Thin-layer chromatography, chiral selectors used

Thin-Layer Chromatography. Chiral stationary phases have been used less extensively in tic as in high performance Hquid chromatography (hplc). This may, in large part, be due to lack of avakabiHty. The cost of many chiral selectors, as well as the accessibiHty and success of chiral additives, may have inhibited widespread commerciali2ation. Usually, nondestmctive visuali2ation of the sample spots in tic is accompHshed using iodine vapor, uv or fluorescence. However, the presence of the chiral selector in the stationary phase can mask the analyte and interfere with detection (43). 

Thin layer chromatography is also used for direct enantiomeric resolution of D,L-arginine, D,L-histidine, d,l-lysine, D,L-valine, and D,L-leucine on silica gel plates impregnated with optically pure (IR, 3R, 5R)-2-azabicy-clo[3,3,0]octan-3-carboxylic acid, which serves as a chiral selector in the pharmaceutical industry. To successfully resolve D,L-amino acids, various combinations of aceto-nitrile-methanol-water were proposed. The spot was detected by ninhydrin (0.2% in acetone). 

Thin-Layer Chromatography. Thin-layer chromatography (lie) offers several advanlages for chiral separations and in Ihc development of new chiral stationary phases. Besides being inexpensive, tic cun be used lo screen mobile phase conditions rapidly (i.e organic modifier content. pH. etc.), chiral selectors, and analytes. Several different analytes may be run simultaneously on Ihe same plale. Usually, no preequilibralion of the mobile phase and stationary phase is required. In addition, only small... 

In contrast, CSPs have achieved great repute in the chiral separation of enantiomers by chromatography and, today, are the tools of the choice of almost all analytical, biochemical, pharmaceutical, and pharmacological institutions and industries. The most important and useful CSPs are available in the form of open and tubular columns. However, some chiral capillaries and thin layer plates are also available for use in capillary electrophoresis and thin-layer chromatography. The chiral columns and capillaries are packed with several chiral selectors such as polysaccharides, cyclodextrins, antibiotics, Pirkle type, ligand exchangers, and crown ethers. 

Lienne et al. [170] resolved the enantiomers of albendazole sulfoxides on a column derived from the 6S )-/V-(3,5-dinitrobcnzoyl)tyrosinc chiral selector. The developed method was applied for the enantiomeric resolution of albendazole sulfoxides in plasma samples. Witherow et al. [171] immersed a commercially available thin-layer plate (thin-layer chromatography) into a solution of N-(3,5-dinitrobenzoyl)-L-leucine solution. The developed plate was used for the chiral resolution of 2,2,2-trifluoro-(9-anthryl)ethanol and l,l -binaphthol enantiomers. 

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). 

For the separation of anionic analytes, the positively charged macrocyclic antibiotic vancomycin is one of the most commonly used chiral selectors. It was introduced by Armstrong and coworkers as a new class of chiral selector in CE, HPLC and thin-layer chromatography for the separation of a wide range of enantiomers [29, 47-52]. It contains multiple stereogenic centers and a variety of functional groups. 

Chiral mobile phase additives provide a more versatile and cost-effective approach for enantiomer separations in thin-layer chromatography. Typically, chemically bonded layers with cyclodextrin and its derivatives, bovine serum albumin, or macrocyclic glycopeptides are used as chiral additives in the reversed-phase mode [59,60,172-178]. For [5- and y-cyclodextrins and their derivatives, a 0.1 to 0.5 M aqueous methanol or acetonitrile solution of the chiral selector is used as the mobile phase. Bovine serum albumin is generally used at concentrations of 1-8 % (w/v) in an aqueous acetate buffer of pH 5 to 7 or in a 0.5 M acetic acid solution, in either case with from 3-40 % (v/v) propan-2-ol (or another aliphatic alcohol), added to control retention. Enantioselectivity usually increases with an increase in concentration of the chiral selector, and may be non existent at low concentrations of the chiral selector. 

The low efficiency and short migration distances typical of thin-layer chromatography limit useful separations to those with relatively large enantioselectivity factors. Absorption by the chiral selector can cause baseline instability and reduced sample detectability for quantitative measurements by scanning densitometry. The chiral sepa-... 

Bhushan, R. and Gupta, D., Resolution of (-P)-ibuprofen using (—)-brucine as a chiral selector by thin layer chromatography, Biomed. Chromatogr., 18,838-840, 2004. 

The two enantiomers of a given amino acid have identical chemical and physical properties in a symmetrical environment. To resolve such a pair of amino acid by chromatography, diastereomers must be formed. Diastereomers can be formed if a chiral reagent (selector) is introduced to either the mobile or the stationary phase. In case of thin-layer chromatography (TLC), latter manner has largely been used for resolution of amino acids, their PTH-, dansyl, and other derivatives [2,4-6]. 

Bhushan and Parsad [65] resolved dansyl amino acids on erythromycin impregnated thin-layer chromatographic (TLC) silica plates. The mobile phase used was different ratios of 0.5 M aqueous NaCl-acetonitrile-methanol. Further, Bhushan and Thiong o [66] achieved the chiral resolution of dansyl amino acids on silica TLC plates impregnated with vancomycin chiral selector. The mobile phase used for this study was acetonitrile-0.5 M aqueous NaCl (10 4 and 14 3, v/v). The chiral recognition mechanisms of antibiotic CSPs in sub-SFC, SFC, CEC, and TLC modes of chromatography were found to be similar to HPLC. 

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