Enantiomer separations analysis

Despite the difficulties caused by the rapidly expanding literature, the use of chiral stationary phases (CSPs) as the method of choice for analysis or preparation of enantiomers is today well established and has become almost routine. It results from the development of chiral chromatographic methods that more than 1000 chiral stationary phases exemplified by several thousands of enantiomer separations have been described for high-performance liquid chromatography (HPLC). 

Schurig V. Enantiomer separation by complexation gas chromatography — Applications in chiral analysis of pheromones and flavours, in Schreier P (ed.), Bioflavour 87, Walter de Gruyter, Berlin, Germany, pp. 35—54, 1988. 

The first gas chromatographic enantiomer separation on a cyclodextrin-based stationary phase was that of the apolar, racemic hydrocarbons a- and /i-pinenc and cis- and trans-pinane on packed columns coated with native a-cyclodextrin dissolved in formamide157. Very soon, it was recognized that alkylated cyclodextrins can be employed in capillary columns for high-resolu-tion enantiomer analysis. Thus, molten permethylated /(-cyclodextrin, hcptakis(2,3,6-tri-0-methyl)-/ -cyclodextrin (Table 2), was used158- 160 at elevated temperatures. 

A full account is also available134-136 on the use of cyclodextrins in gas chromatographic enantiomer separations, including pertinent applications in enantiomer analysis. 

Wittek, M., Vogtle, F., Stuhler, G., Mannschreck, A., Lang, B.M., and Imgartinger, H. (1983) New helical hydrocarbons. VIII. Enantiomer separation, circular dichroism, racemization, and x-ray analysis of... 

As with birds, little work has been done with brominated contaminants in pinnipeds. Walruses (Odobenus rosmarus) from the Canadian Arctic had slightly nonracemic (—)-a-HBCDD for unknown reasons [193]. The only study of chiral brominated flame retardants in pinnipeds reported enantiomer separation of 2,3-dibromopropyl-2,4,6-tribromophenyl ether, the major component of the flame retardant Bromkal 73-5 PE [5]. While this chemical was identihed in blubber and brain tissue of hooded seals and harp seals, enantiomer analysis on these tissue extracts were not performed. 

The chromatographic separation of enantiomers as diastereomers was first developed using GLC (1). Subsequently, many separations using GLC were reported, but modern LC has dominated the field of enantiospedfic drug analysis in recent years. Nevertheless, the arrival of high-resolution capUlary GLC has revived interest in the use of indirect enantiomer separation via this type of chromatography, and today GLC remains important in the analytical separation of enantiomers after derivatization with CDAs. The availability of sensitive detection methods, for example, mass spectrometry, electron capture, etc., enhances the applicability of GLC in indirect enantiospedfic drug analysis. 

Depending on the nature of the substituents on nitrogen, this reaction may give a complex product mixture, but in some cases such derivatization could be the basis of a useful indirect enantiomer separation. When one or two of the three N-substituents are methyl, for example, demethylation may be favored, and the reaction may be usable as a precolumn derivatiza-tion for chromatographic resolution. This approach was used in the analysis of encainide, a tertiary-amine antiarrhythmic drug (73). 

The three general approaches to enantiomer separation entail a chiral stationary phase, a chiral mobile phase, or a chiral reagent. Tandem columns, with reversed and chiral stationary phases, were used to separate 18 D-L pairs of PTC-amino acids in 150 min. OPA-amino acid enantiomers have been separated on both ion-exchange and reversed-phase columns using a sodium acetate buffer with a L-proline-cupric acetate additive. Chiral reagents, such as Marphey s reagent and OPA/IBLC (A-isobutiril-L cysteine), were successfully used for racemization analysis within 80 min. 

W. A. Konig, new developments in enantiomer separation by capUlary gas chromatography, in Analysis of Volatiles Methods, Applications, P. Schreier (ed.), Berhn, 1984, pp. 77-91. 

Modified-C02 mobile phases excel at stereochemical separations, more often than not outperforming traditional HPLC mobile phases. For the separation of diastereomers, silica, diol-bonded silica, graphitic carbon, and chiral stationary phases have all been successfully employed. For enantiomer separations, the derivatized polysaccharide, silica-based Chiralcel and Chiralpak chiral stationary phases (CSPs) have been most used, with many applications, particularly in pharmaceutical analysis, readily found in the recent literature (reviewed in Refs. 1 and 2). To a lesser extent, applications employing Pirkle brush-type, cyclodextrin and antibiotic CSPs have also been described. In addi-... 

Fig. 6.47 Enantio-MDGC analysis of a-ionone. Pre-column separation a(P) ionone (left) enantiomer separation of racemic a-ionone standard (right) (from ref [85, 114])...

E. Francotte, Mobile phase as a key feature for successful analytical and preparative HPLC separation of drug enantiomers, Separation, Purification and Analysis in Drug Discovery, Kungalv (Sweden), 2004. 

The capability of quartz microbalances for the analysis of enantiomers has been demonstrated successfully by Bodenhofer et al. [102]. Cyclopeptides have already been successful as chiral selectors in enantiomer separation by capillary electrophoresis [90, 91], Here, we showed that cyclopeptide 1 exhibits different affinities to D- and L-arginine [99], The differences in sensitivity of the quartz microbalances result in a chiral separation factor of 1.12 [76]. Sensitivity differences of the quartz microbalance coated with 1 for the two enantiomers of arginine are shown in Figure 10.13. 

Aboul-Fnein, H.Y. Van Overbeke, A. Vander Weken, G. Baeyens, W. Oda, H. Deprez, P. Kruif, A. HPLC on Chiralcel OJ-R for enantiomer separation and analysis of ketoprofen, from horse plasma, as the 9-aminophenanthrene derivative. J. Pharm. Pharmacol. 1998, 50, 291 296. 

T. Hanai, Computational chemical analysis of enantiomer separations of derivatized amino acids in reversed-phase liquid chromatography, Internet Electron.]. Mol. Des., 2004, 3, 379-386. 

The introduction of capillary columns for GC analysis produced a breakthrough in the analysis of environmental pollutants due to their high separation efficiency. For environmental applications, fused-silica wall-coated open-tubular columns with internal diameters from 0.1 to 0.32 mm and film thickness of 0.1-0.2 pm, and lengths from 25 to 60 m are currently used. The wide range of stationary phases commercially available with different polarities and high thermal stabiHty provides the tool required to maintain the prominent position of GC in environmental analysis. In addition, the availability of chiral stationary phases gives GC the capability to perform GC enantiomer separations. Table 1 gives the recommended columns used in routine analysis of some selected pollutants. 

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