Cyclodextrins as mobile phases

V Seidel, E Poglits, K Schiller, W Lindner. Simultaneous determination of ochratoxin A and zearalenone in maize by reversed-phase high-performance liquid chromatography with fluorescence detection and/8-cyclodextrin as mobile phase additive. J Chrom 635 227-235,1993. 

The chiral resolution using cyclodextrins as mobile phase additives is also controlled by a number of chromatographic parameters as in case of CSPs. The chiral resolution occurred by the formation of diastereoisomeric inclusion complex formation and, hence, the composition of the mobile phase, pH, concentration of cyclodextrins, and temperature are the most important controlling parameters. 

Cyclodextrins as Mobile-Phase Components for Separation of Isomers by Reversed-Phase High-Performance Liquid Chromatography... 

I. Clarot, D. Cledat, S. Battu, and P. J. P. Cardot, Chromatographic study of terpene derivatives on porous graphitic carbon stationary phase with P-cyclodextrin as mobile phase modifier, J. Chromatogr. A 903 (2000), 67. 

Spencer, B.J. Purdy, W.C. High-performance liquid chromatographic separation of equilin, estrone, and estrone derivatives with cyclodextrins as mobile phase additives. J.Liq.Chromatogr., 1995, 18, 4063-4080... 

Numerous chromatographic studies have also been carried out to investigate possible applications of CyDs as multifunctional drug carriers, discussed in more detail in Chapters 14 and 15. For example the retention of psoralen derivatives was investigated on a C-18 column with HP-) -cyclodextrin as mobile phase additive [62]. Assuming 1 1 stoichiometry, in a 52 48 (v/v) methanol water mixture and temperature —5 °C, the corresponding stability constants were 30, 75, and 40 for the complexes of 8-methoxypsoralen, 5-methoxypsoralen and trimethylpsoralen with HP-yS-CyD, respectively. 

Chen, D, S Jiang, Y Chen and Y Hu (2004). HPLC determination of sertraline in bulk drug, tablets and capsules using hydroxypropyl- 8-cyclodextrin as mobile phase additive. Journal of Pharmaceutical and Biomedical Analysis, 34,239-245. 

The overwhelming majority of chiral separations up to now have been carried out by adding various chiral selectors to the mobile phase rather than by using a chiral plate. For example, enantiomers of amino acids were separated by using alpha- and beta-cyclodextrins as mobile phase additives with bonded Cig and cellulose layers (Cserhati and Forgacs, 1996). 

Lambroussi, V., Piperaki, S. and Tsantili-KakouHdou, A., Formation of inclusion complexes between cyclodextrins as mobile phase additives in RP-TLC, and fluoxetine, norfluoxetine, and promethazine, J. Planar Chromatogr, 12 124,... 

Because the steric effect contributes to the complex formation between guest and host, the chiral resolution on these CSPs is affected by the structures of the analytes. Amino acids, amino alcohols, and derivatives of amines are the best classes for studying the effect of analyte structures on the chiral resolution. The effect of analyte structures on the chiral resolution may be obtained from the work of Hyun et al. [47,48]. The authors studied the chiral resolution of amino alcohols, amides, amino esters, and amino carbonyls. The effects of the substituents on the chiral resolution of some racemic compounds are shown in Table 6. A perusal of this table indicates the dominant effect of steric interactions on chiral resolution. Furthermore, an improved resolution of the racemic compounds, having phenyl moieties as the substituents, may be observed from this Table 6. ft may be the result of the presence of n—n interactions between the CCE and racemates. Generally, the resolution decreases with the addition of bulky groups, which may be caused by the steric effects. In addition, some anions have been used as the mobile phase additives for the improvement of the chiral resolution of amino acids [76]. Recently, Machida et al. [69] reported the use of some mobile phase additives for the improvement of chiral resolution. They observed an improvement in the chiral resolution of some hydrophobic amino compound using cyclodextrins and cations as mobile phase additives. 

Enantiometric mixtures of nimodipine were separated on (3-cyclodextrin-bonded silica gel plates. The plates were developed with light petroleum/ ethyl acetate/methanol, methanol/1% triethylammonium acetate (pH 4.1), or methanol/acetonitrile/1% triethylammonium acetate as mobile phases. Spots were identified by illumination at 365 nm, or by exposure to iodine vapor [11]. 

