Gel permeation chromatography (GPC) is a type of size exclusion chromatography (SEC), that separates analytes on the basis of size. The technique is often used for the analysis of polymers. As a technique, SEC was first developed in 1955 by Lathe and Ruthven.[1] The term gel permeation chromatography can be traced back to J.C. Moore of the Dow Chemical Company who investigated the technique in 1964 and the proprietary column technology was licensed to Waters Corporation, who subsequently commercialized this technology in 1964.[2] GPC systems and consumables are now also available from a number of manufacturers, including Agilent Technologies. It is often necessary to separate polymers, both to analyze them as well as to purify the desired product.

When characterizing polymers, it is important to consider the polydispersity index (PDI) as well the molecular weight. Polymers can be characterized by a variety of definitions for molecular weight including the number average molecular weight (Mn), the weight average molecular weight (Mw) (see molar mass distribution), the size average molecular weight (Mz), or the viscosity molecular weight (Mv). GPC allows for the determination of PDI as well as Mv and based on other data, the Mn, Mw, and Mz can be determined.

In physical and organic chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an inconsistent size, shape and mass distribution is called non-uniform. The objects can be in any form of chemical dispersion, such as particles in acolloid, droplets in a cloud,[1] crystals in a rock,[2] or polymer molecules in a solvent.[3] Polymers can possess a distribution of molecular mass; particles often possess a wide distribution of size, surface area and mass; and thin films can possess a varied distribution of film thickness. [citation

needed]

IUPAC has deprecated the use of the term polydispersity index having replaced it with the term dispersity, represented by the symbol Đ(pronounced D-stroke[4]) which can refer to either molecular mass or degree of polymerization. It can be calculated using the equation ĐM= Mw/Mn, where Mw is the weight-average molar mass and Mn is the number-average molar mass. It can also be calculated according to degree of polymerization, where ĐX = Xw/Xn, where Xw is the weight-average degree of polymerization and Xn is the number-average degree of polymerization. In certain limiting cases where ĐM = ĐX, it is simply referred to as Đ. IUPAC has also deprecated the termsmonodisperse, which is considered to be self-contradictory, and polydisperse, which is considered redundant, preferring the termsuniform and non-uniform instead.[4]

Capillary electrophoresis is an analytical technique that separates ions based on their electrophoretic mobility with the use of an applied voltage. The electrophoretic mobility is dependent upon the charge of the molecule, the viscosity, and the atom's radius. The rate at which the particle moves is directly proportional to the applied electric field--the greater the field strength, the faster the mobility. Neutral species are not affected, only ions move with the electric field. If two ions are the same size, the one with greater charge will move the fastest. For ions of the same charge, the smaller particle has less friction and overall faster migration rate. Capillary electrophoresis is used most

predominately because it gives faster results and provides high resolution separation. It is a useful technique because there is a large range of detection methods available.1

Field-flow fractionation, abbreviated FFF, is a separation technique where a field is applied to a fluid suspension or solution pumped through a long and narrow channel, perpendicular to the direction of flow, to cause separation of the particles present in the fluid, depending on their differing "mobilities" under the force exerted by the field. It was invented and first reported by J. Calvin Giddings.[1] The method of FFF is unique to other separation techniques due to the fact that it can separate materials over a wide colloidal size range while maintaining high resolution. Although FFF is an extremely versatile technique, there is no "one size fits all" method for all applications.

Flow field-flow fractionation (AF4) channel cross section, where the rate of laminar flow within the channel is

not uniform. It travels in a parabolic pattern with the speed of the flow, increasing towards the centre of the

channel and decreasing towards the sides.

In field-flow fractionation the field can be asymmetrical flow through a semi-permeable membrane, gravitational, centrifugal, thermal-gradient, electrical, magnetic etc. In all cases, the separation mechanism is born from differences in particle mobility (electrophoretic, when the field is a DC electric field causing a transverse electric current flow) under the forces of the field, in equilibrium with the forces ofdiffusion: an often-parabolic laminar-flow-velocity profile in the channel determines the velocity of a particular particle, based on its equilibrium position from the wall of the channel. The ratio of the velocity of a species of particle to the average velocity of the fluid is called the retention ratio.