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Gel Permeation and Size Exclusion Chromatography
Ewing's Analytical Instrumentation Handbook, Third Edition
Marcel Dekker Inc.
Lugar: New York; Año: 2005; p. 825 - 867
Size exclusion chromatography (SEC) fractionates the polymer molecules according to their size in solution (with the larger sizes eluting first and the smaller at the end), and then analyzes the different eluting fractions. Ideally, this is a purely entropy-controlled fractionation, and it should not involve any enthalpic interactions. Since its appearance in the mid-60´s, the technique has greatly improved and it has been widely investigated. The older denomination: gel permeation chromatography stemmed from the fact that the original column packings were soft organic gels. The name gel filtration chromatography has been traditionally applied to designate the analysis of hydrophilic biopolymers, and it is also still in use today. SEC is the main technique for determining the molar mass distribution of both synthetic and natural polymers. Other SEC applications include the determination of copolymer chemical composition, of chain branching, of polymer additives, and the pretreatment of biopolymers. The older equipment only included on-line or instantaneous concentration detectors such as the differential refractometer and the single-wavelength UV spectrophotometer. As such, the technique is relative or nonabsolute, in the sense that an independent molar mass calibration is required to transform the chromatogram abscissas (either elution time or elution volume) into molar mass. More recently, the development of "molar mass-sensitive detectors" such as on-line specific viscometers and light scattering sensors, have converted SEC into a more absolute technique for determining the MMD, and also make it possible to indirectly determine other polymer characteristics such as chain branching. Unfortunately, the high cost of these more sophisticated sensors explains their relatively scarce diffusion. The main difficulties of SEC are: a) even under ideal conditions, the fractionation is by hydrodynamic volume rather than by molar mass; b) there is an upper molar mass limit of fractionation, above which all molecular species emerge together; and c) undesirable effects such as secondary (nonentropic) fractionations and band broadening difficult the quantification of macrostructural characteristics. Present trends in SEC include combining the size-controlled fractionation with other mechanisms such as adsorption (for the determination of the chemical composition distribution) or eluability (for determination of the amorphous-crystalline fractions). More recently, true molar mass sensors such as MALDI (matrix-assisted laser desorption ioinization) and mass spectrometers have also been attached to size-exclusion chromatographs; but this instrumentation and data interpretation are still under development. All of these newer developments are outside the scope of the present review, that is strictly limited to SEC.