Two gradient runs

In 1996, Snyder and Dolan elaborated the linear solvent strength (LSS) approach allowing the evaluation of log few from a single gradient run and its precise determination from two gradient runs [48]. From a practical point of view, LSS is the most useful approach to describe theoretical aspects and LSS gradients are convenient for optimization studies. Several commercially available optimization software packages which are able to predict resolution or retention in RPLC are currently based on this approach. 

More accurate values of S and log few could be obtained from two gradient runs. 

Theoretical backgrounds of the determination of log Aw by gradient elution HPLC were elaborated recently by Snyder and Dolan [24]. The approach has been developed to determine both log Aw and p/f , of analytes by means of two gradient runs [25]. 

If two gradient runs are used, in which only the gradient time is varied (e.g. 20 and 60 min), the resulting retention data can be used to predict retention as a function of mobile phase composition in the corresponding isocratic separations. Linear plots of log k against mobile phase composition can be constructed. In those cases where... 

Values of Kand Z derived in this fashion are shown in Table VI. We see that values of K and Z obtained in these two ways agree closely, and are also in good agreement with experimental values obtained from isobratic data (Table VI). We can also obtain values of and c from two gradient runs. 

Vahies of Kg fin two gradient runs Column dead volume (ml) equal to %F)... 

Fio. 2. Isocratic retention (k ) venus mobile-phase composition (<(>) as derived from le-versed-pbase gradient runs ((, values) for curved plots of log k versus p. (a) Results for two gradient runs (igi and toj), — best-fit linear-curve approximation to log A versus, (b) Smilar results for three gradient runs (foi fo>)- Quarry a/. (17). 

DUferenoe in vaiuei for adjacent tends 1 and 2 Difference in 6/, values for two gradient runs [Eq. (83)]... 

Figure 18.14 represents an example of the computer-assisted optimization of linear gradient run time and temperature. It was necessary to perform four preliminary experiments which included two different temperatures (50 and 60 °C) and two gradient run times (17 and 51 min). From the data of these four runs the optimum conditions were calculated by the computer to be 57 °C and 80 min. The simulated chromatogram of this proposal is shown on top of Fig. 18.14. Some peak pairs are not resolved better than with R = 0.7. Since not all peaks are of equal scientific importance, it was decided that peaks 8, 9, 12, 13, 15, and 16 did not need to be resolved. With this facilitation the computer proposed 55 °C and 54 nun (nuddle). The real chromatogram is almost identical with the simulated one (bottom).

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