Centrifugation Techniques

When scientists want to isolate a specific type of biological tissue or other material, they often use the process of centrifugation. This involves spinning the sample at high speeds to cause the different components of the sample to separate out into layers based on their mass or density. By repeating this process at different speeds, with slightly different techniques, researchers can remove specific cells, organelles or molecules.

Pellet

• One of the most important aspects of centrifugation is pelleting. Spinning samples at high speeds causes the gravity acting on the sample to increase. This compresses the heavier particles in the sample into a hard pellet at the bottom of the centrifuge tube. The speed determines which sizes of particles will end up in the pellet. Faster speeds correspond with a wider range of particles in the pellet, and slower speeds will pelletize only the largest particles.

Supernatant

• After a sample has been centrifuged, the remaining liquid that has not become part of the pellet is known as the supernatant. It consists of smaller, lighter particles that were not pulled to the bottom by the increased gravity. This supernatant can then be collected and used for testing or, if necessary, subjected to further spinning to continue to isolate a target material. Supernatants are often removed from the top of the centrifuge tube with a pipette or micropipette.

Isolating Based on Mass

• One of the most commonly used techniques to separate particles based on mass is differential centrifugation. In this process a sample is first spun at a relatively low speed. The supernatant is then extracted and spun at a higher speed. A pellet will form in this tube as well, and scientists will continue to remove and spin the supernatants at higher speeds and for longer times to get smaller and smaller particles, until they have isolated a smaller sample containing the target material.

Isolating Based on Density

• Although differential centrifugation can get rid of most materials larger than the target, the particles that are smaller still remain. To further isolate the target, scientists can isolate based on the density of the particles. In this process a centrifuge tube is filled with layers of media of different densities, from most dense to least. When the sample is placed on top and centrifuged, the particles separate into different layers based on their densities. This way the scientists can get just the material they want.