Positional isomer separation

Using these equations, the KI values for four known pheromone components are estimated to be 1807.7 for Me2,Me5-17 H, 1790.6 for Me5,Me9-17 H, 1788.6 for Me5,Mell-17 H, and 1783.5 for Me7, Mell-17 H. These positional isomers separately elute from a capillary GC column in a coincidental order with these KI values [105,106],indicating the usefulness of IEX(opt) for the pheromone research. 

Other interesting examples of positional isomer separation involving NPC are (a) the separation of dihydrodipyridopyridopyrazines, a new family of antitumor agents, on a silica Nucleosil 50 A-lOpm column [43] and (b) the separation of celecoxib isomers by Chiralpak AD column [44], NPC was also employed successfully for the resolution of (a) four configurational isomers of a steroidal calyx pyrrole [45], (b) regio- and stereoisomers of eicosanoids [46], (c) retinal and retinol isomers [47], and (d) several ElZ isomers pairs of vitamin A [48],... 

When the two coordinating centres are not in separate ions but are joined by bridging groups, the isomers are often distinguished as coordination position isomers as is the case for ... 

H-NMR spectroscopy can be used to determine alkenesulfonates in mixtures [115]. Under normal conditions, 1-alkenesulfonate shows a signal separated from the other positional isomers [122]. Moreover, the utilization of a lanthanide shift reagent makes possible even the separation of the signals of isomeric alkenesulfonic acids and hydroxyalkanesulfonic acids as their methyl esters [124]. 13C-NMR spectroscopy, which is not as quantitative, simply gives the cis/trans ratio of the main positional isomer. 

Hardy, M. R. and Townsend, R. R., Separation of positional isomers of oligosaccharides and glycopeptides by high-performance anion-exchange chromatography with pulsed amperometric detection, Proc. Natl. Acad. Sci. U.S.A., 85, 3289, 1988. 

Fujiwara, S. and Honda, S., Effect of addition of organic solvent on the separation of positional isomers in high-voltage capillary zone electrophoresis, Anal. Chem., 59, 487, 1987. 

Hamai S. and Sakurai H., Capilary electrophoretic separation of positional isomers of hydroxynaphthalenecarboxylic acids through the formation of inclusion complexes with P-cyclodextrin, Anal. Chim. Acta, 402, 53, 1999. 

Principles and Characteristics Traditional analytical approaches include off-line characterisation of isolated components, and the use of several chromatographic separations, each optimised for a specific spectroscopic detector. Neither LC-NMR nor LC-MS alone can always provide complete structure determinations. For example, MS may fail in assigning an unequivocal structure for positional isomers of substituents on an aromatic ring, whereas NMR is silent for structural moieties lacking NMR resonances. Often both techniques are needed. 

On a capillary GC analysis, the separation of positional isomers of epoxy compounds is generally well accomplished by a high polar column, such as DB-23, rather than by a low polar column, such as DB-1. For the positional isomers, a different elution order depending on the kinds of column has not been reported. In the case of two mono epoxides derived from Z6,Z9-dienes, 6,7-epoxides elute slightly faster than 9,10-epoxides [72,170],but the separation is insufficient even on the high polar column. Three monoepoxides derived from Z3,Z6,Z9-trienes elute in the order of 6,7-, 3,4-, and 9,10-epoxides [9]. The former two isomers are sufficiently separated on the high polar column, while the elution of the latter two isomers overlaps [71]. For each positional isomer of diepoxides derived from the Z3,Z6,Z9-trienes, two diastereomeric... 

If a Ce reaction component of known skeletal configuration has been separated by (say) GPC from a reaction mixture, it will consist of a mixture of 13C positional isomers which we may designate X, Y, Z. . ., with corresponding mole fractions x, y, z. . . . If a (n = 4, 5) is the ion abundance determined for this mixture, it follows that... 

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]. 

Silver nitrate may be incorporated in the adsorbent slurry (25 g l-1) giving a final concentration of about 5% in the dry plate. The silver ions bind reversibly with the double bonds in the unsaturated compounds, resulting in selective retardation, and the lipids are separated according to the number and configuration (cis or trans) of their double bonds. This technique is extremely useful in fatty acids, mono-, di- and particularly triacylglycerol analyses when even positional isomers may be resolved. Borate ions may also be incorporated in the silica gel and these plates are used to separate compounds with adjacent free hydroxyl groups. 

Argentation thin-layer chromatography is an extemely useful procedure for the separation of methyl esters of fatty acids. Saturated fatty acids have the highest Rf values, which decrease with the increasing degree of unsaturation, and for a particular acid, the trans isomer usually travels ahead of its corresponding cis isomer. The solvents most commonly used contain hexane and diethyl ether (9 1) although a mixture of 4 6 is used to separate compounds with more than two double bonds. In order to separate positional isomers of the same acid, conditions must be carefully controlled and multiple development in toluene at low temperatures is often necessary. 

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