Chiral mobile-phase additives

Two mechanisms for chiral separations using chiral mobile-phase additives, analogous to models developed for ion-pair chromatography, have been... 

An alternative model has been proposed in which the chiral mobile-phase additive is thought to modify the conventional, achiral stationary phase in situ thus, dynamically generating a chiral stationary phase. In this case, the enantioseparation is governed by the differences in the association between the enantiomers and the chiral selector in the stationary phase. 

High Performance Liquid Chromatography. Although chiral mobile phase additives have been used in high performance Hquid chromatography (hplc), the large amounts of solvent, thus chiral mobile phase additive, required to pre-equiUbrate the stationary phase renders this approach much less attractive than for dc and is not discussed here. 

A82846B [24] and LY307599 [25] were developed as chiral selectors for CE LY333328 [26] and A35512B [27] were applied as chiral mobile phase additives in narrow-bore HPLC. 

Guo, Z., Wang, H., and Zhang, Y., Chiral separation of ketoprofen on an achiral C8 column by HPLC using norvancomycin as chiral mobile phase additives, J. Pharm. Biomed. Anal, 41, 310, 2006. 

Sharp, V.S. et al.. Enantiomeric separation of dansyl amino acids using macrocyclic antibiotics as chiral mobile phase additives by narrow-bore high-performance liquid chromatography. Chirality, 16, 153, 2004. 

Sun, Q. and Olesik, S.V., Chiral separation by simultaneous use of vancomycin as stationary phase chiral selector and chiral mobile phase additive, J. Chromatogr. B, 745, 159, 2000. 

Chiral resolution by HPLC can by divided into three categories (1) a direct resolution using a chiral stationary phase (CSP) (2) addition of a chiral agent to the mobile phase, which reacts with the enantiomeric analytes (chiral mobile phase additive method (CMPA)) (3) an indirect method that utilizes a precolumn diastereomer formation with a chiral derivatization reagent (Misl anova and Hutta, 2003). 

CSPs and chiral mobile phase additives have also been used in the separation of amino acid enantiomers. Another technique that should be mentioned is an analysis system employing column-switching. D-and L- amino acids are first isolated as the racemic mixture by reverse-phase HPLC. The isolated fractions are introduced to a second column (a CSP or a mobile phase containing a chiral selector) for separation of enantiomers. Long et al. (2001) applied this technique to the determination of D- and L-Asp in cell culture medium, within cells and in rat blood. 

In recent years, for analytical purposes the direct approach has become the most popular. Therefore, only this approach will be discussed in the next sections. With the direct approach, the enantiomers are placed in a chiral environment, since only chiral molecules can distinguish between enantiomers. The separation of the enantiomers is based on the complex formation of labile diastereoisomers between the enantiomers and a chiral auxiliary, the so-called chiral selector. The separation can only be accomplished if the complexes possess different stability constants. The chiral selectors can be either chiral molecules that are bound to the chromatographic sorbent and thus form a CSP, or chiral molecules that are added to the mobile phase, called chiral mobile phase additives (CMPA). The combination of several chiral selectors in the mobile phase, and of chiral mobile and stationary phases is also feasible. 

HPLC using chiral mobile-phase additives... 

The development of a plethora of HPLC CSPs in the 1980s and 1990s has, to a large extent, made the use of chiral mobile-phase additives (CMPAs) redundant in most modem pharmaceutical analytical laboratories [23]. Before this period, chiral selectors were used routinely as additives in HPLC, but are now only used for a small number of specific applications [23]. CMPAs are used to form... 

Prbpy, 5-(2-MePr)bpy and 5-(2,2-Mc2Pr)bpy have been prepared and characterized. The mer-and /uc-isomers of each complex have been isolated by use of cation-exchange column chromatography as the steric requirements of the R group increase, the percentage of the /uc-isomer decreases. Enantiomers of [Ru(5-Prbpy)3] + were separated on SP Sephadex C-25. Electro-kinetic chromatography has been used to separate the enantiomers of [Ru(104)3] " anionic carboxymethyl-/ -cyclodextrin was employed as the chiral mobile phase additive. ... 

AM Stalcup, NM Agyei. Heparin a chiral mobile-phase additive for capillary zone electrophoresis. Anal Chem 66 3054-3059, 1994. 

Beads = pure polymeric particles with similar chiral information to the corresponding sorbent (CSp) coated on silica gel CE = capillary electrophoresis CSP = chiral stationary phase CMPA — chiral mobile phase additive MEKC = micellar electrokinetie capillary chromatography. 

In contrast to the various CSPs mentioned so far, but still based on covalently or at least very strongly adsorbed chiral selectors (from macromolecules to small molecules) to, usually, a silica surface, the principle of dynamically coating an achiral premodified silica to CSPs via chiral mobile phase additives (CMPA) has successfully been adapted for enantioseparation. The so-called reverse phase LC systems have predominantly been used, however, ion-pairing methods using nonaqueous mobile phases are also possible. 

Case II dynamically coated CSP via a chiral mobile phase additive = (S)-CMPA ... 

Two mechanisms foe chiral separations using chiral mobile-phase additives, analogous to models developed for ion-pair chromatography, have been proposed lo explain the chiral selectivity obtained using chiral mobile-phase additives. In one model, the chiral mobile-phase additive and the analyle enamiomers form diaslereomeric complexes in solution. As noted previously, diaslereomers may have slightly different physical properties such as mobile phase solubilities or slightly different affinities for the stationary phase. Thus, the chiral separation can be achieved with conventional columns. 

SH Lee, TS Oh, HW Hae. Enantiomeric separation of free amino acids using Af-alkyl-L-proline copper (II) complex as chiral mobile phase additive in reversed phase liquid chromatography. Bull Korean Chem Soc 13 280-285, 1992. 

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,... 

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