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In many lab protocols, a specific centrifugation rate and time is usually required.

For example, DNA precipitation protocols usually require that the precipitated DNA be spun at 12,000 g for 30 minutes.

However, Sigma-Aldrich's guide to centrifugation states that the rate of sedimentation is proportional to the centrifugal force.

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This seems to suggest that as long as the centrifugation is carried out for longer, the total amount of sedimentation will remain constant at a slower speed.

For example, if the lab did not possess a centrifuge capable of reaching 12,000 g, the same sample can be sedimented to the same extent by spinning for 90 minutes at 4,000 g. Is this true? If not, why is this the case? Are there any other factors that come into play when a different centrifugation speed is used?

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  • $\begingroup$ The sedimentation rate will not be constant. Test tubes in the centrifuge are usually angled with respect to the axis of rotation. Particles that settle towards the bottom of the test tube will be increasing their radius wrt the axis of rotation, so they will experience ever higher forces as they do this, and their sedimentation rate will increase as a result. In addition, assuming that your lab is doing business with customers, "time is money", so supervisors want the job done quickly. Hence, they prefer 12,000 g rather than 4000 g. $\endgroup$ Aug 7, 2015 at 14:48

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