Possibilities for Optimizing Concentration Gradients

In theory, it is possible to derive the optimal conditions for a gradient elution with a sodium hydroxide eluant from the functional dependence of log (Vms-Vd)/Vd from log R. However, as mentioned before, this applies only to simple linear gradients with the initial eluant ion concentration of zero, which is rarely used for practical purposes. Much shorter analysis times are obtained when the gradient run starts at a higher eluant ion concentration than zero. Furthermore, gradient programs with different ramps, sometimes combined with isocratic periods, have to be developed to obtain optimal selectivity and speed of analysis. A mathematical description of the retention is impossible in all these cases, because the resulting equation for the calculation of the retention volume would be far too complex. 

However, Eq. (98) can be employed to predict trends. Refer to Fig. 3-151 in which log (Vms-Vk) / Va is plotted as a function of log R for various inorganic and organic anions. It appears from this representation that the elution order of anions with different valencies depends on the steepness of the gradient ramp. In case two anions with different valency co-elute, a separation of both components can be accomplished by employing a steeper ramp, whereas the ion with the higher valency elutes first. 

A quantitative statement regarding the increase of the elution strength when switching from one eluant to another may be derived from the equation proposed by Fritz et al. [102] for calculating the backgrormd conductivity of a mobile phase with fully dissociated eluant ions  

Consider two eluants of different elution strength containing the same eluant anions but different cations with equivalent conductances Xi and X. Under isoconductive conditions, the background conductivity, Gi, of the weaker eluant equals that of the stronger eluant Gz. Given Gi = G2, Eq. (100) may be rearranged, so that the concentrations, Ci and Cz, of both eluants are related to the corresponding equivalent conductances  

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