Anal. Chem. 2000, 72, 4521-4528
Chiral Separations Timothy J. Ward
Department of Chemistry, Millsaps College, Jackson, Mississippi 39210 Review Contents Reviews Capillary Electrophoresis (CE) Thin-Layer Chromatography (TLC) Subcritical and Supercritical Fluid Chromatography Gas Chromatography Liquid Chromatography Capillary Electrophoresis (CE) Cyclodextrin Antibiotics Miscellaneous Thin-Layer Chromatography Supercritical Fluid Chromatography and Related Techniques Gas Chromatography High-Performance Liquid Chromatography Cyclodextrin CSPs Teicoplanin and Vancomycin CSPs Ristocetin A CSP Other Macrocyclic Phases Polysaccharide CSPs Crown Ether-Based CSPs Various Other CSPs Other Related Chromatographic Methods Literature Cited
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REVIEWS This review covers developments and applications in chiral separations from October 1997 to December 1999. This review is not intended to be a comprehensive review of all papers published. This article focuses on major developments in the field of chiral separations as well as representative applications. The review is restricted to English language literature. Three reviews that appeared earlier in 1997, one on supercritical fluid chromatography (SFC), one on gas chromatography (GC), and one on highperformance liquid chromatography (HPLC), were included since no other reviews of significance were published during the time period covered by this article. Capillary Electrophoresis (CE). Enantioresolution of pharmaceutical compounds by capillary electrophoresis continues to be a popular topic with numerous reviews being published between October 1997 and December 1999. While most of the reviews covered broad classes of chiral selectors, several examined specific classes only such as proteins, cyclodextrins, crown ethers, or macrocyclic antibiotics. One of the more comprehensive reviews by Verleysen and Sandra, cites 360 references discussing the use of chiral selectors such as charged cyclodextrins, crown ethers, polysaccharides, proteins, micelles, macrocyclic antibiotics, and ergot alkaloids in CE (1). A review with 223 references summarizing the present state of the art in chiral separations by 10.1021/ac000841o CCC: $19.00 Published on Web 08/11/2000
© 2000 American Chemical Society
capillary zone electrophoresis (CZE) was compiled by Vespalec and Bocek (2). In a review with 111 references, Fanali et al. outlined the methods for the enantioseparation of drugs using native cyclodextrins, derivatized cyclodextrins, and antibiotics as chiral selectors in CE, the parameters affecting these separations, and the use of these methods on real samples such as serum, urine, and pharmaceutical formulations (3). In an earlier review, Desiderio and Fanali described the use of the glycopeptide antibiotics, the ansamycins, kanamycin, streptomycin, fradiomycin, and vancomycin analogues (4). Various chiral selectors, direct separations in two-phase systems, and practical applications were summarized by Vaspalec and Bocek (2). Bojarski and Aboul-Enein reviewed the electrophoretic and chromatographic enantioseparation of cardiovascular drugs (5). Enantioseparations using the following CE methods were reviewed by Chankvetadze: capillary zone electrophoresis, capillary electrokinetic chromatography, partial filling of the CE capillary, nonaqueous separations, and the coupling of chiral CE with mass spectrometry (6). A number of reviews examined specific chiral selectors or methods used in CE. Hui and Caude reviewed the structure and physicochemical properties of the macrocyclic antibiotics and the effects of different operating parameters such as pH, the antibiotic used, the background electrolyte, the organic modifier, and the addition of micelles (7). Enantiomer separations of primary amines using crown ether as the chiral selector was reviewed by Kuhn (8). Two reviews deal specifically with the use of cyclodextrins and their derivatives as either chiral reagents in the buffer (9) or as stationary phases in capillary electrochromatography (CEC) (10). Hatajik provided an overview of the advantages and limitations of CEC for the chiral separation of pharmaceuticals (11). The use of proteins as chiral resolving agents were reviewed by Hage (12) and Lloyd (13). Natural and synthetic surfactants in micellar electrokinetic chromatography (MEKC) were reviewed by Camilleri (14) and Riekkola (15) and the use of polysaccharides by Nishi (16). Thin-Layer Chromatography (TLC). An overview of current chiral separations of drugs by TLC and their potential in the analysis of drug racemates was summarized in a review by AboulEnein et al. (17). Chiral TLC techniques for the separation and quantification of drugs, both in pharmaceutical dosage forms and in biological material are presented. Subcritical and Supercritical Fluid Chromatography. A review by Wolf and Pirkle with 72 references was written on the application of several packed column chiral stationary phases (CSPs) used with subcritical and supercritical fluid chromatography (18). Racemates from many different compound classes were reported enantioresolved, and both analytical and preparative separations were reviewed. Selector-analyte interactions involved in enantiodiscrimination were discussed, as well as the effect of Analytical Chemistry, Vol. 72, No. 18, September 15, 2000 4521
