Solvent Strength and Selectivity

When multiple development is performed on the same monolayer stationary phase, the development distance and the total solvent strength and selectivity values (16) of the mobile phase (17) can easily be changed at any stage of the development sequence to optimize the separation. These techniques are typically fully off-line modes, because the plates must be dried between consecutive development steps only after this can the next development, with the same or different development distances and/or mobile phases, be started. This method involves the following stages ... 

It is important to know the influence of the physicochemical parameters of the mobile phase (dipole moment, dielectric constant, and refractive index) on solvent strength and selectivity. The main interactions in planar chromatography between the molecules of the mobile phases and those of solutes are caused by dispersion forces related to the refractive index, dipole-dipole forces related to the dipole moment, induction forces related to a permanent dipole and an induced one, hydrogen bonding, and dielectric interactions related to the dielectric constant. Solvent strength depends mainly on the dipole moment of the mobile phase, whereas the solvent selectivity depends on the dielectric constant of the mobile phase. 

Buffer Salt Solvent Strength and Selectivity Solvent strength and selectivity are influenced by the nature of the counterion i.e., its value. A change in buffer salt may also change the mobile phase pH. 

A number of models have been proposed to describe the solution formation process [505-509], some of which can be extended to Include chromatographic processes and other solvent-dependent phenomena. In terms of chromatographic aiqplications the most useful are the solubility parameter concept, solvatochromic parameters and Snyder s solvent strength and selectivity... 

As well as density the solvent strength and selectivity of the fluid will affect both the solubilizing power of the fluid for specific solutes and retention in SFC. Solvatochromic indicators have been used to rank fluids commonly used in SFC in terms of... 

Glajch, J. L., Kirkland, J. J., Squire, K. M., and Minor, J. M., Optimization of solvent strength and selectivity for reversed-phase liquid chromatography using an interactive mixture-design statistical technique, /. Chromatogr., 199, 57, 1980. 

Finally, we will summarize here all the correlational equations and experimental solvent parameters required for predictions of solvent strength and selectivity in LSC, and discuss their significance in terms of mobile-phase optimization strategies. 

S. D. West, The prediction of reversed-phase HPLC retention indices and resolution as a function of solvent strength and selectivity,/. Chromatogr. Set. 25 (1987), 122-129 and S. D. West, Correlation of retention indices with resolution and selectivity in reversed-phase HPLC and GC, J. Chromatogr. Set. 27 (1989), 2-12. 

Kord, A.S. Khaledi, M.G. Controlling solvent strength and selectivity in MLC Role of organic modifiers and micelles. Anal. Chem. 1992, 64, 1894-1900. 

Eorgacs, E. Cserghati, T. Solvent strength and selectivity on porous graphitised carbon column separated by a spectral mapping technique using barbiturates as solutes. Anal. Sci. 2001, 17, 307-312. 

Retention mechanisms of adsorption chromatography have been extensively studied. There are two popular models for this process. The displacement model, originally proposed by Snyder, treats the distribution of solute between a surface phase, usually assumed to be a monolayer, and a mobile phase as a result of a competitive solute and solvent adsorption. A treatment of this model, including the significance of predictions of solvent strength and selectivity in terms of mobile-phase optimization strategies, has been published by Snyder (81). 

Solvent strength and selectivity are the properties commonly used to classify liquid stationary phases as selection tools for method development in gas chromatography [29,102-104]. Solvent strength and polarity are often used interchangeably and can cause confusion. Polarity is sometimes considered to be the capacity of a stationary phase for dipole-type interactions alone, while more generally solvent strength is defined as the capacity of a stationary phase for all possible intermolecular interactions. The latter definition is quite sensible but unworkable because there is no substance that is uniquely polar that might be used to probe the polarity of other substances. Indirect measurements of polarity, such as those scales related one way or another to the... 

For chromatographic applications, the most useful models of solvent properties are the solubility parameter concept, Snyder s solvent strength and selectivity parameters, solvatochromic parameters and the system constants of the solvation parameter model for gas to liquid transfer. The Hildebrand solubility parameter, 8h (total solubility parameter), is a rough measure of solvent strength, and is easily caleulated from the physical properties of the pure solvent. It is equivalent to the square root of the solvent vaporization energy divided by its molar volume. The original solubility parameter concept was developed from assumptions of regular solution behavior in which the principal intermolecular interactions were dominated by dispersion forces. 

Solvent strength and selectivity parameters based on Snyder s selectivity triangle. (Si is an empirical solvent strength parameter for reversed-phase chromatography)... 

The retention mechanism and solvent selectivity have been studied most carefully with alumina or silica as stationary phases. The knowledge of both for bonded phases used in normal-phase chromatography is much more limited. Nevertheless, it is safe to assume that similar selectivity rules for solvent strength and selectivity can be applied, especially since the results obtained for alumina and silica correlate well with each other. 

A.S. Kord and M.G. Khaledi, Controlling Solvent Strength and Selectivity in MLC Role of Organic Modifiers and Micelles, Anal Chem., 64 1894 (1992). 

There are also other trial and error approaches, the simplest of which is the so-called spot test. The sample is applied as several spots on a TLC plate. Then specified volumes of different solvents are applied to the centers of the spotted samples. The resulting circular chromatograms can give preliminary information about solvent strength and selectivity required for separation of the sample. With modern instruments for sample application this test can be automated. However, actual optimization of the mobile phase must still be performed in a suitable chromatographic chamber. 

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