Linear-solvent-strength theory

The basis for linear solvent strength theory is the assertion that the logarithm of the capacity factor is linearly related to the solvent strength. This is expressed in the Equation 87 90... 

The practical consequence of linear solvent strength theory is that, in principle, it is possible to estimate the retention times of all peaks at any... 

The conceptual basis for understanding the connection between isocratic and gradient elution is well established and is called "linear solvent strength theory".22 27 Linear solvent strength theory proposes that, for a given solute, mobile phase, and column, if one measures the retention time of an analyte at two organic component concentrations, it will be possible to predict the retention time with any other mobile phase composition. The k value that would be observed in pure water, kw, is related to the actual k by the relationship... 

The regression coefficients of descriptors denote the system (combination of mobile and stationary phases) response to these interactions. These coefficients can be measured, however the procedure is time consuming and inappropriate for practical purposes. According to the linear solvent strength theory (LSST) the retention of the analyte depends on the volume fraction (cp) of the organic modifier in binary mobile phase systems ... 

A. Wang and P.W. Carr, Comparative study of the linear solvation energy relationship, linear solvent strength theory, and typical conditions model for retention prediction in reversed-phase liquid chromatography. J. Chromatogr.A 965 (2002) 3-23. 

Moreover, in various experiments it was found that at a constant total counterion concentration in the eluent the dependence of the retention factors on the organic modifier content tp largely follows the linear solvent strength theory (LSS) (Equation 1.5)... 

Similarly, in gradient elution HP-RPC, resolution optimization can take advantage of the relationship between the gradient retention time of an analyte (expressed as the median retention factor k) and the median volume fraction of the organic solvent modifier, Ip, in regular HP-RPC systems based on the concepts of the linear solvent strength theory,1 31 such that... 

Equations (16.12) and (16.13) are very important, since they easily can be used to predict the influence of any operational parameter on the steepness factor, h, and therefore on the analysis time, efiSciency, and resolution. However, they are based on the validity of Equation (16.10). It has been shown that some deviations occur for some compounds and chromatographic systems (6), especially when retention is not governed solely by hydrophobic interaction. This is, for example, the case when the solutes are strongly basic and the stationary-phase acidity is high. Nevertheless, it is always possible to modify the form of the mobile-phase variation with time in order to maintain the applicability of the linear-solvent-strength theory [Equation (16.1)]. As we have seen above, this type of gradient offers a considerable help in the fundamental understanding of the retention behavior of the solutes and in the optimization of a separation. 

A simple way to estimate the appropriate isocratic conditions from the result of a gradient elution chromatogram is provided by the theory of linear solvent strength (LSS) gradients of Snyder (for a review, see ref. [528] or [527]). By definition, an LSS gradient obeys the following relationship ... 

The behavior of chromatographic columns operated in gradient elution, under linear conditions i.e., assuming linear isotherms for all the solutes) has been studied theoretically by numerous authors [2,4-10]. The most comprehensive treatment is that based on the linear solvent strength (LSS) theory of Snyder et al. [5,6]. This theory has formd widespread acceptance [7,8] and has been extended to include the contributions of the various mass transfer resistances to band broadening [9-11]. It assumes the injection of infinitesimal pulses of a feed and a linear gradient of the volume fraction of a mobile phase modifier, cf). 

A decease in the solvent strength provokes collapse transition in an isolated star polymer, similar to that in a linear chain. Scaling theory predicts a progressive deswelling (collapse) of the star polymer as a function of decreasing solvent... 

These large increases in rate might be attributed to the operation of a neutral salt effect, and, in fact, a plot of log k versus the square root of the ionic strength, fi, is linear. However, the reactants, in this case, are neutral molecules, not ions in the low dielectric constant solvent, chloroform, ionic species would be largely associated, and the Bronsted-Bjerrum theory of salt effects51 52, which is valid only for dilute-solution reactions between ions at small n (below 0.01 M for 1 1 electrolytes), does not properly apply. 

Mobile phase parameters that have to be optimized include the salt type, concentration, gradient shape, pH, temperature, and possibly the addition of a surfactant or organic solvent [368-372]. The change in free energy on protein binding to the stationary phase is determined mainly by the contact surface area between the protein and stationary phase and by the salt type determined by its ability to increase the surface tension of aqueous solutions. Solvophobic theory predicts that in the absence of specific salt-protein interactions and at sufficient ionic strength the logarithm of the retention factor is linearly dependent on the surface tension of the mobile phase, which in turn, is a linear function of the salt concentration Eq. (4.13)... 

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