General elution problem solutions

While such large changes in a are limited to compounds of quite different structure, it is likely that some preparative separations can be greatly facilitated by maximizing a in this way without limit. In analytical separations, however, values of a larger than 5-10 can actually be detrimental, because they result in too large a difference in retention times for the two solutes (the general elution problem, Ref. 1). 

To illustrate the general elution problem and its solution, let us consider the following situation. Consider a 20-component mixture with capacity factors k of the components forming a geometrical progression and exponentially dependent on the modifier concentration (molar or volume fraction c), in accordance with the Snyder-Soczewinski model of adsorption [2]. The log k versus log c plots of the 20 solutes are given in Fig. 1, which has a parallel Rf axis subordinated to the right-hand-side log k axis. It can be seen that no isocratic eluent can separate all the components. A pure modifier [c = 1.0 (100%)] separates well solutes 1-7, and the less polar solutes are accumulated near the solvent front for c = 0.1 (10%), solutes 8-14 are... 

In 1977 Krishnamurthy and Subramanian published an exact theoretical analysis of FFF [19], based on their generalized dispersion theory. Without touching on the details of a complicated mathematical treatment, with the aid of which they solved the problems of both the separation and dispersion processes that occur in the FFF channel during the complete separation from the injection to the elution, let us only say in general that their solution makes it possible to explain some experimental artefacts in detail. These artefacts could not be explained by means of the non-equilibrium theory of FFF mentioned above, which is based on some asymptotic assumptions. Perhaps the most important discrepancy between the theory and the experimental data is that the zone spreading that is observed is considerably larger than the spreading predicted by the theory. 

Ion-exchange columns can be substituted into the general HPLC instrument shown in Eigure 12.26. The most common detector measures the conductivity of the mobile phase as it elutes from the column. The high concentration of electrolyte in the mobile phase is a problem, however, because the mobile-phase ions dominate the conductivity, for example, if a dilute solution of HCl is used as the mobile phase, the presence of large concentrations of H3O+ and Ck produces a background conductivity that may prevent the detection of analytes eluting from the column. 

While the first two difficulties have been overcome there is no general solution available for the problem of incompatibility of mobile-phase composition. LC-MS systems are more complicated than GC-MS, as the eluted substances are mostly involatile, co-eluted with solvent, and frequently not efficiently ionised by El or Cl processes. Solutions to the problem are various, including surface ionisation (SIMS, FAB, FD, HSI,... 

This problem has been discussed by Snyder (8,10) and Scott 9, II). A general solution is difficult to give since it would depend oh the composition of the sample mixture, e.g., the concentration of the la eluting components, and the detection limit, which varies in liquid chromatography with the chemical nature of the sample component. Therefore some arbitrary assumptions have to be made. From Eq. (26), using the maximum permissible sample size given by Eq. (73), we can write foi the retention volume, Vr, the following expression ... 

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