Elutropic series

The choice of solvent or a mixture of solvents used in TLC is solely guided by two important factors (a) the nature of the constituent to be separated i.e., whether it is polar or non-polar and (b) the nature of the process involved i.e., whether it is a case of adsorption or partition chromatography . It has been observed that the rate of migration of a substance on a given adsorbent depends upon the solvent used therefore, the latter may be arranged in order of the elutive power, usually termed as the elutropic series as shown in the following Table 28.1. 

From actual experimental results it has been established beyond any reasonable doubt that the mixtures of two or three solvents of different polarity mostly offer distinct and much improved separation as compared to chemically homogeneous solvents. Table 28.2 records the elutropic series of one and two component solvents. 

RELATIVE STRENGTH OF SOME SELECTED SOLVENTS ON SOME ADSORBENTS ELUTROPIC SERIES. REPRODUCED WITH PERMISSION FROM [43]... 

The elutropic series for a number of solvents is given in Table 2. The solvents are arranged in increasing polarity, with n-pentane the least polar. This is the order of ability of these solvents to dissolve polar organic compounds and to dislodge a polar substance adsorbed onto either silica gel or alumina, with n-pentane having the lowest solvent power. 

Two models have been proposed to describe the process of retention in liquid chromatography (Figure 3.3), the solvent-interaction model (Scott and Kucera, 1979) and the solvent-competition model (Snyder, 1968 and 1983). Both these models assume the existence of a monolayer or multiple layers of strong mobile-phase molecules adsorbed onto the surface of the stationary phase. In the solvent-partition model the analyte is partitioned between the mobile phase and the layer of solvent adsorbed onto the stationary-phase surface. In the solvent-competition model, the analyte competes with the strong mobile-phase molecules for active sites on the stationary phase. The two models are essentially equivalent because both assume that interactions between the analyte and the stationary phase remain constant and that retention is determined by the composition of the mobile phase. Furthermore, elutropic series, which rank solvents and mobile-phase modifiers according to their affinities for stationary phases (e.g. Table 3.1), have been developed on the basis of experimental observations, which cannot distinguish the two models of retention. 

Table 3.1 Elutropic series for some modifiers commonly used in normal-phase and reversed-phase liquid chromatography, arranged in order of increasing solvent strength...

Solvents can be grouped for adsorption chromatography into a so-called elutropic series according to their elution strengths. The choice of solvent system is made in relation to the polarity of the adsorbent and the mixture to be separated, utilizing the elutropic solvent series. This series orders solvents according to their hydrophilic or hydrophobic character. A representative elutropic series beginning with the most hydrophobic solvent is as follows ... 

To experimentally determine a PC mobile phase for an unknown, a number of solvents from the middle part of the elutropic series (see TLC Section 5.2.3.3) are tried, after equilibrating the paper with water vapor. If the Rp value is too high, the mobile phase is too polar relative to the solutes and a solvent closer to the top of the table should be used. If the Rp value is too low, the solvent is too hydrophobic compared to the solute, and a more polar solvent is needed. The R p region between... 

Various workers have recorded elutropic series, e.g. Trappe [4], Snyder [5], Strain [6], BickofT [7] and Knight and Groennings [8]. Generally, for polar adsorbents such as alumina and silica gel, the strength of adsorption increases with the polarity of the adsorbate. For carbon the order is reversed. Trappe s elutropic series found that the eluting power of a series of solvents for substances adsorbed in columns such as silica gel decreased in the order ... 

Solvent strength describes the composition of a liquid mobile phase in terms of its ability to elute components in normal and reverse phase chromatography see elutropic series. 

Thin layer chromatography, TLC a form of planar chromatography having a coating of a solid stationary phase, e.g. silica gel or alumina, on a plate of glass or inert plastic. The liquid mobile phase moves up the plate by capillary action, reverse phase TLC plates are available where a liquid stationary phase is immobilised on the solid support. The polarity of the mobile phase is carefully chosen to obtain the best separation see elutropic series. 

Table 1 Elutropic series of some pure solvents used as mobile phases in liquid chromatography and the relevant values of e, m, and P ...

As noted in the discussion of TLC, the strength of the adsorption of an organic compound to the solid support depends on the polarity and nature of the adsorbent as well as on the nature of the functional groups present in the molecule. When normal-phase column chromatography is performed, a polar stationary phase such as alumina or silica gel is used in combination with organic solvents as the mobile phase or eluant. Under these conditions, the elutropic series described for TLC in Section 6.2 applies. 

To prepare TLC mobile phases, solvents from the elutropic series are blended into binary or ternary mixtures of the correct strength. In most cases, the strength of a solvent mixture will be intermediate between the strengths of the two (or more) components of the mixture. In addition to the proper strength, the mobile phase must also provide adequate selectivity to achieve the required separation. 

The following series of solvent blends provides a regular increase in e" from 0 to 0.95. A variety of these can be used instead of single solvents of the elutropic series for initial screening of optimum solvent strength ... 

To modify selectivity after locating the optimum 8 , the most polar component is substituted in whole or part by one just above or just below in the elutropic series. For example, acetonitrile-toluene (80 20) can be changed to acetonitrile-nitromethane-toluene (40 40 20). 

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