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