temperature and the nature of polar modifiers added to carbon dioxide mobile phases. Gas Chromatography. A review of types of gas chromatographic chiral stationary phases as well as available liquid chromatography chiral stationary phases was summarized by Allenmark and Schurig (19), with topics including types of gas chromatographic CSPs, CSPs based on hydrogen bonding, CSPs based on coordination, CSPs based on inclusion, chiral-phase systems in liquid chromatography, analytical and preparative applications, temperature effects, and enantiomerization and chiral inversion taking place in thermally labile chiral molecular structures. Bicchi et al. published a review dealing with the application of cyclodextrin derivatives as chiral selectors for direct GC enantioseparation of volatile enantiomers in the essential oil, aroma, extract, and flavor fields (20), with the applications grouped by analytical technique as follows: capillary gas chromatography and capillary gas chromatography/mass spectrometry, twodimensional gas chromatography, capillary gas chromatography/ isotope ratio-mass spectrometry, and liquid chromatography/ capillary gas chromatography. Liquid Chromatography. A review by Bojarski with 758 references summarizes important developments in chiral separations in liquid chromatography (21). Both direct and indirect separations are discussed, as well as electrophoretic enantioseparations are considered. The use of cyclodextrins as chiral selectors in direct HPLC separations of drugs and metabolites in crude biological samples are reviewed by Deng et al. (22) CAPILLARY ELECTROPHORESIS Cyclodextrin. Cyclodextrins and their derivatives are the most common chiral selectors used in CE chiral separations. While native cyclodextrins are still used in chiral applications, derivatized cyclodextrins appear to be more commonly used with greater success. Wang and Khaledi separated pharmaceutical enantiomers with anionic sulfated β-cyclodextrin in nonaqueous CE (23). They reported that higher electric field and higher ionic strengths used in nonaqueous systems results in better peak shapes and higher separation efficiencies. Izumoto and Nishi carried out enantiomeric separations of drugs using mixtures of anionic sulfated β-cyclodextrin and netural cyclodextrins under acidic conditions, producing superior separations over the use of neutral cyclodextrin alone (24). Wedig and Holzgrabe resolved the enantiomers of several tropa alkaloids using sulfobutyl ether β-cyclodextrin and heptakis(2,3-di-O-acetyl-6-sulfato)-β-cyclodextrin in a basic medium (25). Enantioseparations were carried out on pharmaceutical mixtures using netural and anionic sulfobutyl ether-β-cyclodextrin by Skanchy et al. (26). Other applications using sulfated β-cyclodextrin include the separation of racemic terbutaline (27), the resolution of voriconazole stereoisomers (28), and the separation of β-blockers with two chiral centers (labetalol and nadolol) (29). A number of cationic cyclodextrins have been used for enantioseparations. O’Keefe et al. and later Haynes et al. used a persubstituted cationic β-cyclodextrin derivative (heptakis(6methoxyethylamine-6-deoxy)-β-cyclodextrin) for the chiral separation of nonsteroidal antiinflammatory drugs and phenoxypropionic acid herbicides (30, 31). Quaternary ammonium β-cyclodextrin was used by Wang and Khaledi for the resolution of various chiral compounds (32, 33). They found the electroosmotic flow can be 4522
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reversed by reducing the analyte-cyclodextrin (CD) complex, making the cationic cyclodextrin especially useful for separating compounds such as tricyclic amine that bind strongly to neutral and anionic cyclodextrins. Methylated cyclodextrin derivatives have shown to be quite successful in a number of applications. The enantiomers of velaxfaxine and O-desmethylvenlafaxine were resolved with a combination of carboxymethylated β-cyclodextrin and R-cyclodextrin by Rudaz et al. (34). The same group also used β-cyclodextrin and methylated-β-cyclodextrin in coated and uncoated capillaries to resolve the enantiomers of aminoindanol and aminoindan (35). They found that the maximum separation of the isomers occurred when the pH of the buffer was equal to the mean pKa of the isomers. Salvador et al. evaluated the composition of several commercially available methylated β-cyclodextrins using previously developed analytical methodologies (36). They calculated binding constants of amphetamine derivatives with various methylated cyclodextrins using different calculation methodologies to demonstrate that different commercially available methylated cyclodextrins often exhibit significantly different enantioselectivities. Otsuka et al. also noted significant differences in enantioselectivity when using dimethyl-β-cyclodextrin from different suppliers (37). Using carboxymethyl-β-cyclodextrin, Chankvetadze et al., demonstrated the potential of flow-counterbalanced capillary electrophoresis (FCCE) to increase selectivity for binary mixtures (38). The following applications using methylated cyclodextrin derivatives were reported: pressure supported CEC using capillaries packed with permethyl-β-cyclodextrin (38, 40); separation of hexobarbital using a dual chiral recognition cyclodextrin derivatives (41); resolution of local anesthetic drugs using Tween 20 and methyl-β-cyclodextrin (42); nonaqueous separation of basic analytes using heptakis(2,3-dimethyl-6-sulfato)-β-cyclodextrin (43, 44); determination of aspartic acid in biosamples (45); determination of salbutamol enantiomers and its application to dissolution samples (46); separation of numerous racemic drugs (47); and determination of binding constants between enantiomers of orciprenaline and methyl-β-cyclodextrin (48). Various chiral separations have been achieved using native cyclodextrins or neutral derivatized cyclodextrins such as hydroxypropyl-β cyclodextrin. Mey et al. performed a chiral separation of amfepramone using hydroxypropyl-β-cyclodextrin (49). Hydroxypropyl-γ-cyclodextrin was used to analyze the enantiomers of thiopental and its oxybarbiturate metabolite, pentobarbital, in human plasma (50). Pak et al. used hydroxypropyl-β-cyclodextrin to resolve the enantiomers of propranolol and some of its metabolites (51). MEKC was used successfully with β-cyclodextrin to separate the enantiomers of racemorphan (52). Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was used for confirmation in the method developed to measure racemorphan in human urine. Pesek et al. etched the walls of fused-silica capillaries with ammonium hydrogen diflouride and then modified the walls with a chiral selector using the silanization/ hydroxysilation method (53). They used β-cyclodextrin, lactone, and naphthlethylamine as chiral selectors in open tubular capillary electrokinetic chromatography to separate a number of racemic solutes. β-Cyclodextrin was used to resolve the enantiomers of dansylated amino acids with various organic modifiers by Yang and Yuan (54). Chiral separations were carried
out on a number of drugs using a β-cyclodextrin polymer and native β-cyclodextrin by Ma et al. The β-cyclodextrin polymer showed better enantioselectivity, and the authors proposed that this is due to an intramolecular synergistic effect in the β-cyclodextrin polymer (55). Thalidomide and its hydroxylated metabolites were enantioresolved using four randomly substituted charged cyclodextrin derivatives by Meyring et al. (56). The pHindependent binding constants of alanylphenylalanine and leucylphenylalanine stereoisomers with β-cyclodextrin in the presence of urea were measured by varying the concentration of β-cyclodextrin (57). While numerous applications with respect to analytical-scale separations have appeared in the literature, the ability to perform preparative-scale separations has lagged behind. The potential for classical gel electrophoresis for chiral separations using sulfated cyclodextrins to separate the enantiomers of terbutaline was demonstrated by Stalcup et al. (58). They discussed possible adavntages and limitations to using this approach. Using capillary isotachophoresis (ITP) operated in a discontinuous fraction mode, a preparative-scale separation for the enantiomers of 2,4-dinitrophenyl-DL-norluecine was developed using β-cyclodextrin by Kaniansky and workers (59). The simulated moving bed (SMB) technology, first conceived for large bulk-scale separations in the petrochemical industry, has found increasingly new applications for the pharmaceutical industry (60). Azevedo discussed modeling, simulation, and design of the operation of a SMB plant in order to separate a binary chiral mixture. A simple, systematic method for screening CE separation conditions for basic enantiomers when using modified cyclodextrins was developed by Liu and Nussbaum (61). Nussbaum also examined the effect of the total cyclodextrin concentration in dualCD systems with respect to peak shape and resolution and considered the influence of temperature, capillary diameter, and current on separations (62). Fillet et al. used a combination of chiral selectors to explain why certain dual cyclodextrin systems show enantioselectivity when single chiral selectors are not effective, and in most cases, the selectivity could be predicted (63). Deng et al. developed a method for modeling and optimizing the enantioselectivity in CEC for neutral and ionic compounds when using an octadecylsilane ODS column and β-cyclodextrin in the run buffer (64). Molecular imprint polymers have been used as highly selective stationary phases for open tubular liquid chromatography (OT-LC) and CEC (65). Methods were developed to bond the inner walls of fused-silica capillaries with thin films of molecular imprinting polymers. Both D- and L-dansyl phenylalanines were enantioresolved in OT-LC and CEC. Antibiotics. While the majority of CE-based applications still utilize cyclodextrins and their derivatives, the macrocyclic antibiotics have proven to be extremely useful for chiral separations. They are the most frequently used class of chiral selectors to achieve enantiorecognition after cyclodextrin. Ekborg-Ott et al. analyzed the parameters affecting the use of the macrocyclic glycopeptide antibiotic avoparcin as a chiral selector in CE, and the use of avoparcin was compared to other macrocyclic antibiotics used in CE including ristocetin A, teicoplanin, and vancomycin (66). Using vancomycin as a chiral selector, Desiderio et al. employed a method in which the capillary is filled just to the detection window (hence the term, partial filling method) and found that sensitivity
