Density, separation techniques based

Since proteins are polymers of amino acids, the chemical nature of the amino acid side chains and the order of the amino acids play an important role in establishing the biological properties of the active protein. Proteins may differ from each other according to size, charge density, shape and biological activity. Similarly, protein purification schemes require a similar diverse combination of separation techniques based on the various physicochemical properties of proteins. 

In addition, improved separation of fine particles has resulted from recent advances in density-based separation techniques. This includes the development of several new water-only devices, such as various concentrating spirals, the Kelsey Jig, the Multi-Gravity... 

Specificity is the most important requirement in gas analysis. Techniques dependent on the physical properties of the gas molecules, such as thermal conductivity, density, viscosity, and sound velocity, generally have insufficient specificity to differentiate a single gas in a mixture of gases, and therefore must incorporate in the procedure some type of preliminary separation. Vapor phase fractionation (gas chromatography) is an example of a popular analytical technique based upon a physical property (thermal conductivity) of the gas that requires preliminary separation of the gases by means of special columns (molecular sieve, silica gel, etc.). 

Centrifugation is a technique that separates materials based upon their difference in densities, the rate of separation being amplified by applying a rotational force. The force is called a centrifugal force, and the apparatus providing the rotational force is called a centrifuge. 

Magnetic fluids can be used as a high-density solution for the sink-and-float separation of solids in suspension. This separation technique for non-ferrous metals is based on the anomalous viscosity increase of a magnetic suspension as a function of applied field. The magnetic fluid consists typically of magnetite particles with an average diameter of 100 A in water, with 15-25 wt% of ferrite. To prevent particle aggregation, a surfactant such as kerosene and oleic acid, or a polymer is usually... 

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