Another approach is electrochromatography with capillary columns packed with an achiral stationary phase, preferentially a reversed-phase type material. The chiral SO is added to the background electrolyte, and may be adsorbed onto the stationary phase by a secondary equilibration process. Enantioseparations in this additive mode have been reported with cyclodextrin type SOs )504-507) and with a chiral ion-pair agent derived from quinine 1508) as mobile phase additives. 

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. 

Aqueous and methanolic solutions of cyclodextrins have been employed as mobile phases in high performance liquid chromatography (HPLC), and as stationary phases by bonding the cyclodextrin to silica packing by several workers (1-8). They have been shown to be especially suitable for separation of structural isomers, cis-trans geometric isomers, and enantiomers. Cyclodextrins (CD) are toroidal-shaped, cyclic, oligosaccharides made up of o-l,4 linked,... 

In conventional reversed phase HPLC, differences in the physicochemical interactions of the eluate with the mobile phase and the stationary phase determine their partition coefficients and, hence, their capacity factor, k. In reversed-phase systems containing cyclodextrins in the mobile phase, eluates may form complexes based not only on hydrophobicity but on size as well, making these systems more complex. If 1 1 stoichiometry is involved, the primary association equilibrium, generally recognized to be of considerable importance in micellar chromatography, can be applied (11-13). The formation constant, Kf, of the inclusion complex is defined as the ratio of the entrance and exit rate constants between the solute and the cyclodextrin. Addition of organic modifiers, such as methanol, into the cyclodextrin aqueous mobile phase should alter the kinetic and thermodynamic characteristics of the system. This would alter the Kf values by modifying the entrance and exit rate constants which determine the quality of the separation. 

P.H. Kuijpers, T.K. Gerding and G.J. de Jong, Improvement of the Liquid Chromatographic Separation of the Enantiomers of Tetracyclic Eudistomins by the Combination of a P-cyclodextrin Stationary Phase and Camphor-sulphonic Acid as Mobile Phase Additive, J. Chromatogr., 625(1992) 223. 

ABSTRACT. The copper(n) complexes of p-cyclodextrins functionalized with aliphatic or pseudoaromatic amines were used for the chiral recognition of unmodified amino acids. Molecular recognition, assisted by non-covalent interactions, was proved by means of thermodynamic and spectroscopic (c.d., e.p.r. and fluorescence) measurements. A cis-disposition of amino groups seems to assist enantiomeric selectivity. The copper(II)-p-cyclodextrin complexes can be used as mobile phase additives in HPLC to separate enantiomeric mixtures of unmodified aromatic amino acids. 

The application of cyclodextrins (CDs) in high-performance liquid chromatography (HPLC) as mobile-phase additives and components of stationary phases is briefly discussed. The advantageous separation characteristics of CDs in various HPLC technologies ate demonstrated using pharmaceuticals, environmental pollutants as analytes. 

Anigbogu et al. [158] studied the effects of methanol, r-butyl alcohol (TBA), and cyclopentanol (CP) on anthracene and pyrene retention on a C)8 column (A = 255 nm) using 50% to 70% methanol in water containing 3 mM ) -cyclodextrin and 1% TBA or CP. On the basis of retention effects, the authors speculate that TBA and CP assist in the formation of a cyclodextrin/pyrene complex and conclude that TBA and CP may be effectively used as mobile phase modifiers in these fiised-ring systems. Schuette and Warner [159] conducted a similar study on the effects of N pentanol on or y-cyclodextrin/PAH complexes. A solution of 0.1 M 1-pentanol with 5mM y- or j -cyclodextrin increased the fluorescence emission intensity markedly in Ihe 430-490 nm range. Chromatographic selectivity and efficiency were enhanced and the detection limits were lowered by nearly an order of magnitude. 

Water-soluble cyclodextrin complexes of water-insoluble monomers were achieved by stirring a slight excess of cyclodextrin derivatives, in most cases RAMEB, and different monomers in water. Several methods were applied to the characterizations of the host- fuest complexes. The most important behavior of the complexes is that the hydrophobic guest monomers become water-soluble by being included into cyclodextrin. Reaction control by thin-layer chromatography (TLC) with methanol as mobile phase, followed by UV detection and iodine development, shows complete conversion to the corresponding cyclodextrin complexes. The Rf values of the complexes are significantly different from the value of imcomplexed monomers and cyclodextrin (Table 1). 

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