Clearing factor

In centrifugation the clearing factor or k factor represents the relative pelleting efficiency of a given centrifuge rotor at maximum rotation speed. It can be used to estimate the time t (in hours) required for sedimentation of a fraction with a known sedimentation coefficient s (in svedbergs):

$t = \frac{k}{s}$

The value of the clearing factor depends on the maximum angular velocity ω of a centrifuge (in rad/s) and the minimum and maximum radius r of the rotor:

$k = \frac{\ln(r_{\rm{max}} / r_{\rm{min}})}{\omega^2} \times \frac{10^{-13}}{3600}$

As the rotational speed of a centrifuge is usually specified in RPM, the following formula is often used for convenience:

$k = \frac{2.53 \cdot 10^5 \times \ln(r_{\rm{max}} / r_{\rm{min}})}{(\rm{RPM}/1000)^2}$

Centrifuge manufacturers usually specify the minimum, maximum and average radius of a rotor, as well as the k factor of a centrifuge-rotor combination.

For runs with a rotational speed lower than the maximum rotor-speed, the k factor has to be adjusted:

$k_{\rm{adj}} = k \left( \frac{\mbox{maximum rotor-speed}}{\mbox{actual rotor-speed}} \right)$²

References

• Beckman Coulter lab resources and manuals
• Appendix F of the Cell Biology Laboratory Manual