Geometrical isomers, chromatographic separation

Thin Layer Chromatographic Isomer Separation. The geometrical isomers were separated by spotting a chloroform solution of the product on a silica gel plate and eluting with a 15 1 v/v chloroform/methanol solution. The spots or bands were dissolved in methanol and the solutions were evaporated to dryness. 

Chromatography of isomers Chromatographic separation of geometric (cis-trans) and optical (I and S) enantiomers on adsorbents that have been impregnated with compounds having the ability to complex preferentially... 

Since all the physical properties of two given enantiomers are the same in the absence of a chiral, or optically active, medium, their chromatographic resolution needs a different approach from the relatively simple separation of geometrical isomers, stereoisomers or positional isomers. Two methods are used. The older technique of indirect resolution, requires conversion of the enantiomers to diastereoisomers using a suitable chiral reagent, followed by separation of the diastereoisomers on a non-chiral GC or LC stationary phase. This technique has now been largely superseded by direct resolution, using either a chiral mobile phase (in LC) or a chiral stationary phase. A variety of types of chiral stationary phase have been developed for use in GC, LC and SFC(21 23). 

Armstrong, D.W. and DeMond, W., Cyclodextrin bonded phases for the liquid chromatographic separation of optical, geometrical, and structural isomers, J. Chromatogr. Sci., 22, 411, 1984. 

Carotenoids can be converted into mixtures of geometrical isomers under appropriate conditions, the most common being iodine catalyzed photoisomerization. This produces an equilibrium mixture of isomers, in general the all-trans isomers predominates. These isomers in an isomeric mixture cannot be measured separately by simple spectrophotometric determination. The usual method of subsequent measurement would be chromatographic separation, diode-array detection, and spectral analysis. In the absence of any definitive data on extinction coefficients for cfv-isomcrs, they are quantified against the all-trans isomer. Modem procedures involve the direct synthesis of c/.v-carotcnoids. 

Heath, R.R. and Dolittle, R.E., Derivatives of cholesterol cinnamate a comparison of the separations of geometrical isomers when used as gas chromatographic stationary phases, J. High Resolut. Chromatogr. Chromatogr. Commun., 6, 16, 1983. 

In the case of analytes with identical elemental formulas ( true isobars ) as positional or geometrical isomers (e.g. 11-hydroxycortisol and 21-hydroxycortisol or testosterone and epi-testosterone)—a discrimination of analyte and interfering compound is not possible with any mass analyzer—even if enabling highest mass resolution. Potentially, the disintegration pattern of isomers may be different, allowing analytical discrimination but in most cases chromatographic baseline separation of analyte and isomer prior to their MS/MS detection is required for unequivocal quantitative measurement. 

The chromatographic separation of positional isomers (26-31), geometrical isomers (27,32-36) and enantiomers (37-49) has been achieved by utilizing the concerted action of inclusion complex formation, additional primary and secondary hydrogen-bond formation and steric hindrance effects between the solutes and the cyclodextrins (11,12,14-23,50). There is an abundant literature on the analytical applications of cyclodextrin-silicas (13-50), but not on their preparative chromatographic use. 

Separations involving cis/trans isomers can also be accomplished by employing NPC. An example of this application is the separation of tricyclic antidepressant doxepin, which is marketed as a mixture of geometric isomers in a cis/trans ratio of 15 85 [41], When a spherisorb silica column is used with a hexane-methanol-nonylamine mobile-phase system, the cis isomer of doxepin elutes first. The structures of the two isomers and the chromatographic separation are shown in Figure 5-7. NPC has also been successfully employed in the separation of cis/trans isomers of steroids. Four diastereomers... 

A GC-MS method was used in the study of the analytical characteristics of volatile Al(dik)3 and Cr(dik)3, dik = acac and tfac. Aluminum and chromium chelates were well separated on a fused silica capillary column of 25 m length and 0.20 mm i.d. with electron impact ionization MS detection, and yielded the unique base peak pattern corresponding to the loss of one ligand from the molecular ion. Cr(tfac)3 exhibited a pair of chromatographic peaks which were found to produce nearly identical mass spectra, pointing to the existence of two geometrical isomers. ... 

Where a stable isotope-labeled standard is unavailable, the analyst can use either a chemically similar homologue (e.g., incorporating an additional methylene different m/z values to monitor) or a chemically similar analogue (e.g, geometric isomer, same m/z values to monitor) that will need to be chromatographically separated... 

Raymond and co-workers have synthesized and separated optical and geometrical isomers of simple tris hydroxamate chroniium(III) and tris phenolate chromium(III) or rhodium(III) complexes and assigned the absolute configurations of these isomers based on criteria such as chromatographic behavior due to differences in dipole moment, theoretical symmetry considerations, and X-ray crystallographic data 174). The absolute configuration of isomers of chromium(III) complexes of ferrichrome, ferrichrysin 175), ferrioxamine B and Dx 176), rhodotorulic acid 177), enterobactin 178),... 

Two such closely related compounds as cis and trans isomers of azobenzene have been separated by exploiting the difference in polarities of the two isomers. It has already been mentioned earlier that as the polarity of a sample component increases, its retention time on an LSC column also increases. Thus, cis-azobenzene (I), which is the more polar one of the two azobenzenes, is retained on the chromatographic column longer than trans-isomer (II) whereby separation of the two geometrical isomers results. 

Resolution problems in the GC analysis of FA esters prompted many investigators in the past to develop capillary techniques to search for more selective stationary phases, and ultimately, to combine both approaches whenever required. In fact, FA esters were among the first substances (beyond hydrocarbons) that were successfully chromatographed on stainless steel capillary columns [355]. The most difficult separations involve different geometrical isomers, and the presence and positions of unsaturated carbon-carbon bonds. Such separations are non-trivial and justify the effort of numerous laboratories to solve these problems. 

Product analysis of irradiated samples attains the ppb level and separation of geometrical isomers is routinely performed. Using gel permeation chromatography the product distribution of y-irradiated n-alkanes has been recently mapped (50). As can be seen in Figure 2, unprecedented resolution of the various isomers can now be achieved with modern chromatographic techniques. Capillary electrophoresis is presently used to determine the product distribution of various cresols and yields as low as 0.1 molecules/100 eV are routinely measured (51). 

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