was significantly improved by removing the strong UV absorbance of vancomycin. Fanali et al. also used the partial filling technique in conjunction with capillary electrophoresis/electrospray mass spectrometry (CE/ESI-MS) (67). Racemic mixtures of tryptophan and dinitrobenzoylleucine were resolved by CEC using the glycopeptide antibiotic teicoplanin (68). Teicoplanin was covalently bound to a silica support and packed into a capillary column with separations being performed in less than 6 min. Fanali et al. investigated Hepta-tyr glycopeptide, a glycopeptide belonging to the teicoplanin family, to resolve several compounds with pharmaceutical and environmental impact (69). Experimental parameters affecting resolution, such as antibiotic concentration, buffer pH, organic modifier type, and capillary temperature, were studied. The macrocyclic antibiotic A35512B was examined as a chiral selector for capillary electrophoresis using 13 racemic dansyl amino acids as test analytes (70). The chiral selectivity of A35512B was evaluated as a function of the run buffer pH, antibiotic concentration, and organic modifier composition. Miscellaneous. Enantioseparations have also been carried out using a number of miscellaneous chiral selectors. Haddadian et al. enantioresolved a number of cationic, anionic, and neutral analytes using two polymeric chiral surfactants (PDCSs) (poly(sodium N-undecanoyl isoleucylvalinate) (SUILV) and poly(sodium N-undecanoyl leucylvalinate) (SULV)) in electrokinetic capillary chromatography (EKC) and, after analyzing a number of parameters, determined that the enantiomeric resolutions were dependent on steric factors and not on the number of stereogenic centers (71). Chiral separations using various polymerized dipeptide surfactants in EKC were investigated, and it was determined the polymers of sodium N-undecylenyl-L-leucine-L-valine (poly(LSULV) provided the best enantioselectivity of the amino acid surfactants studied (72). Bile salt-mediated MEKC was investigated, and it was shown that optimum concentration of bile salts in chiral separations depended on both the aggregation properties of the surfactant and the stability of the analyte-micelle complexes (73). An equilibruim model was proposed in which these two effects were treated separately. Some of the other applications reported were as follows: Verleysen et al. enantioresolved isomeric R- and β-aspartyl dipeptides by CE using 18-crown-6tetracarboxylic acid and carboxymethyl-β-cyclodextrin (74); Lammerhofer and Linder resolved the enantiomers of chiral N-derivatized amino acids by packed capillary electrochromatography with a chiral anion-exchange type chiral stationary phase modified with quinine (75); a method was developed for the enantioseparation of N-protected chiral amino acids using quinine and tert-butylcarbamoylated quinine as chiral selectors in nonaqueous solutions (76); Krause et al. used polyacrylamide and polysaccharide derivatives (poly-N-acryloyl-1-phenylalanine ethyl ester (Chiraspher.RTM) and cellulose tris(3,5-dimethylphenylcarbamate) as chiral stationary phases for chiral separations in normal- and reversed-phase nano-HPLC and capillary electrochromatography (77); enantioseparations of neutral analytes in nonaqueous capillary electrochromatography and capillary HPLC were carried out by Krause et al. using helically chiral poly(diphenyl-2-pyridylmethyl methacrylate) (PDPM) as the chiral stationary phase (78); Ju and El Rassi performed chiral separations on charged chiral solutes using cyclohexyl-pentyl-β-D-maltoside (Cymal 5), a chiral glycosidic surfactant (GS) that produces neutral micelles (79); Analytical Chemistry, Vol. 72, No. 18, September 15, 2000
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fucose-containing glycosaminoglycan (FGAG) and depolymerized holothurian glycosaminoglycan (DHG) (80); pectins were investigated as chiral selective agents to resolve antihistaminic and antimalarial compounds (81); quail egg white riboflavin binding protein was used as a chiral selector in CE by applying a modified partial filling technique (82); heparin was used as a chiral additive for the separation of antihistamines and a dual mechanism involving both inclusion complexation and ionic interactions with heparin was thought to be responsible for the chiral recognition (83); pentosan polysulfate, a semisynthetic polysaccharide, was employed to separate 28 racemic analytes at low pH when the analytes were significantly protonated, indicating that ionic interactions are the dominant associative interactions (84); a variety of drugs were enantioresolved using dermatan sulfate, a complex, polydispersed, sulfate polysaccharide, as a chiral selector (85); an N-(2-hydroxyoctyl)-L-4-hydroxyproline/Cu(II) complex was used as a chiral selector in ligand exchange for R-amino acids (86). THIN-LAYER CHROMATOGRAPHY Recently thin-layer chromatographic plates utilizing a variety of chiral selectors have been used in enantioseparations. Usually enantiomers are directly separated on precoated or impregnated TLC plates, although other techniques such as molecular imprinting