Cyclodextrin-silica stationary phases

Fujimura, K., Ueda, T., and Ando, T., Retention behavior of some aromatic compounds on chemically bonded cyclodextrin silica stationary phase in liquid chromatography, Anal. Chem., 55, 446, 1983. 

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. 

Minority Access for Research careers Program, National Institute of Health, Grant Number 5F31GHL1689 is acknowledged. The authors are grateful to Dr. Thanas Beesley of ASTEC for the beta-cyclodextrin silica stationary phases used in this study. 

Chen,CY,CH Lin and JHYang(2005).Use of chemically bondedj8-cyclodextrin silica stationary phase for liquid chromato graphic separation of structural isomers. Journal of the Chinese Chemical Society, 52(4), 753-758. 

Ching, C.B. Hidajat, K., and Liu, X., Sorption and diffusion of cresols on bonded beta-cyclodextrin-silica stationary phase, Ind. Eng. Chem. Res., 32(11), 2789-2793 (1993). 

Figure 3.11—Separation on a cyclodextrin-boimd stationary phase. Chromatogram of a racemic mixture chemical formula of /f-cyclodextrin (diameter, 1.5 nm cavity, 0.8 nm height, 0.8 nm) partial representation of cyclodextrin bonded to a silica gel bead through an alkyl chain linker arm side view of a cyclodextrin molecule with a hydrophobic cavity.

Silica-base stationary phases have also been employed for enantiomeric separations in CEC [6,72-81]. In the initial work on chiral CEC, commercially available HPLC materials were utilized, including cyclodextrins [6,74,81] and protein-type selectors [73,75,80] such as human serum albumin [75] and ai-acid glycoprotein [73]. Fig. 4.9, for example, depicts the structure of a cyclodextrin-base stationary phase used in CEC and the separation of mephobarbital enantiomers by capillary LC and CEC in a capillary column packed with such a phase. The column operated in the CEC mode affords higher separation efficiency than in the capillary LC mode. Other enantiomeric selectors are also use in CEC, including the silica-linked or silica-coated macrocyclic antibiotics vancomycin [82,83] and teicoplanin [84], cyclodextrin-base polymer coated silicas [72,78], and weak anion-exchage type chiral phases [85]. Relatively high separation efficiency and excellent resolution for a variety of compounds have also been achieved using columns packed with naproxen-derived and Whelk-0 chiral stationary phases linked to 3 pm silica particles [79]. Fig. 4.10 shows the... 

Enantiomer separation of various compounds such as barbituric acids, benzoin, MTH-proline, glutethimide, a-methyl-oc-phenyl-succinimide, y-phenyl-y-butyrolac-tone, methyl-mandelate, l-(2-naphthyl)ethanol, mecoprop methyl, diclofop methyl and fenoxaprop methyl by pressure supported CEC on a permethyl-P-cyclodextrin modified stationary phase was described by Wistuba and Schurig [42-44]. Three different separation beds were used (i) permethyl-P-cyclodextrin was covalently attached via a thioether to silica (Chira-Dex-silica) [42], permethyl-P-cyclodextrin was linked to a dimethylpolysiloxane and thermally immobilized (ii) on silica (Chirasil-Dex-silica) [43] or (iii) on a silica monolith (Chirasil-Dex-monolith) [44], respectively. 

The cyclodextrins can tolerate relatively high buffer concentrations and are stable from pH 3 to pH 14. However, the stability of the silica matrix restricts the pH range from 3. 0 to about 7.0, as silica is significantly soluble at a pH of 8.0 and higher. Cyclodextrin type stationary phases may be operated in the polar or reversed phase mode. As with the other LC stationary phases, mobile phases with high water contents have little dispersive properties and thus the dispersive interactions with the stationary phase can be exploited. Conversely, if strongly dispersive... 

Many racemic mixtures can be separated by ordinary reverse phase columns by adding a suitable chiral reagent to the mobile phase. If the material is adsorbed strongly on the stationary phase then selectivity will reside in the stationary phase, if the reagent is predominantly in the mobile phase then the chiral selectivity will remain in the mobile phase. Examples of some suitable additives are camphor sulphonic acid (10) and quinine (11). Chiral selectivity can also be achieved by bonding chirally selective compounds to silica in much the same way as a reverse phase. A example of this type of chiral stationary phase is afforded by the cyclodextrins. 

Lubda, D., Cabrera, K., Nakanishi, K., Lindner, W. (2003). Monolithic silica columns with chemically bonded b-cyclodextrin as a stationary phase for enantiomer separations of chiral pharmaceuticals. Anal. Bioanal. Chem. 377, 892-901. 

The bonding of cyclodextrins to silica has provided a range of media known as chiral stationary phases (CSPs), which are capable 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. 

