Liquid chromatography column regeneration

Using the purified (S)-3-hydroxycarboxylate oxidoreductase (107) and two forms of NADPH regeneration led to the chiral 3-hydroxy carboxylates shown in Table 28 (83). All products of the bioreduction procedures showed ee values greater than 98 %. On the basis of the optical rotation value reported (108) the synthesized 3-hydroxybutyrate was the (5)-enantiomer. It was assmned that the configuration of the other prepared 3-hydroxycarboxylates was the same. As checked with racemic mixtures of the derivatives of the four 3-hydroxy acids mentioned in Table 28, the applied chiral gas/liquid chromatography columns separated the enantiomers. For details see (83). 

The selection of the counter-ion and its concentration are important for the separation of ionic compounds in reversed-phase and ion-exchange liquid chromatography. The addition of hydrophobic ions is an especially powerful method and several surfactants can be used as hydrophobic counter-ions. The theoretical column efficiency of ion-pair liquid chromatography is much better than that of an ion-exchange column, and the regeneration of a column is much faster. Thus, if we can control ion-pair liquid chromatography, we can solve a separation problem. (The important background sources in this area are listed at the end of the chapter.)... 

However, it can be assumed for most electrochemical applications of ionic liquids, especially for electroplating, that suitable regeneration procedures can be found. This is first, because transfer of several regeneration options that have been established for aqueous solutions should be possible, allowing regeneration and reuse of ionic liquid based electrolytes. Secondly, for purification of fiesh ionic liquids on the laboratory scale a number of methods, such as distillation, recrystallization, extraction, membrane filtration, batch adsorption and semi-continuous adsorption in a chromatography column, have already been tested. The recovery of ionic liquids from rinse or washing water, e.g. by nanofiltration, can also be an important issue. 

Liquid-liquid chromatography is a more efficient separation technique than liquid-solid chromatography. Separation of similar compounds (e.g. members of a homologous series) is carried out more effectively and hence preferably by LLC. Also, LSC suffers from the drawback that usually the column cannot be regenerated for re-use, since the active sites can become poisoned by the irreversible adsorption of highly active compounds. On the other hand, in LLC the immobilized liquid film on the solid support can be essily washed off and replaced without the column having to be replaced. 

Liquid chromatography LC can be used as a cleanup technique. One of the biggest advantages of LC is the possibility of automation. However, LC analytical columns can be deactivated by sample coextractives and regeneration of the columns is time consuming. A solution is to use small precolumns in series with the analytical column this minimizes the effect. Good results are obtained for chlorophenols and other aromatics. 

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