Eluotropic, series

These parameters were developed to provide the chromatographer with a rapid reference to the relative elution strengths of solvents. There are essentially two solvent elution strength scales the eluotropic series and solvent strength parameters. 

Recently, these parameters have been expanded to include a limited number of solvents for the reversed-phase octadecyl (C,g) support. It should be realized however, that ° is based on adsorption equilibrium and that adsorption (except in very specific instances) is not a predominant retention mechanism in reversed-phase separations. That having been stated, regardless of what system die ° parameter is used in conjunction with, it is at best an estimate of the solvent strength for an average solute on a generic support. 

The reason for the wide range in effect between alumina and silica is that the ° value is not only a function of the mobile phase but is dependent upon the solute chemical structure and the support material as well. As a general rule of thumb, the more closely a solvent chemically resembles the support, the stronger an eluent it is (solubility issues withstanding ). For example, hexadecane (Cj ) is a very strong solvent on an octadecyl (C g) support, whereas ethanol (CH3CH2OH) is a very 

TABLE 2.1 Elnotropic Strength of Solvents on Various Sorbents  

Strong solvent on a silica (SiOH) support. (Note that water is such a strong solvent on silica (M alumina that it effectively deactivates the material i.e., neutralizes all the active retention sites on the sur ce through strong hydrogen-bond interactions.) 

Type of chamber N-chamber with chamber saturation 

The solvents are listed in the order of Halpaap s eluotropic series with silica gel as the stationary phase. 

The complete table with a total of 20 chromatographic and physical parameters can be found in [42], 

The stabilizers present in the chloroform were (a) amylene and (b) ethanol 

In the DAB method, the identity test for camomile flowers is also performed using chloroform p.f.a. (DAB nomenclature chloroform R) as the solvent. Even the very low percentage of water in the stabilizer present (ethanol) can lead to the formation of a (3-front on development (Fig. 44a). If, on the other hand, the chloroform is dried over sodium sulfate shortly before placing it in the developing chamber, a thin-film chromatogram without an interfering (3-front is obtained, as shown in Fig. 44b [46]. This example could not be repeated reproducibly, and probably only happens with the 5 x 5cm horizontal chamber (see Section 4.2.1.2). 

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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. 

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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