CENTRIFUGATION
= separation of particles from a solution according to their size, shape, density, viscosity of the
medium and rotor speed
- centrifugal force:
P = m x r x 2
- the particles are usually
m - mass of the particle
r - radius of the centrifuge
 - angular speed
* cells (e.g. blood cells / plasma)
* sub cellular organelles
* viruses
* large molecules (proteins, nucleic acids)
centrifuges
- horizontal or fixed-angle rotor heads
- speed: revolutions per minute (rpm)
(the same rpm doesn´t mean the same centrifugal force in different centrifuges)
- centrifugal force: relative centrifugal force (RCF)
RCF = 1.12 x 10-5 x r x (rpm)2
r - radius of the centrifuge (cm)
units: G (= how many times is the acceleration higher than the gravitational acceleration)
G = 9.81 m x s-2
rpm
RCF
a) usual centrifuges in chemistry:
up to 6000
up to 7300 G
b) microhematocrit centrifuges:
11,000 - 15,000
up to 14,000 G
c) ultracentrifuges:
(refrigerated)
90,000 - 100,000
up to 178,000 G
example:
RCF at least 1000 G for 10 min will give a good separation of clotted blood from serum
* Serum separation tubes contain a silicone gel with specific gravity intermediate to that of serum and
blood coagulum. When these tubes are centrifuged, the gel is displaced up the side of the tube forming
a stable, inert barrier between the serum and the clot.
! ! ! The centrifuge cups in the rotor must be balanced before cenrifugation
Classification of centrifugation
- based on the purpose of centrifugation
1) analytical centrifugation
2) preparative centrifugation
analytical centrifugation
- it involves a measuring of the physical properties of the sedimenting particles
(sedimentation coefficient, MW)
- ultracentrifugation is optimal
- molecules are observed by optical system during centrifugation and projected on to a film or
a computer
preparative centrifugation
- separation of particles from a solution: we obtain two fractions called a sediment (pellet, solid phase)
and a supernatant (liquid phase)
* special kind of the preparative centrifugation:
differential centrifugation (= moving boundary, rate-zonal c.)
- it is used for a separation of cell organelles which differ in size and density
- large, dense structures form a sediment (pellet) in a centrifuge tube faster (low RCF is enough for
separation) than small, less dense ones do
 supernatant obtained from a low speed centrifugation is centrifugated again (a number of
time) using a higher RCF