also are utilized. Deng et al. bonded β-cyclodextrins directly to silica gel and used this as a TLC chiral stationary phase (87). These TLC plates were used for the enantioseparation of 15 pairs of binaphthols and their derivatives. Silica gel plates impregnated with L-lysine and L-arginine were used to resolve some β-blocking agents by Bhushan and Thiongo (88). More adrenergic drugs were separated using molecularly imprinted polymers (MIP) in TLC by Suedee et al. (89). Molecular imprinting involves polymerization in situ in the presence of the imprint molecule. Removal of the imprint molecule leaves a spacially fixed cavity ideally shaped for the imprint molecule. In the preparation of the MIPs in this study, methacrylic acid or itaconic acid monomers were used to imprint (-)-pseudoephedrine or (-)-norephedrine for use as chiral stationary phases. These MIPs were able to enantioresolve several compounds whose structures corresponded to the imprinted molecule. SUPERCRITICAL FLUID CHROMATOGRAPHY AND RELATED TECHNIQUES Super- and subcritical fluid chromatography continues to find use in the chiral separations of thermally labile enantiomers and in seeking to improve resolution compared to HPLC. Wolf and Pirkle discussed the effects of temperature and different polar modifiers added to carbon dioxide mobile phases. Subcritical fluid chromatography and brush-type chiral stationary phases, the Whelk-O 1 and polyWhelk-O columns, were used by Wolf and Pirkle to achieve enantioseparations at low temperatures (90). Several racemates with low enantioselectivity at room temperature were resolved, as well as compounds with low configurational stability at ambient temperatures. Blackwell et al. derived an empirical relationship that related chiral selectivity and mobile-phase modifier properties for carbon dioxide and heptanebased mobile phases and Pirkle-type CSPs (91). Similar results 4524
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were obtained for a heptane-based mobile phase with a cellulosic CSP. Enhanced-fluidity liquid chromatography (EFLC), which uses liquified gases with polar liquids as the mobile phase, was used to resolve several compounds including warfarin (92). The column used was a commercially available macrocyclic antibiotic vancomycin column, Chirobiotic-V. EFLC was compared to HPLC and SFC for normal-phase separations, and separation obtained using EFLC showed the highest resolution. GAS CHROMATOGRAPHY Cyclodextrin-based gas chromatographic columns continue to be widely used for the direct resolution of enantiomers. Schneider and Ballschmiter used alkyl nitrates as chiral and achiral solute probes to investigate the enantioselectivity of a commercially available derivatized β-cyclodextrin column, Lipodex-D, and the structural contributions of the solutes affecting enantioresolution were discussed (93). Schmidt et al. used Lipodex-E, a derivatized γ-cyclodextrin capillary column, in the headspace gas chromatography/mass spectrometry analysis of isoflurane enantiomers in blood samples after anesthesia with the racemic mixture (94). Quattrini et al. studied the gas chromatographic enantioseparation of R-ionone with three different cyclodextrin-based CSPs, and the influence of the concentration of the chiral selector and the feasibility of preparative-scale separation was also discussed (95). In another cyclodextrin-based chiral stationary-phase application, enantiomers of C-3 secondary alcohols, 3-hydroxybutanoates, and cyclic 1,3-dithioacetals were separated by the gas chromatographic columns CP-Chirasil-Dex CB and Chiraldex G-TA, and the absolute configuration of the solutes was predicted from elution order and enzymatic resolution (96). Wiberg et al. investigated four different commercially available derivatized cyclodextrin columns for the enantioresolution of methylsulfonyl polychlorinated biphenyls (97). The columns studied were β- or γ-cyclodextrin columns, which were methylated and/or tert-butyldimethysilylated in the 2,3,6-O positions, and results were obtained using both ion trap mass spectrometry/mass spectrometry (MS/ MS) and electron capture negative ion low-resolution mass spectrometry. The energy of enantioselective interactions of 2-pentanol enantiomers separated on a permethylated β-cyclodextrin gas chromatographic column was calculated by Krupcik et al. (98). This procedure was based on retention data from gas chromatograms obtained by the separation of chiral 2-pentanol and achiral 1- and 3-pentanols on both nonchiral and chiral capillary columns. HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY Direct chiral separations using cyclodextrin-based and macrocyclic antiobiotic-based HPLC columns continue to be common, and enantioseparations on many novel chiral CSPs have also been reported. Polysaccharide, Pirkle-type, and crown ether-based CSPs also have wide use, as well as other commercially available chiral columns. Cyclodextrin CSPs. The enantioselectivity of native β-cyclodextrin and its 2,3-methylated analogue bonded CSPs were compared in both the polar organic and reversed-phase modes (99), and steric interactions and suggested mechanisms of chiral recognition were discussed. A phenyl carbamate-derivatized β-cyclodextrin column was prepared and characterized by Chen et al.
