Eluotropic series optimum

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. 

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. 

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