Isomers, separation using cyclodextrin

Immobilization. The abiUty of cyclodextrins to form inclusion complexes selectively with a wide variety of guest molecules or ions is well known (1,2) (see INCLUSION COMPOUNDS). Cyclodextrins immobilized on appropriate supports are used in high performance Hquid chromatography (hplc) to separate optical isomers. Immobilization of cyclodextrin on a soHd support offers several advantages over use as a mobile-phase modifier. For example, as a mobile-phase additive, P-cyclodextrin has a relatively low solubiUty. The cost of y- or a-cyclodextrin is high. Furthermore, when employed in thin-layer chromatography (tic) and hplc, cyclodextrin mobile phases usually produce relatively poor efficiencies. 

In the recent past separation of isomers has been attempted using aqueous liquid membranes based on p-cyclodextrin. Thus, separation of a mixture of o- and p-nitroaniline (in 80% i-octanol, 20% -heptane) has been studied, with the p-isomer showing a selectivity of 5 at 0.7 molar p-cyclodextrin. Even stereoisomers of stilbene cis and trans) were separated using a 0.02 to 0.2 M cyclodextrin solution, but the selectivity was less than 2 (Mandal et al, 1998). 

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]. 

Isomers of xylene,93 ethyl benzene94 and nitrotoluene95 have been separated using cydodextrins. Individual components of petroleum fractions, such as polynuclear aromatic hydrocarbons, have been identified and quantified using cyclodextrin.96,97... 

The classical types of interactions that play a role in separation using conventional polar and nonpolar stationary phases yield limited possibilities for separation of isomers with similar properties. For a solution of this problem, a high degree of selectivity in the separation process is required. It has been shows that these requirements are met by both cyclodextrins and liquid crystals, used as stationary phases in gas chromatography. 

Optical isomer separations that are carried out on a chiral layer produced from C-18 modified silica gel impregnated with a Cu(II) salt and an optically active enantiomerically pure hydroxyproline derivative, on a silica layer impregnated with a chiral selector such as brucine,on molecularly imprinted polymers of alpha-agonists,or on cellulose with mobile phases having added chiral selectors such as cyclodextrins have been reported mostly for amino acids and their derivatives. Mixtures of sorbents have been used to prepare layers with special selectivity properties. 

Many ionic poly(saccharides), such as heparin, chondroitin sulfates, dextran sulfate, and natural poly(saccharides), such as dextran, dextrin, pullulan, and their charged derivatives have been used as mobile phase additives for the separation of different enantiomers. Figure 10.10 [191,192,205,206]. Dextrins were found to have a wide application range, thought to be due in part to their helical structures. Enantiomer-chiral selector complexes seem to be weaker than for cyclodextrins, and it has not been demonstrated that enantiomer separations obtained by the poly(saccharide) chiral selectors cannot be obtained using cyclodextrins. Natural poly(saccharides) are typically complex mixtures of homologues and isomers, with a composition that can vary for different sources, resulting in differences in enantioselectivity. 

C. Akbay, S.A. Shamsi and I.M. Warner, Separation of monomethyl-benz[a]antracene isomers using cyclodextrin-modified electrokinetic chromatography, J. Chromatogr. A, 910, 147-155, 2001. 

J. Kirschbaum and L. Kerr, Separation of Steroid Epimers and Isomers Using Cyclodextrin HPLC Columns, LC Magazine, 4(1986)30. 

Cyclodextrin. More recently, cyclodextrin-modihed layers have been introduced for use in the. separation of enantiomers. The increasing importance of isomer separation in pharmaceutical applications has led to a demand for these spe-... 

Kirschbaum J, Kerr L 1986 Separation of steroid epimers and isomers using cyclodextrin HPLC columns. LC Mag 4 30-32... 

E. Only Spencer and Purdy applied MEKC specifically to the various E vitamers (173). These investigators added p-cyclodextrins to a buffer containing SDS micelles. The four natural vitamin E homologs, including p- and y-tocopherol, were resolved in 30 min. In contrast, positional isomers could not be separated by reversed-phase HPLC using cyclodextrins as additives to the mobile phase. 

To obtain HBCD diastereoisomer-specific data, it is essential to use LC, since, using GC, the diastereoisomers interconvert at temperatures above 160°C. Therefore, LC—MS/MS using ESI or APCI are versatile tools for the isomer-specific determination of trace levels of HBCDs, monitoring the specific transitions m/z 640.6 to m/z 78.9 and 80.9. An example of separation using permethylated P-cyclodextrin column is shown in Figure 14.1. 

Busby BM, Vigh G (2005) Synthesis of heptakis(2-0-methyl-3-0-acetyl-6-0-sulfo)-cyclomaltoheptaose, a single-isomer, sulfated P-cyclodextrin carrying nonidentical substituents at all the C2, C3, and C6 positions and its use for the capUlaty electrophoretic separation of enantiomers in acidic aqueous and methanolic background electrolytes. Electrophoresis 26 1978-1987... 

Inclusion compounds of the Cg aromatic compounds with tris((9-phenylenedioxy)cyclotriphosphazene have been used to separate the individual isomers (43—47). The Schardinger dextrins, such as alpha-cyclodextrin, beta-dextrin, and gamma-dextrin are used for clathration alpha-dextrin is particularly useful for recovering PX from a Cg aromatic mixture (48,49). PyromeUitic dianhydride (50) and beryllium oxybenzoate (51) also form complexes, and procedures for separations were developed. 

Armstrong and Jin [15] reported the separation of several hydrophobic isomers (including (l-ferrocenylethyl)thiophenol, 1 -benzylnornicotine, mephenytoin and disopyramide) by cyclodextrins as chiral selectors. A wide variety of crown ethers have been synthesized for application in enantioselective liquid membrane separation, such as binaphthyl-, biphenanthryl-, helicene-, tetrahydrofuran and cyclohex-anediol-based crown ethers [16-20]. Brice and Pirkle [7] give a comprehensive overview of the characteristics and performance of the various crown ethers used as chiral selectors in liquid membrane separation. 

The more useful types of chirally active bonded phases are those based on the cyclodextrins. There are a number of different types available, some of which have both dispersive or polar groups bonded close to the chirally active sites to permit mixed interactions to occur. This emphasizes the basic entropic differences between the two isomers being separated. A range of such products is available from ASTEC Inc. and a separation of the d and / isomers of scopolamine and phenylephrine are shown in figure 4. The separations were carried out on a cyclodextrin bonded phase (CYCLOBOND 1 Ac) that had been acetylated to provide semi-polar interacting groups in close proximity to the chiral centers of the cyclodextrin. The column was 25 cm long, 4.6 mm in diameter and packed with silica based spherical bonded phase particles 5pm in diameter. Most of the columns supplied by ASTEC Inc. have these dimensions and, consequently, provide a... 

Only the silica-based stationary phases with covalently bonded alkyl chain, cyano and propylamino ligands have found practical applications in HPLC. Besides these common ligands, the experimental use of naphthalene, pyrene and nitroaromatic as ligands has also been reported. Silica-based stationary phases with covalently bonded cyclodextrins or cyclodextrin derivatives have been frequently employed in the separation and quantitative determination of isomer pairs. 

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