Eluotropic solvent series

Table A-11. Eluotropic solvent series for hydrophilic adsorbents such as alumina or silica, listed in order of increasing eluting power of the solvent , quantitatively measured by Snyder s empirical eluant strength parameter °. ...

As the relative elution power depends not only on the adsorbent, but in many cases on the compound types being separated, there exists no universal eluotropic series of solvent strengths. This series was given by L. R. Snyder". For another eluotropic solvent series, see J. C. Touchstone Practice of Thin-Layer Chromatography, 2 " ed., Wiley, Chichester, 1983. 

For reversed-phase adsorbents such as charcoal or completely silanized silica, this eluotropic solvent series is valid in the reverse order. In this case, the eluting power increases in the following sequence water < methanol < ethanol < acetone < 1-propanol < 1-butanol diethyl ether ethyl acetate < benzene < n-hexane". ... 

III-3 and 111-4) provides several eluotropic solvent series with equal spacing of solvent strength for adsorption on both alumina and silica. 

Figure 3.20 Nomogram illustrating the solvent strength of various binary solvent mixtures for LSC. Vertical (dashed) line illustrates a series of iso-eluotropic solvents (see text). Figure taken from ref. [358]. Reprinted with permission.

The parameter space in the original Sentinel method is restricted to a series of iso-eluotropic solvents, which means that only a very small fraction of all possible quaternary mixtures is considered. This is illustrated in figure 5.27a. 

Relative strength of the solvents on polar adsorbents arranged as an eluotropic series in ehromatographie elution strength order for pure solvents or mixtures are given in literature [13-16]. The eluotropic series of pure solvents are generally referred to a partieular adsorbent. 

Arranging the solvents in separation-strength order, the so-called eluotropic series appeared. This term, introduced by Trappe, was related to the experience with bare silica, where a strong solvent is able to move polar solutes on a polar stationary phase. Later this was improved by the discovery of a direct proportion between the elution strength and the dielectric constant. Because silica is hydrophilic and highly polar, there was a correlation between the eluotropic series and the polarity of a solvent [16,18]. 

In Figure 4.6, some experimental eluotropic series are given. Some disagreements are observed in the order of the solvents because of the action of some secondary factors producing different solvent behavior [16]. 

In the mobile phase selection for the separation of compounds on thin silica gel layers, it is necessary to use not only eluotropic series based on the eluting capacity of the solvent but also eluotropic series of compounds established according to their interaction with silica gel. 

Equation 4.13 and Equation 4.14 were tested for a series of mobile phases on alumina [29-31] and silica gel [32]. Two eluotropic series of solvent binary mixtures for alumina (a = 0.6) and silica gel (a = 0.7) have been calculated by using Equation 4.13, and the obtained data can be used to establish many such series or series of other selectivities [13,28],... 

Eluotropic series can be expressed more precisely by the Snyder parameter e°, given in Table 4.2 for alumina and silica gel. It is important to take into account eluotropic series and know some rules when solvents are used according to this table [16,17] ... 

The eluting power of a solvent is determined by its overall polarity, the polarity of the stationary phase and the nature of the sample components. Table 4.9 lists some widely used solvents in order of their eluting power, this being known as an eluotropic series. In practice, better separations are achieved with the least polar solvent possible and mixtures of solvents are often used to achieve optimum separation conditions. 

Table 28.1 Eluotropic Series of Solvents for Adsorbents Containing Oxygen...

Solvent strength is a measure of the affinity of a solvent for the adsorbent. Complete lists are known as eluotropic series and will include mixtures of solvents as well as the pure solvents above. 

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... 

Table 25-2 Eluotropic series and ultraviolet cutoff wavelengths of solvents for adsorption chromatography on silica...

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. 

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