Principles for Optimization of Enantioselective HPLC Separations

The fundamentals introduced in the previous sections allow some conclusions regarding optimization of enantioselective HPLC separations. 

(1) implies that in order to obtain a sufficient separation factor a, a sufficient difference in the retention factors must be obtained. Therefore, gradients are rarely applied in enantioselective HPLC, wfhich would speed up isocratic separations. 

The equation for (see Section 1.1.3) shows that for the optimization of an enantioselective separation, besides the separation factor aand the retention factor k, a sufficient number of plates N must also be available if baseline resolution is required. For this reason, for some years there has been a trend in enantioselective HPLC towards smaller particle sizes. Today, many CSPs are available as 5 pm materials. 

(7) allows a statement to be made about the temperature dependence of a (Eq. 10) by applying the Gibbs-Helmholz equation (G = H - TS) [3], 

Almost all enantiomer separations show a lower separation factor at higher temperatures when the temperature is increased further, enantiomer separation can be completely suppressed. This means that the entropy term in the above equation approaches the value of the enthalpy term with rising temperature, and that at a certain temperature entropy and enthalpy cancel one another out (In a = 0). For this reason, temperature increase and gradients can be neglected for the optimization of enantioselective HPLC. 

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