Biochemical Techniques and Instrumentation

4.1.6. Rotor categories

Rotors can be broadly classified into three common categories.

1. Swinging-bucket rotors
2. Fixed-angle rotors
3. Vertical rotors

Low-speed rotors are usually made of steel or brass, while high-speed rotors are made of aluminum, titanium or fibre reinforced composites.

1. Swinging bucket rotor

In swinging bucket rotor, the sample tubes are loaded into individual buckets that hang vertically while the rotor is at rest. When the rotor begins to rotate, the buckets swing out to a horizontal position. This rotor is useful when samples are to be separated based on density gradients. This rotor is inefficient for pelleting. 

2. Fixed angle rotor

In fixed-angle rotor, the sample tubes are held fixed at the angle of the rotor cavity. When the rotor begins to rotate, the solution in the tubes reorients to effect the separation. This rotor is most commonly used for pelleting by differential separation of biological particles. It is also useful for isopycnic separations of macromolecules such as nucleic acids.

3. Vertical rotor

In vertical rotor, sealed tubes are held parallel to the axis of rotation. Samples are not separated down the length of the centrifuge tube, but across the diameter of the tube. The isopycnic separation time is shorter in this rotor as compared to swinging bucket rotor. This rotor is not suitable for pelleting applications but is most efficient for isopycnic separations due to the short pathlength. Most common applications are isolation of plasmid DNA, RNA, and lipoproteins.