Gel Permeation and Size Exclusion Chromatography

MEIRA, G. R.; VEGA, J. R.; YOSSEN, M.

Ewing's Analytical Instrumentation Handbook, Third Edition

CRC Press

Lugar: New York; Año: 2004; p. 825 - 867

Size exclusion chromatography (SEC) fractionates polymer molecules according to their sizes in solution (with the larger sizes eluting first and the smaller ones 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-60s, the technique has greatly improved and it has been widely investigated (Provder, 1984, 1987; Wu, 1995; Yau et al., 1979). The older terminology, gel permeation chromatography (GPC), stemmed from the fact that the original column packings were soft organic gels. The name gel filtration chromatography (GFC) 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 (MMD) of both synthetic and natural polymers. Other SEC applications include the determination of copolymer chemical composition, of chain branching, of polymer additives, and the retreatment of biopolymers. The older equipment only included on-line or instantaneous concentration detectors, such as the differential refractometer (DR) and the single-wavelength UV spectrophotometer. As such, the technique is relative, that is, 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 (SV) and light-scattering (LS) sensors, has converted SEC into a more absolute technique for determining the MMD, and also makes 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 distribution. 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 (BB) make the quantification of macrostructural characteristics difficult. Present trends in SEC include combining the sizecontrolled fractionation with other mechanisms such as adsorption [for the determination of the chemical composition distribution (CCD)] or eluability (for the 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 (Fei and Murray, 1996), but this instrumentation and data interpretation are still under development (Macha and Limbach, 2002; Murgasova and Hercules, 2003). All of these newer developments are outside the scope of the present review, which is strictly limited to SEC.