Displacement chromatography complex mixture separation

Displacement chromatography is commonly used for preparative-scale separations, but, because of its focusing or concentrating effect, it also shows potential on the analytical scale, for example, for the concentration of minor components in complex mixtures.24,25 Operationally, displacement chromatography is similar to the step elution process, except that in the displacement process the mobile phase has a greater affinity for the stationary phase than for the sample components, and therefore the components are eluted ahead of the displacer front. The focusing effect of displacement chromatography is due to the fact that the concentration of the displacer determines the concentration of the product bands.26... 

Frontal analysis and displacement have received exhaustive theoretical treatment in numerous original articles [e.g.. Refs. 33-36)] and books on chromatography [e.g.. Refs. 4,6,13,15)]. Today frontal analysis and displacement are essentially obsolete as means for the separation or analysis of complex mixtures. We will therefore limit ourselves to a qualitative... 

In two-way chromatography displacement effects are exploited as wellt. Here, there is no addition of a displacing species. By alternating the direction of flow, the more retained species in the feed serve as the displacer for the less retained species. Two-way chromatography may lead to an elevation of the concentration. However, a serious drawback is that it is far too complex for separation of a multi-component mixture. 

In addition to the dimeric 1,2,4-trithiane derivative 58 (cf. Scheme 8), isolated as a side product in very low yield and characterized by X-ray crystallography (videsupra), 5,6-benzo-l,2,4-trithiin 239 was isolated from the continuous sulfur-carbon displacement reaction of benzopentathiepin 236 <1997H(44)187>. Upon treatment with the phosphorus ylides 237 in CH2CI2, mixtures of benzotetrathiepins 238 and benzotrithiins 239 were obtained in moderate yields (NaH was deemed the base of choice cf. Scheme 69). The new products 238 and 239 were separated easily by high-performance liquid chromatography (HPLC) and the structures characterized by H NMR spectroscopy, whereby the chemical shift of the methine proton was indicative of the cyclic structures that were formed (5.40-5.44 ppm for 238, but 5.96-6.07 ppm for 239). The present desulfurization mechanism is complex and is still unclear because likely intermediates have not been isolated or detected directly by spectroscopic techniques <1997H(44)187>. 

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