CENTRIFUGING — the method of unmixing of particles based on distinction in the speed of their sedimentation under the influence of centrifugal force. When acceleration of centrifugal force exceeds acceleration of gravity (g) in hundreds of thousands of times, C. call ultra-centrifuging (see).
In medicine of C. use for division of blood samples into plasma and uniform elements, including for definition of gematokritny number (see), departments of an urocheras, etc. In a medico-biol. researches C. widely apply to fractionation of homogenates of fabrics, in qualitative and quantification, etc.
During the centrifuging the centrifugal force (F) operating on a particle with the mass of t depends on the speed of rotation of a rotor and distance (g) of a particle from an axis of rotation:
where with — the angular speed of rotation of a separating bowl (I am glad / sec.), p — the speed of rotation of a rotor (rpm).
Except centrifugal force the particle is affected still by the pushing-out force (buoyancy) equal At • r0 • o) 2 •, and friction force equal to f-v where r — density of a particle, Ro — density of solvent, with V — the volume of a particle> — the speed of its movement and / — coefficient of friction. At ustano-
the curling mode of centrifuging of force, acting on a particle, are mutually counterbalanced and then the speed of the movement of a particle is calculated on a formula:
From a formula, follows that rate of sedimentation of a particle depends on its mass and density, density of solvent, size of coefficient of friction. Sedimentation properties of particles are usually characterized by coefficient of sedimentation (S):
measured in svedberga (see Sedimentation). The volume, density and coefficient of friction are connected with the mass of particles. On the basis of empirically established dependences of these sizes and theoretical calculations it is obviously possible, having defined coefficient of sedimentation, to calculate the mass of particles or, for molecules, their molecular weight.
Depending on the purpose distinguish preparative and analytical C. At preparative C. initial biol. material is taken in large numbers (from several tens milliliters to several liters) and the allocated components use for further researches. At analytical C. initial biol. the material which underwent, as a rule, prerefining is taken in small amounts (no more than 1 ml) and subjected to centrifuging on analytical ultracentrifuges (see the Centrifuge). The data obtained as a result of such centrifuging can give an objective idea of purity, a pier. the weight (weight) and structure of the studied substance or substances.
C. as well as ultracentrifuging, is divided into differential, zone and high-speed and equilibrium (isopycnic). In medicine the C is used, as a rule, differential., based on different deposition rates of particles on a bottom of a test tube. This method found big application in hematology for division of blood (see) on plasma and uniform elements. So, by means of differential C. from donor blood receive the fraction enriched with thrombocytes. Specially designed devices use for selective removal from blood of erythrocytes, thrombocytes, leukocytes or plasma by use of centrifuges fractionators of blood. However the fractions received by means of differential C., may contain also impurity of other fractions that it is necessary to consider a lack of a method.
Bibliography: Cm, bibliogr. to St. Centrifuge. I. Ya. Vostrikov.