General Equations of Gradient Elution in HPLC

When a limited range of organic modifier proportions within the mobile phase is considered, the retention changes in RPLC with mobile phase composition are generally adequately described by the linear Soczewinski-Snyder model (Eq. 4) adapted to both isocratic and gradient mode [5]. 

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. 

Retention time in a Unear-gradient separation (tj can be expressed as foUow  

Due to instrumental or laboratory variability, the information given by the experimental gradient retention times (tr) is not sufficient to obtain accurate and repeat-able logP values [38]. As described by Eqs. (4), (6) and (7) the lipophilicity measurement in gradient mode is therefore based on the determination of logfeo and S because both terms are necessary to obtain log few. 

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