Two distinctly different approaches have been used to simulate the movement of bands through a chromatographic column. One example of the first approach Is the Craig distribution model, which replaces the continuous column with a specific number of discrete equilibration processes. Thus It Introduces the concept of (theoretical) plates Into chromatography, but Is not able to explain satisfactorily their significance. The second approach is based on the mass balance equation which can be Integrated numerically over time and space to give the elution profile. This calculation can be performed by using finite difference methods to extrapolate from the concentration value at a given time and position In the column to the next value. In this paper we discuss the physical meaning of the numerical Integration process followed by the finite difference methods. We show that both approaches are equivalent and that the band broadening produced by the different methods, due to "numerical diffusion", can be explained similarly. Since this effect Is sufficiently well-known now, we can introduce a variable amount of additional diffusion and thereby control the overall dispersion.

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SummaryUpon injection of concentrated polymer solutions in size exclusion chromatography, a random peak deformation has been observed and attributed to viscous fingering. In order to characterize this phenomenon, which is due to the difference in sample and eluent viscosities, mixtures of methanol and isopropanol are used as a low-molecular weight model. This system permits the study of viscosity effects in the absence of any adsorption or exclusion. Depending on the percentage of isopropanol added to a pure methanol mobile phase or to a methanol "sample" plug, a wide range of sample and eluent viscosities can be obtained. Statistical moments are used to compare elution profiles and to correlate peak deformation to viscosity differences. Significant loss of efficiency is observed whenever sample and eluent viscosity differ by more than 10 %.