Centrifugation

The old testament tells the story of David, who slew Goliath with a stone thrown by his sling. It was centrifugal force that made the stone fly far faster than if David had simply thrown the stone. Without the sling he might just have made Goliath even madder, with David paying the price.

That would have changed history an even affected our I-Phone contacts list. There would be dozens of friends and family named Goliath but no Davids.

In the lab, centrifugation is one of the principle separation methods. All centrifuges have rotors which hold centrifuge tubes or bottles. As the tube spins, the heavier and larger particles migrate to the bottom of the tube while smaller, lighter particles settle nearer the top of the tube.

If you take a bucket full of big rocks, smaller pebbles and sand and dump it into a swimming pool the big rocks will hit the bottom first, then the pebbles and finally the sand. This is what happens in the centrifuge too. The particle of interest could be the rock and could be the sand, but in either case the researcher can extract just that particle size.

When selecting a centrifuge, RPM’s (revolutions per minute) are not the key parameter. The key parameter is “G” force. For example, If you have two tubes in the same rotor, one and inch long and the other six inches long, the “G” forces will be far greater at the bottom of the longer tube that the shorter one, yet the RPM’s are identical. So, “G” forces are what determines the right unit for the given application.

There are dozens of types and models of centrifuges, and we carry practically all the varieties.

Slow spinning units are needed in the clinical setting (microcentrifuges) and blood bank centrifuge applications. If the “G” forces are too great, the blood cells would disintegrate.

On the extreme other side of the centrifuge spectrum are the ultracentrifuges, often used for protein stratification based on molecular weight. These units can generate 100,000 times “G”.

The forces are so great that the tubes in the rotor must weigh exactly the same as the other tubes in the rotor, or the unit can go off balance with catastrophic consequences. I have a friend who actually saw this happen but luckily everyone made is out of the lab before it exploded, sending pieces of metal flying all over the place. Today these units MUST come with alarms indicating off balance conditions.


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