The eluotropic series

It is clear from the above that not every solvent elutes sample molecules at the same speed. If an ahphatic hydrocarbon constitutes the mobile phase, it becomes very difficult to displace the sample molecules from the active adsorbent sites and the solvent is classified as weak. Tetrahydrofuran, on the other hand, competes strongly for the active sites, leaving less time for sample molecules to be adsorbed and hence they are rapidly eluted. This type of solvent is comparatively strong. 

The elution force or strength of the various solvents is determined empirically and recorded numerically, being represented by the symbol s . The series of weak. 

The most common adsorption isotherm on sihca is of the Langmuir type, the ideal shape of which is as follows  

It is possible to obtain a desired elution strength by mixing two solvents. A choice of 

20% of tert-butylmethyl ether in dichloromethane, 45% of tetrahydrofuran in dichloromethane, 

The influence of mobile phase strength on the separation is shown in Fig. 9.3. Hexane is too weak a solvent for the analysis of this test mixture and the separation needs an unnecessarily long time. r Butylmethyl ether is much too strong. A suitable eluent is the mixture hexane- r butylmethyl ether (9 1) (volume parts). 

The elution force or strength of the various solvents is determined empirically and recorded numerically, being represented by the symbol 8. The series of weak, medium strength and strong solvents thus derived is referred to as the eluotropic series. The eluotropic series of various solvents is given in Section 5.1. 

It is possible to obtain a desired elution strength by mixing two solvents. A choice of binary mixtures is shown in Fig. 9.4. If, for example, a solvent strength of 8 = 0.4 is needed, this can be obtained by the following binary eluents  

Unfortunately the calculation of the strength of a solvent mixture is tedious and cannot be made without computer aid.  

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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]. 

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. 

The following table contains the common solvents used in thin-layer chromatography, with a measure of their strengths on silica gel and alumina. The solvent strength parameter, s°, is defined as the relative energy of adsorption per unit area of standard adsorbent.13 It is defined as zero on alumina when pentane is used as the solvent. This series is what was called the eluotropic series in the older literature. For convenience, the solvent viscosity is also provided. Note that the viscosity is tabulated in cP for the convenience of most users. This is equivalent to mPasec in the SI convention. Additional data on these solvents may be found in the tables on high-performance liquid chromatography. 

A ranking of solvents or solvent mixtures in order of increasing eluent strength for a particular stationary phase is known as an eluotropic series. (In Chapter 4, Fig. 4-7 included an eluotropic series for alumina.) The list in Table 5-3 is the eluotropic series for various compounds on silica (20). Since silica is a polar material capable of hydrogen bonding and ion-exchange... 

Just as we can rank the polarity of the solvent (the eluotropic series) with respect to the packing, the polarity of the molecule can be ranked based upon the presence (and/or position) of the major functional groups that it contains. Table 5-4 ranks some common functional groups in a qualitative fashion. As a rule of thumb, if a molecule has more than one functional group, the one with the largest polarity will determine the polarity of the molecule relative to other molecules in the sample mixture. 

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. ... 

The principle of the variation of solvent composition while holding solvent strength constant was first developed by Neher for separation of steroids. Figure 1 is a representation of the eluotropic series of Neher for application in this fashion. Six solvents, which will act as solvent Si (100% concentration, on the left), are arranged vertically, whereas the same solvents, acting as solvent S2 (100% concentration, on the right of the... 

Several mobile-phase optimization strategies in TLC are based on the use of isoeluotropic solvents (i.e., solvent mixtures of identical strengths but different selectivities). Selecting mobile phase will be achieved based on the eluotropic series (Figs. 1 and 2). These considerations are... 

An eluotropic series is a tool to influence the retention of sample components. If the retention of a product has to be reduced, a solvent of higher ranking in the eluotropic series is chosen. Due to increased competition between product and solvent molecules for the active sites on the adsorbent, the retention factor of the product will automatically decrease. The same holds true for the opposite situation. 

Giddings et al. go on to relate that the chemical effect of the mobile phase in chromatography is manifested in the eluotropic series, which orders the elution strength of various liquids. They compared the elution power of a number of liquids with their solubility parameters, and they found a striking parallel between the two, i.e., Snyder s values correlated very well with Hildebrand s 8. Therefore, Giddings et al. suggested that the elution power of compressed gases may be related to the solubility parameter. But they stated explicitly... 

Despite the widespread use of these eluotropic series, several workers have challenged their general validity and usefulness. Many studies have been reported in which it is quite clear that relative solvent strength can vary drastically for different samples [e.g.. Refs. (15-17) for some striking examples]. Similarly, the eluotropic series observed for different adsorbents often appear markedly different e.g., see the discussion of Ref. (7),... 

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. 

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