Eluotropic series chromatography

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

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. 

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. 

Eluotropic Series of Solvents for Thin-Layer Chromatography Simion Gocan... 

A practical eluotropic series of solvents, based on the expended solubility parameter concept, was reported. This series was defined based on partial specific solubility parameter (5 ) that is equal to the sum of Keeson (5q) and acid-base (2 a b)> which represents the contribution to interaction forces introduced to characterize the solute, the mobile, and the stationary phase in liquid-solid chromatography. Exactly the same two interaction forces define e° and, consequently, there should exist a direct relation between e° and s = o+2 a b- Unfortunately, the general correlation for all the solvents on alumina is poor (r =0.75). 

Subsequent collapse of the packing can be prevented by pumping about 0.51 of solvent through at this maximum pressure level, that used later for chromatography being perfectly adequate. A silica column should be equilibrated with an eluotropic series of solvents (e.g. methanol ethyl acetate —> ferf-butyl-methyl ether —> hexane) if the slurry has been topped up with water. Finally, the column... 

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... 

The considerations on the choice of eluents for straight phase separations on polar bonded phase materials do not differ much from those used in adsorption chromatography and in conventional liquid-liquid chromatography. Non-polar solvents with the addition of polar modifiers are used, and eluotropic series as in Table 4.4.1 are useful in the adjustment of eluting strength. 

An eluotropic series can be used to find an optimum solvent strength for a particular separation. Using a solvent of constant composition is called isocratic elution. If an isocratic solvent is too strong (if the k values for the solutes are too small), a weaker solvent is substituted. On the other hand, if the initial solvent is too weak (the k values are too large), a stronger solvent is selected. This trial-and-error approach to finding the optimum solvent can be done more rapidly by TLC than by column chromatography. 

Several mobile-phase optimization strategies in TLC are based on the use of isoeluotropic solvents (i.e., solvent mixmres of identical strengths but different selectivities). Selecting mobile phase will be achieved based on the eluotropic series (Figs. 1 and 2). These considerations are very general. The selection and optimization of one system of eluent is a more complex problem and must be discussed for each particular system. For example, the separation of 13 phenylurea and s-triazine herbicides was performed by overpressured layer chromatography (OPLC) with a binary... 

ELUOTROPIC SERIES. A series of solvents arranged in order of increasing ability to displace a solute, i.e., increasing polarity in adsorption chromatography. 

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