The Interdepartmental Equipment Facility

Ultracentrifuge

The Sorvall WX 90 ultracentrifuge can run at speeds as high as 90,000 rpm. Rotors spin in an evacuated and refrigerated chamber, to reduce friction and heating of rotor and samples.

Specifications

model - WX 90.
Maximum Speed - 90,000 rpm.
Maximum RCF - 429,643 x g
Speed control Accuracy - 10 rpm
Set Speed - 100 rpm to maximum speed, in 100 rpm increments

Centrifugation

Centrifugation is used to accelerate the rate of sedimentation. Particles in solution may gradually settle even under normal gravity, if they are large enough. The settling rate depends on the density, size and mass of the particles. Gravity however is not enough to sediment cellular organelles and macromolecules and a centrifuge is therefore used to accelerate settling of such particles. Spinning the samples at high speed in the ultracentrifuge creates forces that are up to 500,000 times stronger than gravity. The Relative Centrifugal Force (RCF) is indeed measured in i.e. in multiples of g - the gravitational force.

Particles of different size will travel at different rates under a centrifugal forces, and will take different times to sediment. Centrifugation is indeed used to isolate and separate different classes of particles.

Rotors - fixed angle vs. Swinging Bucket

The rotors we offer for use are defined as either 'fixed angles' or 'swinging buckets' .Particles precipitate faster in a fixed angle rotor, where they are first thrown against the side wall of the tube and then slide down this wall to the bottom. Note that friction with the wall might damage some particles such as chloroplasts, whose outer membrane can be easily disrupted. In swinging bucket rotors, particles travel through the liquid straight down the entire length of the tube. The journey is gentler but takes longer. The rotors available in the lab are listed below.

Rotor Tubes

Regular glass centrifuge tubes cannot be used in the ultracentrifuge as they shatter at speeds exceeding 3,000 RPM. Tubes made of Corex-glass can handle speeds up to 18,000 RPM and polypropylene or polyethylene tubes are good for speeds up to 20,000 rpm. For higher speeds use polyallomer tubes. These tubes are more expensive but they are reusable. They are chemically inert and have smoother walls. Thin-wall tubes must be run full and should be properly capped and sealed. Thick-wall tubes should be at least half full. All tubes should be examined for cracks. Tubes may not leak before the run but the cracks will open under the centrifugal forces.

Speed Calculator

Published procedures involving a centrifugation step usually cite the required speed in g-units (i.e. RCF or Relative Centrifugal Force). The centrifuge speed however, is set in rpm (Revolutions Per Minute). In order to convert g-values to rpm, for one of the rotors available in the lab, use the following table.