Stationary phases cyclodextrin applications

Chiral chromatography methods are considered by many to be superior to conventional methods in that, besides analytical applications, they offer the greatest potential for the preparation of optically pure forms of the isomers [5,27,28]. In these examples the third chiral species is an integral part of the LC (or GC) system and may appear as a plain stationary phase (cyclodextrins),... 

Aroma compounds originate from biosynthetic pathways inside an animal, a botanical body, and other life-forms as well as enzymes and thus frequently carry chiral components within the molecule. Determination of such enantiomeric properties can, in many cases, be accomplished using a GC column with a chiral stationary phase (CSP) application.75-79 These columns, usually called chiral GC column, will provide diastereometric interaction that could lead to resolution of enantiomers. Commercially available chiral GC columns predominantly utilize cyclodextrin derivatives as CSPs. Chiral columns consisting of multiple cyclodextrin derivatives intending synergic effect in resolution property80 are also successful in the market. In practice, these columns are mainly operated as secondary columns in MDGC technique. 

Cyclodextrin stationary phases are applicable to as wide variety of compounds, often without derivatization. Chiral separations by GC are becoming more and more important, and for very difficult separations, it is easy to increase the length of the capillary column to produce the necessary number of theoretical plates. 

Mourier s report was quickly followed by successful enantiomeric resolutions on stationary phases bearing other types of chiral selectors, including native and deriva-tized cyclodextrins and derivatized polysaccharides. Many chiral compounds of pharmaceutical interest have now been resolved by packed column SFC, including antimalarials, (3-blockers, and antivirals. A summary is provided in Table 12-2. Most of the applications have utilized modified CO, as the eluent. 

There is a wide variety of commercially available chiral stationary phases and mobile phase additives.32 34 Preparative scale separations have been performed on the gram scale.32 Many stationary phases are based on chiral polymers such as cellulose or methacrylate, proteins such as human serum albumin or acid glycoprotein, Pirkle-type phases (often based on amino acids), or cyclodextrins. A typical application of a Pirkle phase column was the use of a N-(3,5-dinitrobenzyl)-a-amino phosphonate to synthesize several functionalized chiral stationary phases to separate enantiomers of... 

Walhagen, A., Edholm, L.E. (1991). Chiral separation of achiral stationary phases with different functionalities using P-cyclodextrin in the mobile phase and applications to bioanalysis and coupled columns. Chromatographia 32, 215-223. 

As yet, the number of applications is limited but is likely to grow as instrumentation, mostly based on existing CE systems, and columns are improved and the theory of CEC develops. Current examples include mixtures of polyaromatic hydrocarbons, peptides, proteins, DNA fragments, pharmaceuticals and dyes. Chiral separations are possible using chiral stationary phases or by the addition of cyclodextrins to the buffer (p. 179). In theory, the very high efficiencies attainable in CEC mean high peak capacities and therefore the possibility of separating complex mixtures of hundreds of... 

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. 

Even more generally applicable are GC columns with chiral metal chelates as stationary phases (complexation gas chromatography)26 (Table 6). Quite recently, chiral GC methods have been developed on the basis of cyclodextrin derived stationary phases27. 

A. Berthod, W. Li, and D. W. Armstrong, Multiple Enantioselective Retention Mechanisms on Derivatized Cyclodextrin Gas Chromatographic Chiral Stationary Phases, Anal. Chem. 1992,64, 873 K. Bester, Chiral Analysis for Environmental Applications, Anal. Bioanal. Chem. 2003,... 

Y. Gong and H. K. Lee, Application of Cyclam-Capped (5-Cyclodextrin-Bonded Silica Particles as a Chiral Stationary Phase in Capillary Electrochromatography for Enantiomeric Separations, Anal. Chem. 2003, 75,... 

Cyclodextrins. As indicated previously, the native cyclode.xtrins. which are thermally stable, have been used extensively in liquid chromatographic chiral separations, but their utility in gc applications was hampered because their highly crystallinity and insolubility in most organic solvents made ilium difficult to formulate into a gc stationary phase. However, some functionalized cyclodextrins form viscous oils suitable for gc stationary-phase coalings and have been used either neat nr diluted in a polysiloxane polymer as chiral stationary phases for gc. 

In view of the importance of chiral resolution and the efficiency of liquid chromatographic methods, attempts are made to explain the art of chiral resolution by means of liquid chromatography. This book consists of an introduction followed by Chapters 2 to 8, which discuss resolution chiral stationary phases based on polysaccharides, cyclodextrins, macrocyclic glyco-peptide antibiotics, Pirkle types, proteins, ligand exchangers, and crown ethers. The applications of other miscellaneous types of CSP are covered in Chapter 9. However, the use of chiral mobile phase additives in the separation of enantiomers is discussed in Chapter 10. 

In this section an overview of the numerous methods and principles for the discrimination of enantiomers is given. First, the interaction principles of the polymer-based methods adapted from chromatographic procedures are illustrated. The discrimination of enantiomers was achieved some decades ago by using different types of stationary materials, like cyclodextrins or polymer-bonded amide selectors. These stationary-phase materials have successfully been appointed for label-free optical sensing methods like surface plasmon resonance (SPR) or reflectometric interference spectroscopy (RIfS). Furthermore, various successful applications to optical spectroscopy of the well-established method of fluorescence measurements for the discrimination of enantiomers are described. 

In LC, cyclodextrins have been used in both the mobile and stationary phases, although the latter application predominates using bonded phases. Both normal and reversed phase separations are possible. With polar mobile phases, the cyclodextrin cavity hosts and retains hydrophobic analytes but when large concentrations of organic solvent are used in the mobile phase, solvent molecules occupy the cavity—inclusion of analytes is suppressed and hydrogen bonding and dipole-dipole interactions with analytes predominate. Applications to electrochromatography have also been described. [96]... 