(100). This column was compared to a commercially available derivatized β-cyclodextrin column, and a chiral recognition mechanism was suggested for the normal-phase enantioseparations of some novel organophosphorus compounds. A β-cyclodextrin bonded CSP was used in the enantioseparation of several dipeptides, isoleucine, and a tripeptide using a polar-organic acetonitrile mobile phase and precolumn derivatization with tagging reagents (101). A commercially available β-cyclodextrin CSP was used in the determination of D- and L-modafinil in human plasma after liquid-liquid extraction (102). Native and methylated γ-cyclodextrins bonded CSPs were prepared and evaluated using dansyl amino acids as model solutes (103), and the effect of pH and mobile-phase composition was assessed and optimized for these separations. The enantioresolution of novel atropic R-,Rdisubstituted-β-amino acids was accomplished by direct LC separation on native and derivatized β-cyclodextrin bonded phases, as well as by indirect resolution with precolumn derivatization (104). Teicoplanin and Vancomycin CSPs. The macrocyclic antibiotics teicoplanin and vancomycin were used as chiral stationary phases for HPLC for a variety of separations of amino acids. Unusual amino acids (105) and unusual secondary aromatic amino acids (106) were enantioresolved using a teicoplanin chiral stationary phase for HPLC. The effect of temperature on retention of β-methyl amino acids on a teicoplanin CSP was investigated, and thermodynamic parameters were calculated (107). The unusual amino acids also were resolved in a GC system on a Chirasil-L-Val column (105). Protected amino acids were directly separated without further chemical modification using proteinbased and macrocyclic columns (108). Chiral columns used in this study were Chirobiotic T (teicoplanin), Ultron ovomucoid (OVM), and human serum albumin (HSA). A comparison of enantioresolution of amino acids and their N-tert-butyloxycarbonyl derivatives was made using a teicoplanin CSP (109), and the effect of organic modifier and triethylamine acetate buffer on the separations was studied. Lehotay et al. reported that enantioresolution of R-amino acids was achieved on both teicoplanin and vancomycin CSPs, with teicoplanin giving the better separation (110). A teicoplanin-based CSP was used for the determination of enantiomers of albuterol in plasma by Fried et al. (111). Albuterol and its metabolites were enantioresolved on a teicoplanin CSP in biological matrixes, with tandem mass spectrometry (112). Joyce et al. reported the enantioresolution of salbutamol and its metabolites on a teicoplanin CSP in biological matrixes, with tandem mass spectrometry (112). Vancomycin and teicoplanin CSPs were used in the enantioseparation of semisynthetic ergot alkaloids in reversed-phase HPLC (113), and the mechanism of interaction and separation parameters were discussed. The enantioseparation of 29 racemic aza-analogues of nifedipine-type dihydropyridine calcium channel modulators was evaluated on several different CSPs, including Chirobiotic T and V (teicoplanin and vancomycin), Chiralcel OD-H, ChiraDex, and Whelk-O 1 (114). A 1,2-diphenyl-1,2-diaminoethane-based CSP and two quinine carbamate-based chiral ion exchangers were also included in this study. Several cyclic imides including mephobarbital and thalidomide were enantioresolved on a vancomycin CSP, with both normal-
and reversed-phase conditions examined (115). D’Acquarica et al. achieved direct enantioresolution of carnitine and O-acylcarnitine enantiomers on a laboratory-made teicoplanin CSP (116). The enantiomers of citalopram and its demethylated metabolites were resolved on a commercial vancomycin column following liquid-liquid extraction (117). A vancomycin column was again used to demonstrate the use of EFLC for chiral separations (92). EFLC was compared to HPLC and SFC for normal-phase separations, and separation obtained using EFLC showed the highest resolution. Ristocetin A CSP. The macrocyclic antibiotic ristocetin A was evaluated as a CSP for HPLC by Ekborg-Ott et al. (118). This CSP was found to be complementary to the other macrocyclic antibiotic columns in that if a solute is partially resolved on a macrocyclic column, then a complete resolution is very likely on one of the other macrocyclic columns. The ristocetin A column was found to be extremely viable, with over 230 racemates resolved. The effect of selector coverage and mobile-phase composition on enantioseparations with the ristocetin A column was also studied (119). The macrocyclic antibiotic avoparcin was another new CSP evaluated and found to be complementary to other macrocyclic columns (120). Other Macrocyclic Phases. Ekborg-Ott reported on a new macrocyclic antibiotic CSP based on avoparcin and found it to be complementary to other macrocyclic columns (120). The macrocyclic antibiotic LY333328 was evaluated for use as a chiral mobilephase additive in narrow-bore HPLC for the enantioseparation of nine dansylated amino acids (121). LY333328 concentration, pH, and stationary-phase types were varied and the effects on enantioresolution studied. Polysaccharide