Isomerism separation

Gray, M., Dennis, G.R., Wormell, P., Shalliker, R.A., Slonecker, P. (2002). Two-dimensional reversed phase separations isomeric separations incorporating C18 and carbon clad zirconia stationary phases. J. Chromatogr. A 975, 285-297. 

Commercial LASs are complex mixtures of four individual LASs (C10-C13) with 20 possible positional isomers. Isomeric separation can be achieved by solvophobic association with SDS or host-guest interaction with cyclodextrins. Complete resolution of 19 isomers was achieved using 10 mM phosphate buffer (pH 6.8) containing 40 mM SDS and 30% acetonitrile [4]. LAS isomers in technical products were separated using a-cyclodextrin, but complete resolution of all isomers was not achieved [5]. 

The choice of a stationary phase will depend upon the nature of the carbohydrates to be separated and whereas an OV-17 column (phenylmethyl polysiloxy gum) may give satisfactory isomeric separations, a non-polar phase such as OV-1 (methylpolysiloxy gum) may be more useful for a wider range of carbohydrates (Figure 9.23). 

Akhtar et al. [74] proposed that, in rhodopsin, an acceptor group on the protein forms a charge-transfer complex with the unprotonated Schiff base of retinal furthermore, upon ll-cis to trans isomerization, separation of donor and acceptor moieties would occur and the Schiff base linkage would be exposed to hydrolysis. This model can now be discarded as unrealistic the resonance Raman experiments have shown that it is not an unprotonated Schiff base, but a protonated base which is bound to opsin. Further, this and related models were examined by Komatsu and Suzuki [223] using theoretical calculations, who found that charge-transfer type models cannot satisfactorily explain the red shifts seen in visual pigments. 

Porous graphitic carbon (PGC) has a rigid structure, is chemically stable and has a very hydrophobic surface. It can therefore be used for reversed-phased HPLC, where it displays a similar or greater degree of retention to ODS. It has excellent pH stability and hence ion suppression of either acidic or basic drugs is easily carried out on this material. It displays different selectivity to silica based packings, allowing isomeric separations more typical of unmodified silica to be carried out (Bassler, 1989). 

The illustration of an isomeric separation shown in Figure 12.3 is an organometallic example. It is interesting to note that the mode of LC used is straight-phase. This has largely been superceded by reversed-phase LC for many applications, but although it has its disadvantages for polar, basic compounds (as can be seen), it can provide excellent selectivity which allows some very difficult separations to be achieved even with relatively inefficient columns. 

Penmetsa, K.V. Leidy, R.B. Shea, D. Enantiomeric and isomeric separation of herbicides using cyclodextrin-modified capillary zone electrophoresis. J. Chromatogr.,... 

Gas chromatography, first described by James and Martin in 1952, has become one of the most frequently used separation technique for the analysis of gases, volatile liquids, and solids. The major breakthrough of GC was the introduction of the open mbular column by Golay and Desty in 1958 and the adoption of fused silica capillary columns by Dandeneau and Zerenner in 1979. Today, using of the capillary columns can solve many analytical problems, such as isomeric separation and analysis of complex mixtures of natural products and biological samples. 

Selective para-halogenation of biphenyl 4.5 Chlorination / isomerization / separation... 

Although the inter-isomeric separation energies are important, they alone cannot predict the relative stabilities of the isomers. Owing to very high temperatures, entropy contributions can even over-compensate the enthalpy terms. Hence, the enthalpy-entropy interplay represents an essential feature of fullerenes and metallofullerenes. 

The history of the crystallization of stereoisomers is fascinating. Pasteur noted the visible difference in morphology between crystals of the isomers of ammonium sodium tartrate in 1848. This turned out to be a fortuitous circumstance as ammonium sodium tartrate happens to crystallize as conglomerate crystals, that is, a eutectic mixture containing crystals of the two pure isomers. Such behavior has since been found to be infrequent, although it has proved industrially important. Pasteur separated the crystals manually to achieve the first documented isomeric separation ... 

L. A. Sly, Isomeric separation of Beraprost sodium using an a -acid glycoprotein column, J. 

The iterative and convergent protocol using (E)- and/or (Z)-l,4-diiodo-2-methyl-1-butenes is efficient as well as regio- and stereoselective, requiring hardly any isomeric separation even in the synthesis of a decameric isoprenoid. It permits both iterative and convergent modes of construction of oligomeric isoprenoids in any desired ratio of the two modes of operation to best suit a given synthetic task. At any point of synthesis, either the E- or Z-trisubstituted Cs-alkene unit can be incorporated. This synthetic method promises to find many additional applications in the area of isoprenoid synthesis. 

Such gas-dependent shifts of relative K for two ions reflect differences in size, mass, and polarizability of gas molecules, though the effect on isomeric separations cannot be due to different M. To quantify these phenomena and predict the optimum gas for separation of any two ions, one needs to calculate mobilities of polyatomic ions in any gas. So far, that has been demonstrated only with He, in which attractive ion-gas interactions are weak and even crude models produce accurate fl (1.4.4). The choice of gas is much more important in differential IMS, where relative separation parameters of ions in different gases often differ dramatically (3.4) and not by a few percent as in conventional IMS. 

A prototypical isomeric separation in biological analyses is that of leucine and isoleucine amino acids. Those were (barely) resolved by FAIMS as deprotonated... 

Figure 1 Illustration of the isomeric separation of oligostyrene containing four configurational repeating with a sec-butyl end group on (A) a Cl8 column (250mm x 4.6 mm, Pd 5 pm) and (B) a carbon clad zirconia column (100mm x 4.6mm, Pd 3pm). Both separations were in a mobile phase of 100% acetonitrile at 30°C.

Another application of target metabolomics illustrative of the importance of isomeric/isobaric separation is in the field of pharmacokinetics. Pharmacokinetic studies focus on the absorption and distribution of drugs and their metabolites in biological systems. " A limiting factor in pharmacokinetic research is a requirement of high-throughput screening of isomeric metabolites. IM-MS is a rapid technique where isomeric separations occur within minutes. For example, IM-MS has been used to separate opiate metabolic isomers in less than 1 minute. ... 

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