The above phases represent the most common phases used in solving nearly all of the frequently encountered application problems. There are many other stationary phases which are produced to tune the phase polarity for specific applications. In addition to these phases, there are liquid crystalline, chiral, cyclodextrin, polymers such as polystyrene, divinylben-zene, molecular sieves, and alumina, which are designed for specific separation problems. The chemistry of fused silica deactivation and stationary-phase application, bonding, and cross-linking has been reviewed in detail [3,4]. 

Cvclodextrin-Silica stationary Tfriaaag. since excellent reviews deal with the preparation, properties, and analytical applications of cyclodextrin-silica stationary phases (11,12.14-16), the following paragraphs will discuss these topics very briefly, only to the extent that the information will be used in the rest of this chapter. 

Chiral CEC will be discussed in detail later in the book but is included here to exemplify the application of the high efficiencies obtained with electro-driven techniques which makes them attractive for chiral analysis where selectivity factors are sometimes small. CE has made use of chiral additives in the electrolyte whilst LC tends to utilise chiral stationary phases. Both options have been explored for chiral CEC [27,28,77]. The small amount of packing material necessary for capillaries allows the use of chiral stationary phases that would be prohibitively expensive for standard LC. Cyclodextrins, proteins, antibiotics and molecular imprinting have all been used to form chiral stationary phases [78-80]. After some less than encouraging peak efficiencies obtained using the chiral CEC approach, much improved chiral resolutions have been achieved using CEC compared to LC or CE [81-83]. 

Although they are extremely useful analytically, the protein based stationary phases 3-6,371 have found little application in preparative HPLC because they suffer from low loading capacity, due primarily to the low number of active sites. The natural macrocylic molecules Cyclodextrin 3 8,3 91 and antibiotics such as Vancomycin 3 10] have shown some promise. Synthetic chiral crown ethers 311 are particularly useful for the separation of chiral primary amines. 

In high performance liquid chromatography (HPLC), the cyclodextrins (12, 27-36) or highly soluble methylated cyclodextrins (37) in the mobile phase, as well as the silica bonded cyclodextrins (38-40) as stationary phase have attained spectacular success. A series of rapid, elegant separations have been published. The field of application of this method seems to be inexhaustible. 

The properties of cyclodextrins were studied using a packed column, whereas the effect of the liquid crystals were enhanced by employing a capillary column The stereoselective properties of these materials as stationary phases were studied with a set of alkylbenzoderivatives The mechanism of the separation is discussed on the basis of the retention data obtained The advantages and drawbacks of these phases are compared with conventional GC stationary phases and analytical applications are discussed ... 

In order to increase the selectivity and efficiency of extraction, novel stationary phases have been developed, particularly in trace analysis. Cyclodextrins, graphi-tized carbon blacks, and conductive polymers such as polypyrrole and poly aniline have been investigated [73]. Sol-gel polymers also seem to be interesting. A particularly important feature is the high thermal stability of these polymers [74]. Recent applications for analysis of biological samples have been described in several articles [75-81]. Review articles present recent developments in methodology, the SPME technique [82-84], and novel coatings [85, 86]. 

Y. Gong, Y. Xiang, B. Yue, G. Xue, J. S. Bradshaw, H. K. Lee, and M. L. Lee, Application of diaza-18-crown-6-capped (l-cyclodextrin bonded silica particles as chiral stationary phases for ultrahigh pressure capillary liquid chromatography, J. Chromatogr. A 1002 (2003), 63-70. 

Practically every type of separation that has been done by the column technique can also be carried out by thin-layer chromatography. Several papers and reviews were published on the various aspects of the technique. In addition to the books on chromatography [17,26-301, an overview of ion-exchange application of TLC was presented by Devenyi and Kalasz 311. Recent results on the separation of enantiomers have been reviewed by Mack, Hauck and Herbert (32.33) (enantiomer. separation on an RP-18 plate, impregnated with copper salt and proline derivative as chiral selectors) and Lepri, Coas and Desideri, using a microcrystalline triacetylcellulose stationary phase, or modified beta-cyclodextrins in the mobile phase 134.35). 

The developments of the Lewis base-modified zirconia and mixed-oxide containing zirconia as stationary phases for high-performance liquid chromatography (HPLC) are reviewed. In this context, the preparation methods of porous spherical zirconia, and zirconia supports for HPLC based on modification with fluoride, phosphate, phos-phonate, carboxylic acid, phenols, and protein, as well as cyclodextrin derivative, are covered. The application of modified-zirconia in capillary electrochromatography (CEC) is also discussed. 

Modified-C02 mobile phases excel at stereochemical separations, more often than not outperforming traditional HPLC mobile phases. For the separation of diastereomers, silica, diol-bonded silica, graphitic carbon, and chiral stationary phases have all been successfully employed. For enantiomer separations, the derivatized polysaccharide, silica-based Chiralcel and Chiralpak chiral stationary phases (CSPs) have been most used, with many applications, particularly in pharmaceutical analysis, readily found in the recent literature (reviewed in Refs. 1 and 2). To a lesser extent, applications employing Pirkle brush-type, cyclodextrin and antibiotic CSPs have also been described. In addi-... 

Direct determination of the enantiomeric purity of an enantiomeric mixture is usually achieved by GC or HPLC on a chiral stationary phase derived from chiral materials such as cellulose and cyclodextrin. NMR analysis in the presence of a chiral shift reagent is also applicable, although with less accuracy than the chromatographic methods. 

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