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Use the k factor in the following equation to estimate the run time t (in hours) required to pellet
particles of known sedimentation coefficient s (in Svedberg units, S).
Run times can be estimated for centrifugation at less than maximum speed by adjusting the k factor
as follows:
Run times can also be estimated from data established in prior experiments if the k factor of the
previous rotor is known. For any two rotors, a and b:
where the k factors have been adjusted for the actual run speed used.
Run Speeds
The centrifugal force at a given radius in a rotor is a function of speed. Comparisons of forces
between different rotors are made by comparing the rotors' relative centrifugal fields (RCF). When
rotational speed is selected so that identical samples are subjected to the same RCF in two different
rotors, the samples are subjected to the same force. The RCF at a number of rotor speeds is provided
in
Table
3.
Do not select rotational speeds in excess of 32,000 RPM. In addition, speeds must be reduced under
the following circumstances:
1. If nonprecipitating solutions more dense than 1.2 g/mL are centrifuged, reduce the maximum
allowable run speed according to the following equation:
where is he density of the tube contents. This speed reduction will protect the rotor from
excessive stresses due to the added tube load.
2. Further speed limits must be imposed when CsCl or other self-forming-gradient salts are
centrifuged, as equation (6) does not predict concentration limits/speeds that are required to
prevent precipitation of salt crystals. Solid CsCl has a density of 4 g/mL, and if precipitated
during centrifugation may cause rotor failure.
description and examples below, show how to reduce run speeds when using CsCl gradients.
LXL-TB-015BB
SW 32 Ti and SW 32.1 Ti Swinging-Bucket Rotors
k
t
=
-- -
s
32 000
=
k
k
--------------------------------------- -
adj
actual run speed
t
k
a
a
=
---- -
------
t
k
b
b
reduced maximum speed = 32,000 RPM)
Figure 2
Preparation and Use
2
1.2 g/mL
--------------------- -
through
Figure
5, together with the
EQ 3
EQ 4
EQ 5
EQ 6
13
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