Sedimentation
Particles in a suspension will settle in the bottom of a vessel over time. This is due to gravity, which is the effect of the earth's gravitational field. This force is expressed as g. Centrifugation increases the rate of sedimentation by spinning the samples and creating a centrifugal force that acts on the particles.
Revolutions per minute (RPM)
The speed of rotation in a centrifuge is expressed as revolutions per minute.
Radius of rotation ®
The distance from the centre of rotation to the position in the centrifuge tube of the material being spun is the radius of rotation measured in centimeters. In examples of centrifugation used in this manual, this is the distance from the centre of the centrifuge to the bottom of the tube.
Relative centrifugal field (RCF)
Relative centrifugal force is the force acting on samples during centrifugation. It is expressed as multiples of the earth's gravitational field (g).
For example 500 g = 500 RCF
Relationship between RCF, RPM and r
RCF, RPM and r are linked by the equation for calculating RCF.
RCF = 11.2 × r (RPM/1000)2 or RCF = 1.12 × 10-5 (RPM)2. This equation can be rearranged to calculate RPM from a given RCF.
RPM = 1000Ö(RCF/11.2 × r)
Summary
In this manual, instructions for centrifugation are given as spinning at a given RCF (g) for a certain length of time. This value can then be used to calculate the correct RPM for the centrifuge available. A Table shows RCF values rounded to the nearest 10 for a typical bench top centrifuge with a radius of 18.6 cm.
Table 9: Relationships between RCF (g) and RPM for centrifuge with radius ® = 18.6 cm
|
RCF g |
RPM |
|
500 |
1 500 |
|
850 |
2 000 |
