General elution problem

The general elution problem in chromatography. Improving the resolution of the overlapping bands in chromatogram (a) results in a longer analysis time for chromatogram (b). 

Separation scientists speak of a general elution problem when asked to develop a universal separation method using chromatography. What is the problem (Hint you may need to consult works by Snyder or Heftmann in the library.)... 

General elution problem For a complex mixture, isocratic conditions can often be found to produce adequate separation of early-eluting peaks or late-eluting peaks, but not both. This problem drives us to use gradient elution. 

The general elution problem. A Normal elution. B Gradient elution. From L. Ft. Snyder, J. Chromatogr. Sci. 8, 692 (1970) Reproduced by permission of Preston Publications. 

Figure 6.1 Illustration of the general elution problem in chromatography. Chromatograms a and b constant elution conditions. Chromatogram c (opposite page) programmed analysis.

The vertical lines a and b correspond to temperatures at which chromatograms would be obtained which are similar to the chromatograms a and b in Figure 6.1. Hence, we are confronted with the general elution problem . 

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). 

Fig. 1.25. An example of ihe general elution problem with ihc separation of a homologous series of l,2-naph(hoylenebenzimida/,olesulphonamide derivatives of ten homologous alkylamines — methyl (/) to n-decyl (/O) — by reversed-phase HPLC on a Liehrosorb RP-18. 10 pm, column (300 x 4 mm i.d.). (A) Isocratic separation with methanol-water 95 5 as the mobile phase. (B) Isocraiic separation with methanol-water 80 20 as the mobile phase. (C) Gradient-elution separation with linear concentration gradient from methanol-water 70 30 to l(X) methanol in 20 min. Flow rale I ml/min. fluorimctric detection, ycx = 365 nm, ytm >410 nm. The numbers of peaks agree with the number of carbon atoms in n-alkylamines.

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... 

General elution problem The compromise between elution time and resolution addressed through gradient elution (for liquid chromatography) or temperature programming (for gas chromatography). 

This is then the general elution problem and there are several ways of dealing with it ... 

For complex mixtures where components to be separated include a wide range of polarities, the k values are too dissimilar, and the peaks tend to crowd together near the start, while later peaks are broadened too greatly to be measured with accuracy (Figure 7.5A). This common difficulty encountered in the analysis of multicomponent samples is referred to as the general elution problem. 

Figure 7.5 The general elution problem. (A) Normal elution, (B) Gradient elution.

Fig. 2-8. Illustration of the general elution problem, (a) tiution with a weak solvent A (b) elution with a strong solvent B (c) elution with a mixture of A and B (d) stepwise elution with A, AS, and B (e) gradient elution beginning with solvent A and ending with B. Reprinted from Chromatographic Reviews (24) by permission of Elsevier...

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