Enantiomer analysis

Chapman, J., Chen, F.A. Implementing a generic methods development strategy for enantiomer analysis. LC-GC Europe, 2001, January, 2-6. 

Methods development strategy for enantiomer analysis using the P/ACE MDQ chiral system, Application information A-1889A, Beckman coulter, www.beckman-coulter.com... 

LSR measurements are generally performed at room temperature. A detailed study of the influence of temperature on the magnitude of ArsAS has shown that the use of chiral LSR at low temperatures offers important advantages for the enantiomer analysis of weakly coordinat-... 

A highly versatile method for enantiomer analysis is based on the direct separation of enantiomeric mixtures on nonraceinic chiral stationary phases by gas chromatography (GC)6 123-12s. When a linearly responding achiral detection system is employed, comparison of the relative peak areas provides a precise measurement of the enantiomeric ratio from which the enantiomeric purity ee can be calculated. The enantiomeric ratio measured is independent of the enantiomeric purity of the chiral stationary phase. A low enantiomeric purity of the resolving agent, however, results in small separation factors a, while a racemic auxiliary will obviously not be able to distinguish enantiomers. 

The direct gas-chromatographic method is especially suited as an analytical tool for enantiomer analysis when no sample derivatization is required. In the absence of diastereomeric effects between enantiomers ( EE-effect )25, the chiral compound may be investigated in situ, i.e., without isolation and purification using a minute amount of sample, e.g., by head-space analysis with 10 9 g using flame ionization detector (FID) or 1 (F 11 g using selected ion monitoring (GLC-MSSIM). 

The prerequisites for the use of gas chromatography as an analytical tool in enantiomer analysis, i.e., quantitative resolvability, substrate volatility and thermal stability, tend to restrict its general use. The development of supercritical fluid chromatography (SFC) for enantiomer... 

In general, enantiomer analysis by complexation gas chromatography can be performed without substrate derivatization. Aliphatic diols, however, have been separated as boronates or acetonides151. 

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. 

With enantiomer analysis, however, a linear detector response is indispensible. Thus, for the correct determination of. say, 0.1 % of an enantiomeric impurity, linearity within a concentration range of at least three orders of magnitude is required. It is generally accepted that the flame ionization detector (FID) does fulfill this requirement, but it is recommended that the linear detector response is verified via dilution experiments31. In contrast, the linear response range of the electron capture detector is low, being only two to three orders of magnitude. 

The applications of chiral polysiloxanes in enantiomer analysis in various fields of chemistry have been extensively reviewed6,124,128. Noteworthy is the use of Chirasil-Val (in both enantiomeric forms) for trace enantiomer analysis in the realm of EPC and enzymatic transformations31109. Enantiomeric impurities down to levels as low as <0,005% have been determined31185 (Figures 24a and 24b). 

Figure 24a. Trace enantiomer analysis of derivatives of hydroxy acids on n-Cbirsail-Val [20 m x 0.3 mm (i.d.) glass capillary column, between 45 and 115 C. 0.45 bar hydrogen]185.

For trace enantiomer analysis involving small peak separation factors a, a multiscanning procedure has been devised (Figure 25)186. 

Applications of enantiomer analysis by complexation gas chromatography in pheromone chemistry189 and in flavor research190 have been reviewed. The quantitative kinetic resolution of c -2-ethyl-3-methyloxirane by enzymatic hydrolysis to (21 ,3/ )-2,3-pentanediol, catalyzed... 

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

These experiments demonstrate the high accuracies of these two complementary methods of modem enantiomer analysis. 

Other more conventional detectors that might ostensibly outperform CD in selectivity are nmr and mass spectrometry, and in fact they do for the analysis of diastereomers, although quantitation is a much more difficult task. They cannot compete with chiroptical methods for the distinction between enantiomers. In nmr detection, derivatization to diastereomers is a prerequisite to enantiomer analysis, and chiral forms of lanthanide reagents can been used with good effect [16,17]. For the analysis of mixtures by either nmr or mass spectrometry, total chromatographic separation is a necessity, so the completeness of the baseline separation is the limiting step not the detector. In contrast CD can be applied to the analysis of enantiomers in mixtures in methods that require no prior separation. 

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. 

As with polar bears, wolverines and Arctic foxes also appear to have sizable capacity for biotransforming POPs, as evidenced by enantiomer analysis. Wolverines captured from Iceland and the Canadian Arctic had enrichments of (—)-PCBs 136 and 149 in livers, with mean EFs of 0.41 and 0.46, respectively [268]. While these enrichments were similar to those of a-HCH (mean EF of 0.42) and heptachlor epoxide (mean EF of 0.55), they were not as stereoselective as those of /ra 5-chlordane (mean EF of 0.65) and oxychlordane (mean EF of 0.71) [268]. Populations of Arctic foxes in Iceland feeding mostly on marine mammals had much higher POP concentrations in liver tissue than those with a terrestrial diet, but had similar enrichments of (-l-)-chlordane [267]. As with wolverines, Arctic fox livers [268] had modest enrichments of (—)-a-HCH, (+)-PCBs 136 and 149, and El -PCB 95 on Chirasil-Dex, with mean EFs of 0.41, 0.48, 0.54, and 0.55 respectively. These enrichments were again minor compared to that of the OC compounds, which had mean EFs of 0.61 for cw-chlordane, 0.89 for /ra 5-chlordane, 0.68 for oxychlordane, and 0.73 for heptachlor epoxide. Depletions of labile analytes compared to recalcitrant compounds... 

Artifacts in EE can also occur for peaks that are not fully resolved, as is often the case in enantiomer analysis of POPs. In this case, deconvolution of enantiomer peaks using least-squares fitting of chromatographic data to mathematical models accounting for non-ideal chromatography can provide more accurate and precise results than conventional integration techniques [340]. Of course, full chromatographic resolution of peaks is always desired, but this may not be possible or feasible. 

In contrast to the aforementioned fullerenes, C76 is a chiral molecule containing 30 different types of carbon-carbon bond. In this molecule five different pyracylene-type carbon-carbon bonds repeat to form chrysene-shaped units. Kinetic resolution of this fullerene has been achieved via asymmetric osmylation in the presence of a cinchona based chiral ligand (see Section 4.4.4.1.1., ligand 1 d/2 d, Table 5). The calculated enantiomeric excess of the recovered material (after 95% conversion) is >97%, whereas the regenerated C76, formed by tin(II) chloride reduction of the osmylated material (after 33 % conversion), is enriched in the opposite enantiomer. Analysis of the local curvature of the C76 molecule indicates that Os04 should selectively add to two of the 30 types of bonds86. 

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