Biomedical Engineering Reference
In-Depth Information
5 Centrifugation
5.1 Speed vs Centrifugal Force Graphs
If particles with densities (specific gravity) larger than the sur-
rounding liquid are distributed in a solution or suspension, and
if a force acts which overcomes random thermal motion of these
particles, the particles will move to form a stable pellet. The ve-
locity of sedimentation depends on the size of the particle, on the
density difference between particle and solution, on the viscosity
of the solution, and on the force which causes the movement: the
gravitation or the centrifugal force.
Molecules are surrounded by and filled with liquid (mostly
water). Because at first view liquids are not compressible, the size of
moleculesdoesnotchange,evenathighpressure.Butlargebiologic
particles, such as cells or cellular organelles (even proteins), are no
bodies uniformly filled with liquid. High pressure produced by high
centrifugal forces may irreversibly deform these organelles up to
disruption.
The force produced in a rotating body is given as multiples of
earth's gravitational field (9.807 m
/
s
2
), and relative centrifugal field
(rcf) is calculated by
rpm
1000
2
·
ω
r
2
=
=
rcf
1119
·
r
·
[x g]
g
ω
=
·
π
·
rpm
/
2
60
r: distance of a particle or part of a centrifuge rotor from the
axis of rotation (mm);
ω
: angular velocity (radians s
−1
); g: earth's
s
−2
); rpm: revolutions per minute (min
−1
)
In a centrifuge rotor g
max
is the maximal centrifugal force at
maximal distance from rotor axis r
max
(at the bottom of the tube),
g
min
is the minimal force at the top of the tube (r
min
), and the
arithmetic mean of both is called g
av
. The difference of r
max
and
r
min
is the sedimentation path length.
In the biochemical lab mostly two types of centrifuge rotors are
used. Fixed-angle rotors have tubes with defined angle relative to
rotor axis. These angles range from 18 to 45
◦
.Withanangelof0
◦
,
i.e., the tube is parallel to the axis, the fixed-angle rotor is a so-called
vertical rotor. The other type of rotor is the swinging-bucket rotor.
gravitational field (m
·
