Enantiomer separations with impregnated layers

Often chromatographers have asked the TLC plate manufacturers if they could make chiral TLC layers. This would allow them to determine which of the five classes of chiral columns would be most useful for their enantiomer separations. Unfortunately, such layers would be very costly since a TLC plate is used only once. Only one plate is available, typically with a chir in its name. It is composed of a layer that is impregnated with proline and copper(II) ion to allow complexation of amino acids and amino alcohols for enantiomer separation. Unfortunately, this... 

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). 

Bhushan and Ali (1987) tested amino acid separations on silica gel layers impregnated with various metal salts. Bhushan and Reddy (1989) reported the separation of phenylthiohydantoin (PTH) amino acids on silica gel with new mobile phases. Laskar and Basak (1988) de.scribed a new ninhydrin-based procedure that produced different colors and good sensitivity for amino acid detection. Bhushan and Reddy (1987) reviewed the TLC of PTH amino acids. Gankina et al. (1989) described a unidimensional multistep silica gel HPTLC method for the separation and identification of PTH and dansylamino acids. Bhushan et al. (1987) developed numerous solvent systems for effective separations of 2,4-dini-trophenyl-(DNP) amino acids. Bhushan (1988) reviewed the TLC resolution of enantiomeric amino acids and their derivatives. Kuhn et al. (1989) reported the amino acid enantiomer separation by TLC on cellulose of d- and L-tryptophan and methyltryptophan. Guenther (1988) determined TLC-separated enantiomers by densitometry. 

Impregnation of layers with a chiral selector for use in enantiomer separations has usually been carried out by mixing the compound with silica gel to prepare a slurry for in-house preparation of the layer. For example, Bhushan and Martens [18] reported use of this method for preparing layers containing selectors such as (-F)-tartaric acid or (-F)-ascorbic acid, erythromycin, vancomycin, L-lysine and L-arginine, and (—)-brucine. 

CHIRAL PHASE. A layer used for separation of optically active compounds (enantiomers). The most widely used chiral phase is prepared from C-18 (octa-decyl) chemically bonded silica gel impregnated with a Cu(II) salt and an optically active enantiomerically pure hydroxyproline derivative and operates with a ligand-exchange mechanism. 

Chapter 3 through 6 deal with the commercial and noncommercial stationary phases used for the direct and indirect enantioseparations by means of TLC and with the chiral modifiers of mobile phases, which are used exclusively in direct separations. Chapter 3 describes the commercial chiral and nonchiral sorbent materials and commercial precoated layers used in chiral separations. Thus, it deals with silica gel native and esterified cellulose chiral plates (reversed phase plates impregnated with a chiral selector) and C-18, C-18W, diol, diphenyl, and C-2 chemically bonded silica gel. At the end of this chapter, the author discusses the quantification of enantiomers by using densitometry, depending on the type of the stationary phase employed. 

Originally, we attempted to separate two pairs of enantiomers, namely, S,R-( )-ibuprofen and 5,/ -( )-2-phenylpropionic acid [1]. The two samples were dissolved in 70% ethanol and then the respective solutions were spotted with the aid of an automatic sampler on to the adsorbent layer. Thin layer chromatographic conditions that are best suited for separation of the APA enantiomers involve silica gel impregnated with L-arginine, which is kept in the cationic form, due to a properly fixed pH value (<4.8). The mobile phases used were the ternary mixtures of acetonitrile (ACN), methanol (MeOH), and H2O (plus several drops... 

Alkaloids, for example, quinine, are widely applied as catalytic reagents in chiral organic syntheses and also in the production of HPLC stationary phases for chiral separations. Cinchona alkaloids also induce the enantioselection of chiral fluoro-compounds analyzed by FNMR spectroscopy [32]. (-)-Brucine and (-)-berberine mixed with silica gel were used for preparation of thin-layer plates to resolve racemic amino acids. Other optically active pure enantiomers of natural compounds are applied for impregnation of TLC plates (-l-)-tartaric acid, L-aspartic acid [33]. (—)-Menthol is used for preparation of diastereomeric derivatives in indirect enantiomeric separation of, for example, 2- and 3-hydroxy acids [34]. 

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