What are LFERs and Why can they be Profitable in HPLC

The retention factor k, a common measure of chromatographic retention, depends on the equilibrium constant K describing the partition of the analyte between the mobile and stationary phases and the so-called phase ratio O, which is the ratio of the capacities of the two phases to accommodate an analyte (see Eq. 1). 

The retention factor thus follows the rules of equilibrium thermodynamics, whereby the degree of retention is controlled by the change in the Gibbs free energy of the analyte molecule on going from the mobile into the stationary phase. This is in accordance with the general description of chemical equilibria in terms of standard free energies AG (Eq. 2) with universal gas constant R and absolute temperature T). 

The Hammett equation is regarded as the origin of linear free energy relationships (LFER). It correlates the rates of hydrolysis of differently substituted benzoic acid esters with the acidities of the equivalently substituted benzoic adds. A linear relationship between the free energy for the add dissociation and the free activation energy for the ester hydrolysis is assumed. Both energies are sensitive to the inductive (electron-shifting) effects of a particular substituent, which are embodied in the substituent constant a, an example of a molecular descriptor. The fundamentals of the Hammett theory are summarized in Fig. 2. 

Analogously to the Hammett relationship, and the more complex relationships between structure and reactivity derived from it, chromatographers equally strive for appropriate descriptions of retention with the help of linear free energy 

Relationship between the rate of hydroiysis of a substituted ester and that of its non-substituted form and. k 

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