How to convert Centrifuge RPM to RCF or G-force?
Translational research is finding its way into all phases of clinical trials which means more and more research nurses are finding themselves with the task of collecting and processing samples. These samples are very valuable resources and crucial to the overall success of research.
This requires many research nurses to learn and develop knowledge and skills in the lab not only to work safely in a laboratory setting but also to maintain integrity of the samples processed. It can feel challenging and overwhelming when first reading instructions and lab manuals to guide us in processing samples. When you are lucky, the lab handbook gives you the speed at which your samples must be spun in the same language spoken by your centrifuge, either RPMs or G-force. But what do you do when it doesn’t? The answer is easier than you think…
The force exerted on a particle in a centrifuge is a simple function of the rotation speed of the centrifuge and the radius of rotation. The actual equation is:
RCF or G-force= 1.12 x R x (RPM/1000)²
R is the radius of rotation measured in millimetres. For example in the photograph below R is 240mm.
R can be measured at the top of the tube (Rmin), the middle of the tube (Rav) or the bottom of the tube (Rmax). If your protocol does not specify, you can use chose but if you are trying to pellet something out, you should probably use Rmax as the pellet forms at the bottom of the tube.
Particles in a suspension will settle in the bottom of a vessel over time, this is called sedimentation. The particles fall to the bottom due to gravity, in the same way that herbs in a salad dressing settle at the bottom of the bottle. This force is expressed as G. Centrifugation increases the rate of sedimentation (i.e. accumulation of red and white bloods cells in the bottom of a blood tube) by spinning the blood samples and creating a centrifugal force that acts on the particles (in this case, red and white blood cells).
RPM stands for ‘revolutions per minute’. This is how centrifuge manufacturers generally describe how fast the centrifuge is going (i.e. revolving). The rotor, regardless of its size, is revolving at that rate. The force applied to the contents, however, varies according to the size of the centrifuge as a larger centrifuge will have a longer radius and a smaller centrifuge will have a shorter radius.
For example, when revolving at 2000 RPM, a larger centrifuge with a longer radius length will spin samples at a higher g-force than a smaller centrifuge with a shorter radius length.
If you know at what g-force you need to spin your samples and you can measure the radius on your centrifuge, you can figure out the speed or RPM you need to set your centrifuge by using a Nomograph, as set out below.
You may also use this equation if you can’t access a nomograph.:
G-force = 0.000001118 x R x RPM²
When you know the G-force at which a sample should be spun, you can measure the radius of your centrifuge and determine the revolutions per minute at which to set your centrifuge.
The take home message is that centrifugation speeds quoted in RPM will only be constant for centrifuges with the same rotor radii. If you use an RPM setting from a protocol where someone used a centrifuge with a different radius from yours, you will get a different G-force. Often the difference will not be significant enough to affect the sample but it is always good practice to standardise as much as possible how samples are processed at various institutions using different equipment.
Sample collection and processing are very important aspects of clinical research and the research nurse’s skills in the laboratory are crucial not only in maintaining a safe practice but also in ensuring sample integrity. Results of studies are dependant on the quality of the collection and processing of these samples and good lab skills help research nurses achieve this.