CSPs. Polysaccharide-based stationary phases, whether commercially available or synthesized, continue to find use as CSPs in HPLC. Carbamate derivatives of cellulose or amylose were evaluated as potential CSPs, including structural contributions to the mechanism of chiral selectivity (122). β-Blockers were enantioresolved on an amylose tris(3,5-dimethoxyphenyl carbamate) chiral HPLC column, with validated methods developed for metoprolol and betaxolol (123). Another derivatized polysaccharide CSP was used to enantioresolve a chiral morphanthridine analogue. This method used amylose tris(3,5-dimethylphenyl carbamate) as a CSP (124). The first direct enantioseparation of gossypol was reported on a CSP consisting of cellulose tris(3,5-dimethylphenyl carbamate) coated onto microporous aminopropylsilica (125). Propranolol and several analogues were enantioresolved using two cellulose-type CSPs, Chiralcel OF and Chiralcel OC, and one amylose-type CSP, Chiralpak AD (126). The amylose column showed baseline resolution for all compounds studied, while the cellulose columns showed a significant decrease in enantioselectivity. The effects of the structure of the CSPs on enantioseparation was discussed. A Chiralpak AD column was used in the enantioseparation of trans-(-)-paroxetine in bulk pharmaceutical preparations (127) and in the enantioresolution of the antitumor agent SCH 66336 in Cynomolgus monkey plasma (128). Derivatized amylose CSPs, Chiralpak AS and Chiralpak AD, were used to enantioresolve melatonin ligands receptors (129). The effect of alcohol concentration in the normal mobile phase and substitution effects on enantioseparation were studied. A Analytical Chemistry, Vol. 72, No. 18, September 15, 2000
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Chiralcel OD-R cellulose column was used in the enantiomeric separation of tramadol and its active metabolite in human plasma (130), and the method was validated and used in a pharmacokinetic study. Polysaccharide CSPs used with polar organic solvents were used in the preparative enantioresolution of numerous pharmaceutical intermediates and final products (131). Sadler et al. developed a method for the separation of indenestrol A and B isomers and enantiomers using a Chiralcel OJ column (132). Peng et al. reported the separation and quantification of the enantiomers of two aminohydantoin compounds using polysaccharide-type CSPs (133). Ceccato et al. employed a Chiralcel OD-R CSP for the simultaneous determination of pirlindole enantiomers and dehydropirlindole (134). Derivatized cellulose columns were also used in the enantioresolution of ketoprofen from horse plasma (135), tiagabine (136), and trimipramine in human serum (137). Crown Ether-Based CSPs. Crown ether-based CSPs, both commercially available and newly synthesized, continue to see use in the area of enantioseparations. Van Dort reported the direct enantioresolution of the four stereoisomers of m-hydroxyphenylpropanolamine on a Crownpak CR(+) column (138). The separation of hydrophobic amino compounds was investigated using a Crownpak CR(+) column with the addition of crown ether, cyclodextrins, cations, etc., to the mobile phase to create a dynamically coated CSP (139). A new crown ether-based CSP was prepared by Ho-Hyun et al. and used to enantioresolve various natural and unnatural R-amino acids and their derivatives (140). Another novel crown ether-based column was prepared by binding optically active di-tert-butylpyridino-18-crown-6 ligand to silica gel (141). This column was used to enantioresolve four selected racemic organic ammonium perchlorates. Various Other CSPs. Various additional references to other CSPs in HPLC exist in the literature, with many involving unique applications using commercially available columns and others involving the preparation of novel CSPs. A quick survey of some of these papers follows. Abu-Lafi and Turujman tested the reproducibility of the separation of astaxanthin stereoisomers on commercially available Pirkle brush-type covalent L-leucine CSPs obtained from different manufacturers and found significant differences (142). Brush-type chiral selectors for new CSPs were synthesized from (R)-1naphthylethylamine bound to 2,4,5,6-tetrachloro-1,3-dicyanobenzene and used to resolve 23 racemic analytes (143). A brushtype CSP based on L-(3,5-dinitrobenzoyl)leucine was used to screen 108 4-aryl-1,4-dihydropyrimidine enantiomers as potential chiral selectors (144), with the lead compound used in a semipreparative chiral column for the separation of several π-acidic compounds. A series of homoisoflavonoids that showed in vitro anti-rhinovirus activity were enantioresolved using a brush-type stationary phase, the Whelk 01 column, as the CSP in HPLC (145). Malyshev and Vinogradov reported a convenient method for incolumn synthesis of π-acceptor CSPs from halogen-substituted 3,5-dinitrobenzoylamides (146). Sotalol was the subject of a temperature study using a immobilized cellobiohydrolase I CSP. Sotalol was found to undergo a temperature-induced inversion of elution order (147). Various pharmaceuticals including thalidomide, glutethimide, primaquine, and others were also enantioresolved on a commercially available avidin protein column (148). Brandsteterova and Wainer described 4526
