Chromatographic modes chiral

Several chromatographic modes will be reviewed in this respect, and most will make use of a chiral support in order to bring about a separation, differing only in the technology employed. Only countercurrent chromatography is based on a liquid-liquid separation. 

The mechanisms described above form the basis for the chromatographic modes described in Chapter 2, namely, normal-phase, reversed-phase, size-exclusion, ion-exchange, and affinity chromatographies. However, other modes that are variations of those mentioned above, such as hydrophobic-interaction chromatography (HIC), chiral, ion-exclusion, and ion-pair chromatographies are also used and will be mentioned. 

There are five major chromatographic modes that can be applied to the analysis of solutes in solution normal phase, reversed phase, ion exchange, size exclusion, and affinity. In addition, a variety of submodes exist, such as hydrophobic interaction, chiral separations, ion suppression, and ion pairing. 

Macaudiere, E., Lienne, M., Caude, M., Rosset, R., and Tambute, A. 1989. Resolution of Jt-acid racemates on Jt-acid chiral stationary phases in normal-phase liquid and subcriti-cal fluid chromatographic modes A unique reversal of elution order on changing the nature of the achiral modifier. Journal of Chromatography, 467 357-72. 

The chromatographic modes typically used in speciation analysis are size-exclusion, ion-exchange, ion-pair reversed-phase, reversed-phase and, to a lesser extent, micellar, vesicular, chiral and affinity LC. Detailed descriptions of their capabilities and limitations can be found in a number of comprehensive reviews.i>2,9-ii,i4... 

Normal-phase HPLC usually offers much improved separation of positional isomers or stereoisomers with respect to RPLC. This is also the reason why normal-phase liquid chromatographic mode with nonaqueous mobile phases is often used for the separation of enantiomers on chiral bonded stationary phases. 

The separation of antipodes by chromatography has developed into an important tool for resolution in recent years [1-3]. The newly introduced procedures utilize various chromatographic modes, such as GC, LC, HPLC, TLC [4] and DCCC [5], and imply by necessity at least one chiral phase (stationary or mobile). 

Chapter 7 is devoted to important physicochemical —basically mechanistic — aspects of the direct enantioseparations, carried out by using either CSP or mobile phase. In such cases, the diversity of the involved separation mechanisms is much greater than the most of other chromatographic modes (and, particularly, when compared with the relatively simple physicochemical rules governing adsorption or partition liquid chromatography). Thus, the author of this chapter discusses enantioseparation in terms of the solute-chiral selector complexation constants, stoichiometry and selectivity of complexation, the nature of the binding sites on the stationary phase surface, and, finally, the supramolecular mechanisms of complexation. 

The need to develop and use chiral chromatographic techniques to resolve racemates in pesticide residues will be driven by new hazard and risk assessments undertaken using data from differential metabolism studies. The molecular structures of many pesticides incorporate chiral centers and, in some cases, the activity differs between enantiomers. Consequently, in recent years manufacturers have introduced resolved enantiomers to provide pesticides of higher activity per unit mass applied. For example, the fungicide metalaxyl is a racemic mix of R- and 5-enantiomers, both having the same mode of action but differing considerably in effectiveness. The -enantiomer is the most effective and is marketed as a separate product metalaxyl-M. In future, it will not be satisfactory to rely on hazard/risk assessments based on data from metabolism studies of racemic mixes. The metabolism studies will need to be undertaken on one, or more, of the resolved enantiomers. 

Another method for creating a chiral environment is lo add an optically pure chiral selector to a bulk liquid phase. Chiral additives have several advanlages over chiral stationary phases and continue lo be the predominant mode for chiral separations by tic and capillary electrophoresis (cc). First of all, the chiral selector added to a bulk liquid phase can be readily changed. The use of chiral additives allows chiral separations lo be done using less expensive, conventional stationary phases. A wider variety of chiral selectors are available [ be used as chiral additives than are available as chiral stationary phases, thus, providing the analyst with considerable flexibility. Finally, the use of chiral additives may provide valuable insight into (he chromatographic conditions and/or likelihood ol success with a potential chiral stationary-phase chiral selector. This is particularly important for the development of new chiral stationary phases because of the difficulty and cosl involved. 

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