Instrumentation

Rotors in Centrifuge

Rotors in centrifuge

A centrifuge rotor is rotating unit of the centrifuge, which has fixed holes drilled at an angle. Test tubes are placed inside these holes and rotor spins to aid in the separation of materials.

Rotor is the part of centrifuge for holding the tubes with solution for separation through centrifugation with help of centrifugal field.

There are four types of rotors used in centrifuges:

    • Fixed-angle rotor
    • Swinging-bucket rotor
    • Vertical rotor
    • Zonal rotor

 

Low-speed rotors are usually made of steel or brass & high speed rotors are made from either aluminum, titanium or from fiber-reinforced composites. Titanium rotors are stronger and more chemical resistant than the aluminium rotors. Exterior surfaces of titanium and composite rotors are finished with black polyurethane paint. Titanium buckets and lids of high-performance rotors are usually painted red for identification.

 Rotors in centrifuge

 

Swinging bucket rotor

Swing-buckets can support two types of separations: rate-zonal and isopycnic. Swing-buckets are preferred for rate-zonal separations, because the distance between the outside of the meniscus and the outside of the bottom of the tube is long enough for separation to occur.

Swinging-bucket rotor are used for pelleting, isopycnic studies and rate zonal studies. Tubes are attached to rotor body by hinge pins or a crossbar. The buckets swing out to a horizontal position.

  • Isopycnic studies (separation as a function of density).
  • Rate zonal studies (separation as a function of sedimentation coefficient).

 

Swinging Bucket Rotor

 

When rotors accelerates, solution in tube reorients to lie perpendicular to axis of rotation and parallel to applied centrifugal field. When rotor deaccelerates, tubes fall back to their original position and solution too regains its original orientation.

Longer distance to travel allows better separation. It is easier to withdraw supernatant without disturbing pellet.

 

Fixed angle rotor

Fixed-angle rotors are general-purpose rotors that are especially useful for pelleting subcellular particles and in short column banding of viruses and subcellular organelles. Tubes are held at an angle (usually 14 to 40 degrees) to the axis of rotation in numbered tube cavities.

Under centrifugal field, particles move radially outwards, travel only a short distance before they strike the wall. The particles then slide down the wall and pellet is formed at outermost point of tube. This sliding down makes sedimentation quicker (wall- effect).

As sedimenting particles have short distance to travel before pelleting, they have short run time.

 Rotors in centrifuge

 

Vertical rotor

Vertical rotors hold tubes parallel to axis of rotation therefore, bands separate across diameter of tube rather than down length of tube. Vertical rotors are useful for isopycnic and, in some cases, rate zonal separations when run time reduction is important. Vertical rotors are highly specialized. They are typically used to band DNA in cesium chloride.

Tubes lie parallel to the rotor shaft. As rotor accelerates and centrifugal field is applied, solution within tube reorients through 900. This reorientation makes it lie perpendicular to axis of rotation. As rotor decelerates, solution orients back to original position. Sedimentation in these rotors occurs across diameter of the tube. The particles thus have to travels shortest possible distance and sedimentation is quicker than other rotors.

 

Zonal rotor

A zonal rotor is divided into four quadrants (zones, or sectors). The density-gradient medium is pumped into the sectors while the rotor is turning at low speed. At high speed, a gradient forms with same density at same radius in each sector, in effect creating a series of concentric rings.

Once the gradient is formed, molecules or organelles introduced into rotor through the hub will migrate to radius where they have same density as gradient.

 

Download Link: Centrifuge Rotor.pdf

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