Gel permeation chromatography (GPC) also known as size exclusion chromatography (SEC) is a chromatographic method in which molecules are separated based on their size. This method is most widely used in the analysis of polymer molecular weights (or molar mass). The term GPC was used in the beginning of polymer analysis when people used glass columns filled with gels to perform GPC. Nowadays more and more automated and high pressure liquid chromatographic columns are used. Therefore GPC is an old terminology and size-exclusion chromatography (SEC) is the correct expression for the determination of molecular weights. Chromatography is a family of analytical chemistry techniques for the separation of mixtures. ... In general, a molecule is the smallest particle of a pure chemical substance that still retains its composition and chemical properties. ... Polymer is a generic term used to describe a very long molecule consisting of structural units and repeating units connected by covalent chemical bonds. ...

In SEC, a column (steel cylinder typically 10 mm in diameter and 500 to 1000 mm in length) is packed with a porous material (typically silica or crosslinked polystyrene) and solvent is forced through the column (at rates typically 1 ml/min and pressures of 50 to 200 bar). A sample is dissolved in the same solvent that is running through the column and is then introduced into the solvent stream going through the column. A detector monitors the concentration of sample exiting the end of the column. Inside the column, molecules are separated based on their hydrodynamic volume (the volume the molecule occupies in a dilute solution). For polymers this can vary greatly with the particular solvent and the temperature. By studying the properties of polymers in particular solvents and by calibrating each column setup with samples of known molecular weight, it is possible to get a relative distribution of molecular weights for a given polymer sample. Using this data, it is possible to calculate number average molecular weight, weight average molecular weight, polydispersity, as well as higher order molecular weights to within a useful level of accuracy. Inside the column, molecules are separated by whether or not they can fit within the pore size of the packing material. When columns are created they are packed with porous beads with a specific pore size so that they are most accurate at separating molecules with sizes similar to the pore size. As a molecule flows through the column it passes by a number of these porous beads. If the molecule can fit inside the pore then it is drawn in by the force of diffusion. There it stays a short while and then moves on. If a molecule can not fit into a pore then it continues following the solvent flow. For this reason, in a GPC column, molecules with larger size will reach the end of the column before molecules with smaller size. The effective range of the column is determined by the pore size of the packing. Any molecules larger than all the pores in a column will be eluted together regardless of their size. Likewise, any molecules that can fit into all the pores in the packing material will elute at the same time. A solvent is a liquid that dissolves a solid, liquid, or gaseous solute, resulting in a solution. ... In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. ... The molecular mass of a substance (less accurately called molecular weight and abbreviated as MW) is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12). ... A pore, in general, is some form of opening, usually very small. ... Schematic drawing of the effects of diffusion through a semipermeable membrane. ...