Solvents eluotropic values

The concept of eluotropic strength has been invoked here without a rigorous definition. Snyder (130) developed a series of eluotropic values for solvents by using retention values measured on alumina columns. Colin and Guiochon (86) used a definition similar to that of Snyder to evaluate eluotropic strengths of methanol-water mixtures on various column surfaces. The eluotropic strength, e, was calculated by using the equation... 

Eluents used in reversed-phase chromatography with bonded nonpolar stationary phases are genei ly polar solvents or mixtures) of polar solvents, such as acetonitrile, with water. The properties of numerous neat solvents of interest, their sources, and their virtues in teversed-phase chromatography have been reviewed (128). Properties of pure solvents which may be of value as eluents are summiuized in Table. VII. The most significant properties are surface tension, dielectric constant, viscosity, and eluotropic value. Horvath e/ al. 107) adapted a theory of solvent effects to consider the role of the mobile phase in determinmg the absolute retention and the selectivity found in reversed-phase chromatography. 

The following table provides, for comparative purposes, eluotropic values on bonded octadecyl silane (ODS) and octyl silane (OS) for common solvents.12... 

Binary Solvent Eluotropic Series for Adsorption on Water-Deactivated Alumina (a = 0.6) Calculated Values from Eq. (8-10)... 

The elution of the analytes from a normal-phase sorbent is a function of the eluotropic strength E ) of the solvent. Table 10.7 shows the values of eluotropic strength and polarity of the organic solvents used in normal-phase SPE. The compounds are usually dissolved in the solvents with values less than 0.38 for silica sorbent, and eluted with solvents of values... 

In conclusion, eluotropic values, although derived ftom complex mathematical relationships and difficult to generate, offer a r id means of efficiently investigating different solvents. The use of nomograms is key for multicomponent mobile phases. 

The following table provides, for comparative and interpretive purposes, eluotropic values on bonded octadecylsilane (ODS) and octylsilane (OS) for common solvents used in HPLC (Refs. 1-3). For additional information on common, specific and chiral stationary phases for HPLC, and for solvents, derivatizing reagents, and detectors, see Ref. 3. 

Common solvent systems are shown in Table 2. Although there is a direct correlation between increasing Rf values and increasing the eluotropic strength of the organic solvent, Wilson pointed out that this correlation exists over a very wide range, contrary to what is seen on RP-HPLC. Additionally, Revalues are not particularly sensitive to pH, which makes RP-TLC a particularly robust and simple technique to use. 

The most important features of this equation are the emergence of a eluotropic series, i.e., an eluent-strength series based on e°. The key solvent-strength parameter, e°, which relates to the eluotropic series, was calculated by Snyder (14) for a wide variety of pure solvents. Table 2 lists some values. 

Eqn.(3.73) suggests that any mixture of two solvents with the same ° value (iso-eluotro-pic solvents) will also have the same eluotropic strength. This would allow the application of a similar strategy for the definition of iso-eluotropic multicomponent mobile phase mixtures as was used for RPLC in section 3.2.2.1. In practice, the situation in LSC has proved to be more complicated, because an effect described as solvent localization limits the validity of eqns.(3.72) and (3.73) if polar components (such as acetonitrile or methyl t-butyl ether) are present in the mobile phase. This makes it difficult to calculate the composition of iso-eluotropic mixtures for LSC with sufficient accuracy for optimization purposes [360-363]. 

The eluotropic series of pure solvents is generally referred to a particular adsorbent. The magnitude of the solvent strength parameter, can be defined as the adsorption energy of the solvent per unit of the standard activity surface. AU these ° values are relative to the solvent pentane, for which ° is defined equal to zero. This parameter is defined as a measure of the degree of the adsorption interaction of the solvent with the stationary phase. As a function of this magnitude, the eluotropic series for various polar adsorbents is presented in Table 1. 

It is well recognized that the eluotropic series of solvents, according to Snyder, is suited for monoactive site-type adsorbents, but in the case of multiactive site-type adsorbents (e.g., alumina), their imperfection becomes acute. Thus for multiactive-site-type adsorbents, the eluotropic series sequence and eluent strength values are highly dependent on the class of test solutes employed. ... 

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