Fundamentals of retention

The equilibrium constant for this reaction is called the selectivity coefficient The magnitude of the selectivity coefficient is an 

Substituting the exchange capacity (Cap.) of the column for the stationary phase concentration of the counterion [CS], and k for the ratio of the metal concentration in the stationary phase to the metal concentration in mobile phase [(M ) mS ] an expression is 

Equation 11 can be used for the determination of capacity factors in ion-exchange systems, but only if the eluent is unreactive to the metal eluite cations. This requisite mandates that no strong metal-ligand complexes be formed within the column, and that each metal exist predominately as the free ion M throughout the elution process. 

Accordingly, eqn. 11 cannot be applied to ion-exchange separations which utilize complexing eluents. 

This equation was obtained by substituting [M ] for [M ] in eqn. 10, where [M ] represents the total concentration of metal (free and complexed) in solution. The capacity factor ( j m+) was then substituted for the ratio [M /nS ] [M ]. Sevenich and Fritz tested the validity of this equation and found fairly good agreement for those divalent ions studied. Some trivalent metals were also tested, but these did not show the same degree of correlation as the divalent metals. 

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One of the most powerful features of HPLC stems from the fact that the eluent is not merely a transport medium, it rather contributes significantly to the mechanism of separation. Retention as well as selectivity arise from the combined action of mobile and stationary phase on the solutes. Because of its distinct physicochemical characteristics, a given solvent interacts in a specific manner with the solutes. Its capacity to donate or accept protons or to induce a dipole moment defines the nature of its interaction with the solutes in solution. Slight differences in these interactions complemented by stationary-phase action are often enough to provide the desired selectivity in HPLC. Solvent classification based on their elution strength or polarity represents a classic area of investigation into the fundamentals of retention mechanisms in HPLC (2S-27), and the chapter is not yet closed on its development and refinement (28-30). Here we will present a practical and comprehensive way to exploit the effect that the nature of the solvent has on retention and selectivity in reversed phase HPLC. 

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