Sedimentation Analysis

an aggregate of methods for determining the size of particles in dispersed systems by establishing the sedimentation rate and the parameters of the sedimentation-diffusion or sedimentation equilibrium. Sedimentation analysis makes it possible to determine both the average characteristics of the dispersion and the distribution of particles of the dispersed phase relative to size or mass. The principal methods of sedimentation analysis involve the determination of the sedimentation rate and the determination of the sedimentation-diffusion or sedimentation equilibrium. Other methods include approximation of the sedimentation equilibrium and the sedimentation in a density gradient.

Sedimentation analysis in a gravitational field is used for dispersed media with relatively massive particles, such as suspensions, emulsions, and dusts, with particle sizes of 10^-2 to 10^-4cm. In such cases, the usual procedure is to determine the sedimentation rate; the values sought are found according to the change in the rate of accumulation of the sediment (as is done with cream), the density of a suspension in a column (as with an emulsion), or the concentration of particles at a given level. Instruments for carrying out this method operate by weighing, for example, the precipitate, or by measuring the hydrostatic pressure.

Highly dispersed systems with particle sizes less than 10^-4 cm do not exhibit sedimentation under normal conditions; in such cases, sedimentation analysis is carried out in a centrifugal force field. The use of the centrifuge for sedimentation was proposed by A. V. Dumanskii in 1912. The detailed development of sedimentation analysis methods was carried out by the inventor of the ultracentrifuge, T. Svedberg. The centrifugal accelerations produced in the ultracentrifuge are tens or hundreds of thousands of times greater than the acceleration of the earth’s gravity. This provides for the sedimentation not only of very fine colloidal particles but also of molecules of high molecular weight compounds.

In sedimentation analysis in an ultracentrifuge, the sedimentation constant—the ratio of the sedimentation rate to the acceleration of the centrifugal force field—is used to characterize the particles of the dispersed phase or molecules of a dissolved polymer. The svedberg (l0^-13 sec) has been adopted as the unit of measurement of the sedimentation constant. This constant depends on the mass and shape of the particle or macromolecule and varies from 1 to 200 svedbergs for proteins. Optical measurements are used to determine the sedimentation rate or sedimentation equilibrium in an ultracentrifuge, the sedimentation constant, the mass and size of colloidal particles or macro-molecules, and the polydispersity of the system being analyzed. Such measurements are based on the change in the refractive or transmittance indexes of a solution or colloidal system.

Sedimentation analysis in a gravitational field is widely used to determine the dispersion composition of finely ground materials, soils, and industrial dusts. Sedimentation analysis in a centrifugal force field is used to determine the molecular weight and homogeneity of various polymers, including biopolymers. In biochemistry and molecular biology, sedimentation analysis makes it possible to determine the complex composition of cellular structures, establish the dimensions of viruses, and separate lipoproteins with varying ratios of lipide to protein components.