, proper level of care and maintenance is also important.
Fixed angle rotor
In these rotors (Figure 4), particles move radially outward under the influence of centrifugal field and they travel very short distance before reaching the outer wall of the centrifuge tube. Two factors determine type of fixed-angle rotor required in a centrifuge. These are desired g-force (RCF) and the desired volume of the sample.
Swinging-bucket rotor
Swinging-bucket rotors (Figure 3) are ideal for separating large-volume samples (up to 12 L) at low speeds. A swinging-bucket rotor has the following main parts:
- Rotor body having four or six arms to support buckets attached to the centrifuge drive,
- Buckets placed onto the arms of the rotor body, and
- Trunnion pins used to hold the buckets in place.
Additional accessories are added when needed for specific applications. For example, large-volume rotors offer a wide variety of adapters that can be placed into the buckets to hold desired tube size. Certain buckets offer sealing lids, having bio-containment for potentially hazardous samples.
Particles in centrifugal field fan out radially from the center of rotation rather than sedimenting in parallel lines. This generates convection currents. Strohmaier cells control the convection and swirling effects by slowly accelerating and subsequently decelerating rotor and by using density gradients. Most common benchtop and high-speed centrifuge require a swinging-bucket rotor used in high-throughput protocols, such as batch harvesting of whole cells from growth media, large volume tissue culture processing, high-capacity processing of blood collection tubes, and separation of small range structured viruses. An ultracentrifuge with this rotor can support both rate-zonal (i.e., based on mass or size) and isopycnic (i.e., based on density) separations.
Vertical tube rotor
Vertical rotors are commonly used in ultracentrifugation for isopycnic separations, specifically for banding of DNA in cesium chloride. Vertical rotors have very low K factors (in the range of 5–25), indicating that it has shortest path length for the particles to travel, finally causing sedimentation of particles across diameter of the tube. Thus, sedimentation of particles in vertical tube rotors is quick in comparison to fixed angle or swinging bucket rotors. It is generally used for the isolation of plasmid DNA and calculation of sedimentation co-efficient.
Other rotors
Continuous flow rotors are used to separate small quantities of matter from the large volume of suspension. Elutriator rotor is a type of continuous flow rotor used to separate mononuclear leucocytes in human blood, endothelial cells, fat storing cells in liver, and rat brain cells. A liquid buffer carries rotor for separation and collection of specimen particles by continuous centrifugal elutriation. In this rotor, interior of the rotor itself is employed as a separation chamber.
Zonal rotors are used for large-scale separation of particles on density gradients. They can contain up to 2 liters of solution and can work with tissue samples measured in ounces. Capacity of this rotor is 300cm3 to 2000cm3. They are designed to minimize the wall effect encountered in swinging bucket and fixed angle rotors.