Chromatographic peaks computer simulations

Figure 1.5—Theoretical plate model. Computer simulation of the elution of two compounds, A and B, chromatographed on a column with 30 theoretical plates (KA = 0.5 KB = 1.6 MA — 300 pg Mb = 300 pg) showing the composition of the mixture at the outlet of the column after the first 100 equilibria. As is evident from the graph, this model leads to a non-symmetrical peak. However, when the number of equilibria is very large, and because of diffusion, the peak looks more and more like a Gaussian distribution.

The appearance of anomalous HO elution patterns has been interpreted In two wqys (ref. 48). The first explanation is based on computer simulations of HD elution profiles of ligand-mediated association-dissociation equilibria of the macromolecule (ref. 41). The initially narrow band of N departs from the top of the chromatographic column and starts to equilibrate with the solution of L to form complexes ML, M2L etc. just as in the normal HD runs. Simultaneously, the concentration of M and its complexes decrease continuously as their band spreads, and these complexes will dissociate. Typically, a trailing elution profile of the ost peak is formed. The exact peak shape and size depends on the mode and strength of associations and on the concentration of components (ref. 41). The accordance of the theoretical predictions with actual experimental model systems has not been tested. 

Better peak shape simulations take into account the basic van Deemter-Golay equations to compute the degree of peak broadening that would occur under the set of isothermal conditions, or with each simulation step for temperature programming. A more intensive and accurate approach uses the Giddings-Golay equation (22,23), which includes additional compensation terms for carrier-gas expansion. In either case, ultimately the chromatographer must transfer an optimized set of conditions into an instrument and evaluate the efficacy of the optimization procedure. 

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