Gel permeation chromatography
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Gel permeation chromatography (GPC) is a separation technique based on hydrodynamic volume (size in solution). Molecules are separated from one another based on differences in molecular size. This technique is often used for polymer molecular weight determination. GPC is performed in organic mobile phases and is used mostly for synthetic polymers. GFC (gel filtration chromatography) is defined as sized based separation performed in an aqueous mobile phase and is typically applied for protein analysis or for water soluble polymers. A broader term which encompasses both of these separation methods is size exclusion chromatography (SEC).
GPC is accomplished by passing the sample through a porous media. Larger molecules can not access some of the pores and exit the column more rapidly. Smaller molecules penetrate into more of the porous structure and elute at longer retention times. Figure I shows an example of this process and the path which two molecules of different size take when passing through a GPC column. The red molecule can not fit within the pores of the gel while the blue molecule fits into all the pores of the gel increasing the time required to pass through the column. It is this filtration effect which causes separation by size. Figure II shows an example of the instrumentation used to accompish a GPC separation.
GPC is the most commonly used method for polymer molecular weight and polydispersity index. This process is done by first analyzing a series of standards of known molecular weight. The retention time for these standards is then used to create a calibration curve. Figure III shows an example of this process in which standards were run and the resulting retention time used to make the calibration curve. The retention time for an unknown material can then be determined based on the retention volume at which it elutes. The results of this analysis are typically displayed as the molecular weight distribution an example of which is shown in Figure IV.
GPC differs from most other methods of chromatography in that separations are accomplished without interactions between the stationary phase and the sample. Interactions are undesirable and prevent a purely size based separation. This can be contrasted with the more common reverse phase chromatography (RP) in which separation is accomplished based on differences in the hydrophobic (van der waals) interactions between the column and sample components.