Page 23 - Subyek Encyclopedia - Encyclopedia of Separation Science

18 I / CENTRIFUGATION/ Derivatization

centrifuges were introduced in 1910, further acceler- where m is the mass of the object and g is the acceler-
ating centrifuge development. Svedberg’s invention of ation due to gravity.
the analytical ultracentrifuge in 1923, operating at In an idealized case of a free-falling object being
10 000 rpm and equipped with transparent observa- accelerated by gravity in a vacuum, the velocity of the
tion windows, marked another milestone in centri- object would exhibit a uniform rate of increase. How-
fuge technology. In the 1940s, the isolation of the Rrst ever, for a real-world case of an object falling through
subcellular components by centrifugal techniques not air, or more appropriately for our purposes, settling
only served to revolutionize our knowledge of the in a liquid medium, there are two forces that oppose
structure, composition and function of intracellular the gravitational force; the buoyancy force, F b , and
components, but demonstrated the potential of cen- the frictional force, F f .
trifugal methods for biomedical research. Although Buoyancy force The buoyancy force was Rrst noted
temporarily abandoned in 1943 in favour of a gas- by Archimedes, who showed that a particle sus-
eous diffusion process, industrial-scale gas cen- pended in a Suid experiences an upwards force that is
trifuges were rapidly developed during World War II equivalent to the weight of the Suid displaced:
in an effort to enrich or separate uranium iso-
topes. In 1943, Pickels was the Rrst to employ a suc- F b "m M g"V p M g [2]
rose-based density gradient to measure particle sedi-
mentation rates. Density gradient centrifugation was where m M is the mass of the Suid medium displaced,
further reRned in the 1950s by Brakke, who applied V p is the volume of the particle ("volume of the dis-
the concept to puriRcation and characterization of
placed Suid), and M is the density of the displaced Suid.
viruses, and by Anderson and co-workers at Oak 5
At pressures up to several bars (1 bar"10 Pa), the
Ridge National Laboratory, who designed a series of
buoyancy force in air or other gaseous media can be
zonal centrifuge rotors for separation of subcellular
neglected to a Rrst approximation with respect to the
particles and viruses. More recent advances have
net gravitational acceleration experienced by solids
been characterized by signiRcant improvements in
or liquids. However, in a liquid medium, the buoy-
materials and equipment and a broadening range
of applications. ancy force is substantial. Since the volume of the
settling material is equal to the volume of the Suid
Today, centrifuges are routinely used in a variety of
disciplines ranging from large-scale commercial ap- being displaced, the net gravitational force experi-
enced by the particle is proportional to the differ-
plications to laboratory-scale scientiRc research. The ence between the mass of the particle and that of the
number of centrifuge designs and conRgurations used displaced medium. Thus, assuming gravity sedi-
in the mineral, petrochemical, chemical, medical, mentation of a spherical particle with radius r and
pharmaceutical, municipal/industrial waste, dairy, volume of 3 r , eqn [1] can be rewritten to show the
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food, polymer, energy and agricultural industries (to net gravitational effect, F g-net :
name a few) seem almost as numerous as the applica-
tions themselves. An in-depth description of centri- F g-net " 3 r ( p ! M )g" 3 r ( p ! M ) 980 cm s 2
3
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fuge designs and applications is, therefore, well beyond
the scope of this treatise. Instead, this article will [3]
present the reader with an introduction to the theory of
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centrifugation, an overview of the various types of where M is the density of the medium (g cm ); p is
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centrifugal separations, and a description of selected the particle density (g cm ); and r is the particle
rotor/centrifuge designs and their more common radius (cm).
applications. For those instances in which the medium density is
greater than the density of the material in suspension,
the net effect is negative, that is, particles would
Theory experience a net upward force in such instances and
would tend to rise through the medium.
Sedimentation by Gravity
A particle suspended in a liquid medium of lesser Frictional force, F f In addition to the buoyancy
density tends to sediment downward due to the force force, the movement of a particle through a Suid
of gravity, F g . Newton showed that an object is accel- medium is hindered by the viscosity of the medium, ,
erated by the gravitational force according to the as described for a spherical particle by Stokes’ equa-
relation: tion:

F g "mg"m 980 cm s 2 [1] F f "6 r(dx/dt) [4]