Liquid chromatography eluotropic series

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 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). 

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. 

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