All aldehydes used in the experiment were freshly distilled or washed with aqueous NaHC03 solution to minimize the amount of free acid. Chiral HPLC was performed using a chiral OJ-H column (0.46 cm x 25 cm, Daicel industries) with a water 717 auto sampler and a UV-vis detector (254 nm). The eluting solvent used was different ratios of hexane and 2-propanol. Chiral gas chromatography analysis was performed in a Shimadzu auto sampler with cyclodextrins columns as chiral stationary phase (fused-silica capillary column, 30 m X 0.25 mm x 0.25 gm thickness, /3-Dex-120 and /3-Dex-325 from Supelco, USA) using He as a carrier gas (detector temperature 230 °C and injection temperature 220 °C). 

In an attempt to change and broaden the capabilities of the vancomycin CSP, the glycopeptide was derivatized with (R)- and (S )-(l-naphthylethyl) isocyanate (NEIC) and then bonded to a silica-gel support [48]. A variety of chiral compounds was tested on the two composite stationary phases and the results were compared with the ones obtained using the underivatized vancomycin CSP. The advantages of the NEIC derivatization were not as obvious or substantial as they were in the case of cyclodextrin phases [49]. Moreover, the exact chemical structures of the synthesized NEIC derivatives of vancomycin were not reported. 

Capillary gas chromatography (GC) using modified cyclodextrins as chiral stationary phases is the preferred method for the separation of volatile enantiomers. Fused-silica capillary columns coated with several alkyl or aryl a-cyclo-dextrin, -cyclodextrin and y-cyclodextrin derivatives are suitable to separate most of the volatile chiral compounds. Multidimensional GC (MDGC)-mass spectrometry (MS) allows the separation of essential oil components on an achiral normal phase column and through heart-cutting techniques, the separated components are led to a chiral column for enantiomeric separation. The mass detector ensures the correct identification of the separated components [73]. Preparative chiral GC is suitable for the isolation of enantiomers [5, 73]. 

Many types of chiral stationary phase are available. Pirkle columns contain a silica support with bonded aminopropyl groups used to bind a derivative of D-phenyl-glycine. These phases are relatively unstable and the selectivity coefficient is close to one. More recently, chiral separations have been performed on optically active resins or cyclodextrins (oligosaccharides) bonded to silica gel through a small hydrocarbon chain linker (Fig. 3.11). These cyclodextrins possess an internal cavity that is hydro-phobic while the external part is hydrophilic. These molecules allow the selective inclusion of a great variety of compounds that can form diastereoisomers at the surface of the chiral phase leading to reversible complexes. 

Cyclodextrin stationary phases utilize cyclodextrins bound to a solid support in such a way that the cyclodextrin is free to interact with solutes in solution. These bonded phases consist of cyclodextrin molecules linked to silica gel by specific nonhydrolytic silane linkages (5,6). This stable cyclodextrin bonded phase is sold commercially under the trade name Cyclobond (Advanced Separation Technologies, Whippany, New Jersey). The vast majority of all reported hplc separations on CD-bonded phases utilize this media which was also the first chiral stationary phase (csp) developed for use in the reversed-phase mode. 

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,... 

To establish chiral separation method for donepezil hydrochloride enantiomers by capillary electrophoresis (CE) and to determine the two enantiomers in plasma [39], alkalized plasma was extracted by isopropa-nol-n-hexane (3 97) and L-butefeina was used as the IS. Enantioresolution was achieved using 2.5% sulfated-beta-cyclodextrin as chiral selector in 25 mmol/1 triethylammonium phosphate solution (pH 2.5) on the uncoated fused-silica capillary column (70 cm x 50 fim i.d.). The feasibility of the method to be used as quantitation of donepezil HC1 enantiomers in rabbit plasma was also investigated. Donepezil HC1 enantiomers were separated at a baseline level under the above condition. The linearity of the response was evaluated in the concentration range from 0.1 to 5 mg/1. The linear regression analysis obtained by plotting the peak area ratio (A(s)/A(i)) of the analyte to the IS versus the concentration (C) showed excellent correlation coefficient The low limit of detection was 0.05 mg/1. The inter- and intra-day precisions (RSD) were all less than 20%. Compared with chiral stationary phase by HPLC, the CE method is simple, reliable, inexpensive, and suitable for studying the stereoseletive pharmacokinetics in rabbit. 

Chirality of derivatized cyclodextrin was used for recognition of stereoisomers. Phenylazobenzoyl modified y-cyclodextrin was anchored onto silica gel used as stationary phase in HPLC and photoresponsive chromatographic behavior of dansyl amino acid enantiomers was studied [64],... 

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]. 

In the initial experiments reported here we did not attempt to optimize the separation in terms of yield and production rate. Rather, cur intent was to demonstrate that displacement chromatographic separations are feasible on a chiral stationary phase, cyclodextrin-silica, and gather preliminary information regarding the structure of displacers which cam De used with cyclodextrin-sil icas. The method development sequence described in the previous paragraph will be followed in the discussion of the results. 

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. 

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