Interpretive methods for selectivity optimization

For the optimization of programmed solvent LC the Sentinel method starts by establishing a suitable binary methanol-water gradient. The approach of Snyder described above may be used for this purpose. For example [627], a gradient from 20 to 100% methanol (in water) in 20 minutes may be the result. 

By analogy with the isocratic Sentinel optimization procedure a series of 7 gradients (all of the same duration time) can now be defined. These gradients are shown in figure 6.13 and the initial and final compositions are listed in table 6.4. 

The individual retention times of all solutes in a 14-component sample mixture were measured and used to calculate resolution values (eqn.1.14, because eqn.1.22 is invalid) between each pair of peaks in the chromatogram. The largest value for the limiting resolution (max Rsmin eqn.4.25) was used as the optimization criterion. 

Initial and final compositions of the 7 linear gradients shown in figure 6.13. All 7 gradients have the same duration. 

It will be clear that the interpretive procedure described here allows the recalculation of the resolution surfaces (and the response surface) after the retention times of the individual solutes have been obtained from the chromatogram at the predicted optimum (figure 6.14), so that an iterative optimization procedure, in which the accuracy of the resulting optimum is improved, is also possible. 

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