Centrifugation speed is primarily measured in Revolutions Per Minute (RPM), indicating how many times the rotor spins in one minute.
While RPM is the common way to express speed, it's important to understand its implications and limitations.
Understanding RPM
- Definition: RPM simply quantifies the number of full rotations the centrifuge rotor completes every minute.
- Direct Indication: It tells you how fast the rotor is physically spinning.
The Importance of Relative Centrifugal Force (RCF)
Although RPM is a common measurement, Relative Centrifugal Force (RCF), often expressed as 'g' (times the standard acceleration due to gravity), is a more accurate and relevant measure for determining the effectiveness of centrifugation. RCF accounts for the radius of the rotor, providing a standardized measure of the force applied to the sample.
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Why RCF is Better: The force experienced by the sample is what actually separates components. Since different centrifuges have rotors of different sizes (radii), the same RPM will generate different forces. RCF normalizes this.
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RCF Calculation: RCF is calculated using the following formula:
RCF = 1.118 x 10-5 x r x (RPM)2
Where:
- RCF is the relative centrifugal force in g
- r is the radius of the rotor in centimeters (cm)
- RPM is the revolutions per minute
Converting Between RPM and RCF
Many centrifuges allow you to set either RPM or RCF. If you only have RPM available and need a specific RCF, you must know the radius of your rotor. Online calculators and nomograms are available to simplify the conversion.
In Summary
Centrifugation speed is commonly measured in RPM, but RCF provides a more accurate representation of the force applied to the sample, taking into account the rotor radius. RCF is crucial for replicating separation protocols across different centrifuges.
