Elution power, relative

When the solvent has nearly reached the top of the adsorbent layer, the components should be well separated. The relative distance travelled by the components can be increased by using a solvent of higher or lower polarity in the order of increasing eluting power ... 

Fig. 27. The dependence of In K.d/M for polybutadienes of different molecular weight on the relative elution power of binary solvents...

As the relative elution power depends not only on the adsorbent, but in many cases on the compound types being separated, there exists no universal eluotropic series of solvent strengths. This series was given by L. R. Snyder". For another eluotropic solvent series, see J. C. Touchstone Practice of Thin-Layer Chromatography, 2 " ed., Wiley, Chichester, 1983. 

The relative effectiveness of an organic modifier in MLC appears to be directly related to its own ability to partition and bind to the micellar pseudophase. That is, the better the organic modifier binds to the micellar assembly, the greater its ability to alter the retention of solutes. Eq. 5.9 is also valid for hybrid micellar eluents (Fig. 5.5). For these eluents, solute binding constants to micelles and the partitioning into the stationary phase both decrease as a result of the addition of the modifier, especially for highly hydrophobic solutes. However, the K m/Kas ratio increases and, therefore, the elution power of the mobile phase is greater. 

The choice of eluent is a compromise between eluting power and eluent concentration. The choice of the eluent is dependent on the relative affinities of the eluent ion and the s lmple ions for the resin. Both the ion species should have roughly equal affinities for optimum separation. AgNOg, HCl, NaHCO I, and NaOH are most commonly used eluents. 

Some detectors can give additional information about the elutes (the eluted solutes). One example is the gas chromatograph—mass spectrometer (GC-MS), which produces a mass spectrum of each compound as well as its mass and location in the chromatogram. This powerful means of detection can be used when standard samples are not available to help determine the identities of the solutes. A beam of ions bombards each compound as it emerges from the chromatograph. The compound breaks up into ions of different masses, providing a spread of narrow peaks instead of one peak for each compound. The relative amount of each fragment is determined and used to help identify the compound. 

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