Column broadening, size exclusion

The main disadvantage of size-exclusion chromatography is its low peak capacity, which arises from the small elution volumes of the peaks. In addition because peak dispersion is small, extra-column contributions to band broadening in size-exclusion chromatography have to be kept to a minimum. The relative contribution of extra-column band broadening can be minimized by the use of small particles = 3-10 pm) and long (L = 50 100 cm), wide-bore (d = 8-10 mm) columns. 

As previously indicated, this discussion is organized for chromatograms from very narrow polymer standards for which we can consider that the effect of molecular weight distribution is negligible and for which the unique separation process is size exclusion. With these limitations, the contribution to band broadening is conveniently separated into extra column effects, eddy dispersion, static dispersion, and mass transfer. In the most classical chromatographic interpretation, extra-column effects are not discussed and the three other contributions are considered as Gaussian, so there is simply the addition of their variances. The number of theoretical plates is defined as N = VJaY and the influence of v, the linear velocity of the eluent, is summarized by the so-called Van Deemter equation ... 

Sato et al. [752] did a similar study on poly(styrene/methyl methacrylate) copolymer (PMMA). A 100/0-> 60/40 acetonitrile/DCM gradient was used with a polystyrene column. As pore size decreased, significant band broadening occurred as the retention mechanism became less dominated by adsorption while size exclusion began to play an equally important role in the separation process. 

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