Phases through mobile-phase additives

Enantiomeric separation through mobile-phase additives is a very powerful method that unfortunately lost partially its use with the advancements in chiral stationary phase technology. With the development of capillary electrophoresis, however, this technique faces a revival showing its power to separate enantiomers. 

The book provides systematic and detailed descriptions of the numerous approaches to chiral resolution. The first chapter is an introduction to basic concepts of molecular chirality and liquid chromatography. Chapters 2 through 9 discuss the chiral resolution of various classes of chiral stationary phases. Chapter 10 deals with chiral resolution using chiral mobile phase additives. These discussions elaborate the types, structures, and properties of the chiral phases,... 

Chromatographic enantioseparations can be achieved when the diastereoisomeric interaction is established via a precolumn covalent derivatization, or a non-covalent association formed within the chromatographic system through the use of chiral mobile phase additives (primarily an LC technique) or chiral stationary phases. 

It is well known that a chiral environment is essential for the enantiomeric resolution of racemates. In CE, this situation is provided by the chiral compounds used in the BGE and is known as the chiral selector or chiral BGE additive. Basically, the chiral recognition mechanisms in CE are similar to those in chromatography using a chiral mobile-phase additive mode, except that the resolution occurred through different migration velocities of the diastereoisomeric complexes in CE. The chiral resolution occurred through diastereomeric complex formation between the enantiomers of the pollutants and the chiral selector. The formation of diastereomeric complexes depends on the type and nature of the chiral selectors used and the nature of the pollutants. 

A third approach to modulate silanol effects is through mobile-phase pH control. At pHs below 4, most silanol groups will be protonated and less available as cation exchange sites. Addition of acetic or formic acid to the mobile phase is useful for this purpose. Buffer concentrations of about 25 mM will promote retention of buffer cations rather than analyte molecules, but can promote ion suppression. 

The heart of liquid chromatography lies in the highly selective chemical interactions that occur in both the mobile and stationary phases. It is now possible to rapidly separate compounds whose difference in free energy of transfer between the mobile and stationary phases is only a few calories per mole. Columns exhibiting virtually every type of possible selectivity exist—from shape selectivity to charge selectivity to size selectivity to enantio-selectivity. It is also possible to generate additional selectivity through clever manipulations of the mobile phase additives that interact with the solute in the mobile phase can create unique selec-tivities in columns that do not show that type of selectivity. 

Resolution of enantiomers of amino acids through MR has been found to be simple and rapid on RP TLC plates without using impregnated plate or a chiral mobile phase additive. Enantiomers of glutamate and aspartate... 

In the analytical chromatographic process, mixtures are separated either as individual components or as classes of similar materials. The mixture to be separated is first placed in solution, then transferred to the mobile phase to move through the chromatographic system. In some cases, irreversible interaction with the column leaves material permanently attached to the stationary phase. This process has two effects because the material is permanently attached to the stationary phase, it is never detected as leaving the column and the analysis of the mixture is incomplete additionally, the adsorption of material on the stationary phase alters the abiHty of that phase to be used in future experiments. Thus it is extremely important to determine the ultimate fate of known materials when used in a chromatographic system and to develop a feeling for the kinds of materials in an unknown mixture before use of a chromatograph. 

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