Elution order reversal

The elution order of the resolved enantiomers can be controlled by the ligand. Generally, the D-ligand selector gives an elution order reversed from the L-ligand selector elution order, but this is not always the case [249],... 

Figure 11.23 Qualitative prediction of the elution order reversal of cis- and frans-androsterone. (Left) Anal5dical and preparative sample sizes. High concentration band profiles calculated with the LeVan-Vermeulen isotherm model. Inset experimental data [23], Stationary phase silica modified with a pH 6.8 phosphate buffer. Mobile phase (9 1) acetonitrile-dichloromethane, 0.98 mL/min. Samples, cis-androsterone 0.026 and 5.2 mg, trans-androsterone 0.15 and 1.8 mg. (Right) Band profiles calculated with the competitive Langmuir model. Reproduced with permission from S. Golshan-Shirazi, J.-X. Huang and G. Guiochon, Anal. Chem., 63, (1991) 1147 (Fig. 8). ( )1991, American Chemical Society.

Figure 18.14 Window diagram. The separation was performed with a linear gradient from 0 to 45% B and the optimum runtime needs to be found out. (a) Gradient in 15 min (b) gradient in 45 min with some elution orders reversed (c) window diagram calculated from the initial two experiments with a linear relationship between retention time and %B assumed the plot shows the resolution / of the peak pair which is critical under the respective conditions it is necessary to use long gradient runtimes to obtain a good resolution (d) optimized chromatogram with 0-45% B in 80 min, but separation is already finished after 45 min and 25% B.

This material is synthesized from 4-(3,5-dinitrobenzamido)-tetrahydro-phenanthrene which is also covalently bonded to silica particles 5 pm in diameter. It is used to separate the underivatized isomers of drugs such as Ibuprofen and Naproxen. It is a fairly stable phase and is available in both enantiomeric forms allowing elution order reversal if so desired. 

Curves Relating the Corrected Retention Volume of the Two Isomers to Temperature Showing Elution Order Reversal... 

Interesting selectivities were reported from the early publications of MLC, mostly in the form of elution reversals. Peak crossing is often considered proof of enhanced selectivity. However, although elution order reversals are indicative of a unique separation mechanism in MLC, peak crossing in a complex mixture does not necessarily mean an improvement in the overall separation. 

Pirkle and Murray first reported the temperature-dependent elution order reversal in Pl-basic proline-derivatized CSPs in 1993. They used (/ 5)-A-(3,5-dinitrobenzoyl)-a-phenylethylamine as the solute to investigate the response of the chromatographic behavior by changing the temperature on the CSP. 

In reversed-phase LLC the two phases are interchanged and the elution order reversed. This mode is more suitable for hydrophobic analytes that have poor water solubility. For reversed-phase LLC, the support should be hydrophobic to ensure appropriate fixing of the stationary phase liquid. This can be achieved, for instance, through silan-ization of the silica. 

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