Physical fractionation methods electrophoresis

Painstaking attempts to identify the size of the infectious unit have in the past been necessitated for plant viruses, and also for animal viruses for which hitherto no method of assay for single virus particles has materialized. These include chemical fractionation, physical fractionation by means of separation cells in the ultracentrifuge and electrophoresis apparatus, and inactivation of the virus (Lauffer, 181,182,184,299). Because of electron microscopy and the excellent linearity of the plaque count, these methods have not proved necessary with the bacteriophages. 

An important advantage of two-dimensional methods is the solitary migration of each fraction on its own path. In contrast with unidimensional or zone electrophoresis where each fraction migrates its own distance but over a common path, no contamination of fractions is possible (Fig. 40). If the substrate is sufficiently long and wide and a high intensity electrical field is applied, it becomes possible to separate fractions reasonably completely from each other. If the physical forces involved are deployed skillfully, the electrical field can be driven up so that rapid and wide separations are obtained. This point is especially important in star electrophoresis, whereas in collecting electrophoresis mastery of the physical forces provides an excellent continuous preparative tool. 

Various technologies have been used to measure plasma lipids and lipoproteins and lipoprotein subfractions, including enzymatic, immunochemical, and chemical precipitation reagents, and physical methods, such as ultracentrifugation, electrophoresis, column chromatography, and others. Such methods have been reviewed extensively. As mentioned earlier, however, the cholesterol content of any particular lipoprotein class can vaiy somewhat from individual to individual. Moreover, although different methods of lipoprotein separation may produce similar lipoprotein fractions, they usually do not produce identical fractions, giving rise to systematic biases between methods that purport to measure the same component. The present discussion focuses primarily on methods and procedures commonly used in clinical practice for lipid and lipoprotein measurements. 

Analytical Techniques and Physical Methods.— The mapping of oligonucleotides and nucleic acid digests on cellulose or cellulose-polyethyleneimine has been described recently, and columns of mercurated dextran or dihydroxyborylcellulose have been used to fractionate nucleotide mixtures. Electrophoresis on polyacrylamide gels has been advocated as a rapid method for desalting and fractionating mixtures of oligonucleotides. ... 

Relative methods for determining molecular weights require calibrations with standards of known molecular weight and molecular weight distribution. They are influenced by the chemical and physical nature of the polymer and its interactions with solvent molecules. Gel permeation chromatography, fractional precipitation, and electrophoresis are used most frequently, with electrophoresis particularly useful to determine the molecular weight of proteins. 

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