subject: Molecular Weight And Molecular Size By Gpc/sec [print this page] Who wants to measure molecular weights and sizes?
The answer is a surprisingly broad range of people and companies. Molecular weight and size is most often used in identification of proteins, and in characterization of polymers.
Molecular Weight is commonly measured in industries such as::
Pharmaceutical industry
Biotech research
Medical analysis
The food industry
Petroleum and polymer industries
Radius of gyration and Hydrodynamic radius
Molecules are not simple shapes - unlike the fairly compliant, mathematically-uncomplicated atom, which is spherical, molecules are difficult to define as a simple shape. Hence two measurements have become industry standards, and analytical instruments are geared towards obtaining these; the radius of gyration, and the hydrodynamic radius.
Radius of gyration
Scientific instruments can be used to identify the center of mass and dimensions of a molecule; this is its radius of gyration. It is measured directly using static light scattering; however this method has limitations at sizes lower than 10-15nm, and also for large molecules like polysaccharides. Viscometers are the tool which is most often used for determining radius of gyration, using the Flory-Fox equation.
Hydrodynamic radius
This is a behavioral property of a substance - a measurement of size based on how it moves, which means that it is much more useful in industrial or practical settings.
Size exclusion chromatography
Size exclusion chromatography is the method by which scientists determine the molecular size (not the weight!), of a particular substance. Molecules are separated in columns packed with porous substances, which might include glass beads, polystyrene gels, silica gel, etc. Larger molecules elute more quickly through the columns, since the molecules cannot fit into as many spaces.A concentration detector is placed at the bottom of the GPC columns to determine the amount of material of each size fraction. In traditional SEC/GPC systems, operators need to pass known standards through the columns before the sample. By creating a calibration curve of size versus elution time, particle size of unknown samples can be calculated.
In more recent times, addition of other detection techniques such as Static Light Scattering and the Intrinsic Viscosity detector provide direct measurement of molecular weight and size so that traditional calibration techniques are not required.
Gel permeation chromatography
SEC is also commonly referred to by other names. When an aqueous solution is used to carry the sample through the column, the technique is often called Gel Filtration Chromatography, and the name Gel Permeation Chromatography is often used when solvents are used to carry the substance in question through the packed columns.
by: Bryn McDonagh
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