Enantiomeric separations with

Ekborg-Ott, K.H., Wang, X., and Armstrong, D.W., Effect of selector coverage and mobile phase composition on enantiomeric separations with ristocetin A chiral stationary phases, Microchem. J., 62, 26, 1999. 

The enantiomeric separation with chiral mobile phases consists of the addition of an active compound in the mobile phase which is constantly pumped though the chromatographic system. The active ingredient contributes to a specific secondary chemical equilibrium, interacting with the enantiomers in the mobile phase as well as in the stationary phase, leading to the formation of diastereomeric complexes potentially in both phases. This affects the overall distribution of the analyte between the stationary phase and the mobile phase, affecting its retention and the overall enantiomeric separation. The rates of formation of the diastereomeric complexes should be similar to the diffusion rates to minimize excessive chemical contribution to the band-broadening. 

Litsea cubeba oil. Lemon oil washed as residues from production of terpene-free oil is preferably used, as these contain still all components of the pure lemon oil. Also lemon terpenes and heads of distilled grapefruit oils could be found. Blending is done by using synthetic decanal, non-anal, octanal, and citronellal from Corymbia citriodora oil. Detection is made by GC-MS and mainly by multidimensional enantiomeric separation with various methods (see part of methods). Mondello (1998) reports some constituents with chiral ratios as follows (f )-(+)-p-pinene 6.3% (5) ( )-P"pinene 93.7% (f )-(+)-sabinene 14.9% (5)-(-)-sabinene 85.1% (5)-(-)-limonene 1.6% (K) (+)-limonene 98.4% (5)-(+)-terpinen-4-ol 24.7% (/ )-(-)-terpinen-4-ol 75.3% and (5)-( ) a terpineol 75.2% (R)-(+)-a-terpineol 75.2%. Further on, Dugo and Mondello (2011) gave the following data (/ )-(+)-a-pinene (25.5%-31.5%) (5)-(-)- -pinene (68.5%-74.5%) (15,4/ ) ( ) camphene (86.2%-92.4%) (l/ ,45)-(+)-camphene (7.6%-13.8%) (5)-(-)-p-pinene... 

Impregnated layers, 17-18,120-I2I enantiomeric separation with, 122 Indole-3-yIacetic acid (lAA), 784 Indoles, 905-912... 

Kang J et al (2004) A mechanistic study of enantiomeric separation with vancomycin and balhimycin as chiral selectors by capillary electrophoresis. Anal Chem 76 2387-2392... 

For this reason, noncommercial MCTA plates were preferably used for enantiomeric separations with respect to the commercial ones. 

As mentioned previously, cellulosic phases as well as amylosic phases have also been used extensively for enantiomeric separations more recently (89,90). Most of the work ia this area has been with various derivatives of the native carbohydrate. The enantioresolving abiUties of the derivatized cellulosic and amylosic phases are reported to be very dependent on the types of substituents on the aromatic moieties that are appended onto the native carbohydrate (91). Table 3 fists some of the cellulosic and amylosic derivatives that have been used. These columns are available through Chiral Technologies, Inc. and J. T. Baker, Inc. 

Recently, two examples of the separation of enantiomers using CCC have been published (Fig. 1-2). The complete enantiomeric separation of commercial d,l-kynurenine (2) with bovine serum albumin (BSA) as a chiral selector in an aqueous-aqueous polymer phase system was achieved within 3.5 h [128]. Moreover, the chiral resolution of 100 mg of an estrogen receptor partial agonist (7-DMO, 3) was performed using a sulfated (3-cyclodextrin [129, 130], while previous attempts with unsubstituted cyclodextrin were not successful [124]. The same authors described the partial resolution of a glucose-6-phosphatase inhibitor (4) with a Whelk-0 derivative as chiral selector (5) [129]. 

Fig. 2-4. The enantiomeric separation of a-hydroxy/halogen acids on ristocetin A CSP (250 x 4.6 mm) with the same mobile phase composition methanol with 0.02 % acetic acid and 0.01 % triethylamine (v/v). The flow rate was 1.0 mL min at ambient temperature (23 °C).

In supported liquid membranes, a chiral liquid is immobilized in the pores of a membrane by capillary and interfacial tension forces. The immobilized film can keep apart two miscible liquids that do not wet the porous membrane. Vaidya et al. [10] reported the effects of membrane type (structure and wettability) on the stability of solvents in the pores of the membrane. Examples of chiral separation by a supported liquid membrane are extraction of chiral ammonium cations by a supported (micro-porous polypropylene film) membrane [11] and the enantiomeric separation of propranolol (2) and bupranolol (3) by a nitrate membrane with a A/ -hexadecyl-L-hydroxy proline carrier [12]. 

Macaudiere et al. first reported the enantiomeric separation of racemic phosphine oxides and amides on native cyclodextrin-based CSPs under subcritical conditions [53]. The separations obtained were indicative of inclusion complexation. When the CO,-methanol eluent used in SFC was replaced with hexane-ethanol in LC, reduced selectivity was observed. The authors proposed that the smaller size of the CO, molecule made it less likely than hexane to compete with the analyte for the cyclodextrin cavity. 

Column selection remains the most important factor in successful enantiomeric separations. The CSPs most likely to be effective in SFC are those that have been employed under normal phase conditions in LC. In fact, the tremendous body of knowledge that has been accumulated for LC can also guide column selection in SFC [66]. The likelihood of success with a particular CSP can generally be gauged after one or two injections [67]. If no evidence of separation is observed, another CSP should be investigated. 

The resolution of enantiomers by liquid chromatography using chiral stationary phases is based on the formation of reversible diastereomeric complexes of different stability between the sample and stationary phase. Since the formation of the complexes is strongly dependent on the structure of the sample, there are no universal chiral stationary phases. The specific advantages of TLC for enantiomeric separations result from its low cost, convenience and speed (10,97,98). The main limitation, particularly with respect to column liquid chromatography, is the small number of phases currently available. 

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