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a rapid and simple enantioseparation of verapamil and its metabolites in serum samples using an R-1-AGP column (149). Olsen et al. reported the determination of the enantiomeric excess of fluoxetine hydrochloride on an ovomucoid CSP (150). A series of homoisoflavonoids that showed in vitro anti-rhinovirus activity were enantioresolved using a brush-type stationary phase, the Whelk 01 column, as the CSP in HPLC (145). An R-chymotrypsin CSP was synthesized using an in situ immobilization process on an epoxide-derivatized silica gel column, and this CSP was then used to enantioresolve several protected amino acids and other racemates (151). Three chitin phenyl carbamate derivatives were prepared and their usefulness as CSPs was evaluated by Yamamoto et al. (152). One of the chitin CSPs showed high chiral recognition ability for some chiral acidic drugs such as ibuprofen and ketoprofen. Porous graphitic carbon was used in the development of a capillary liquid chromatography CSP based on lasalocid (153). Several amines, acids, amino acids, and alcohols were enantioresolved, and the influence of the solute structure on separation was studied. The chromatographic system was found to be stable and reproducible. A terguride-based CSP was characterized and used in the enantioseparation of profens (154) and chrysanthemic acid and analogous compounds (155). Molecular basket phases as CSPs were synthesized by the immobilization of triethoxysilyl derivatives of calixarenes and used to enantioresolve aromatic and nonaromatic solutes (156). Bile acids were utilized as a CSP to achieve the separation of dansyl amino acids and other enantiomers (157). Brush-type chiral selectors for new CSPs were synthesized from (R)-1-naphthylethylamine bound to 2,4,5,6-tetrachloro-1,3-dicyanobenzene and used to resolve 23 racemic analytes (143). Chiral R- and β-aminophosphonic acids were enantioresolved in a direct LC method using quinine-derived chiral anion exchangers (158). Enantiomeric excess and absolute configurations were also determined and found to correlate well with previous results obtained by 31P NMR spectroscopy. Uniformly sized, molecular imprinted chiral stationary phases for HPLC were prepared by a two-step swelling technique using chiral amides derived from (S)-R-methylbenzylamine as template molecules. The cross-linked polymer network memorized the shape of the chiral template to provide chiral resolution. Also, CSPs with multichiral selectors were prepared using both molecular imprinting and in situ surface modification (159). MIPs with high chiral selectivity for N-R-protected amino acids were synthesized, and the factors influencing the chiral selectivity were investigated (160). Also in this reference, the number of binding sites on MIPs and dissociation constants for the enantiomerMIP complex were detected by frontal chromatography analysis. Other Related Chromatographic Methods. High-speed countercurrent chromatography was used to determine binding constants for a set of dinitrobenzoyl amino acids (161). The chiral selector used was N-dodecanoyl-L-proline-3,5-dimethylanilide. This method could be useful in the design of effective chiral selectors and determination of the mechanism of enantioselectivity. A helically self-assembled chelate as a chiral mobile-phase additive was used in the separation of labetalol stereoisomers (162). An analogy of the biomimetic properties of this system with a helically
moving domain model of G-protein-coupled receptor family was presented. ACKNOWLEDGMENT
The author gratefully acknowledges the support by the National Institutes of Health (Grant R15 AI41182) and Millsaps College. Thanks to Brad Farris and Colette Rabai for their gracious and valuable help. Timothy J. Ward, associate professor of chemistry and chair of the Chemistry Department at Millsaps College (Jackson, MS 39210), received his B.S. degree from the University of Florida and his Ph.D. from Texas Tech University. He joined Millsaps College in 1990, after working at Syntex in the pharmaceutical process control division. His research interests include chiral separations and the characterization of enantiomeric resolution, the development of analytical LC and CE methods, and their application to pharmaceutical and environmental separations.
LITERATURE CITED (1) Verleysen, K.; Sandra, P. Electrophoresis 1998, 19, 2798-833. (2) Vespalec, R.; Bocek, P. Electrophoresis 1999, 20, 2579-91. (3) Fanali, S.; Aturki, Z.; Desiderio, C. Enantiomer 1999, 4, 22941. (4) Desiderio, C.; Fanali, S. J. Chromatogr., A 1998, 807, 37-56. (5) Bojarski, J.; Aboul-Enein, H. Y. Biomed. Chromatogr. 1999, 13, 197-208. (6) Chankvetadze, B. TrAC, Trends Anal. Chem. 1999, 18, 48598. (7) Hui, F.; Caude, M. Analusis 1999, 27, 131-7. (8) Kuhn, R. Electrophoresis 1999, 20, 2605-13. (9) Vigh, G.; Sokolowski, A. D. Electrophoresis 1997, 18, 2305-10. (10) Schurig, V.; Wistuba, D. Electrophoresis 1999, 20, 2313-28. (11) Hatajik, T. D.; Brown, P. R. J. Capillary Electrophor. 1998, 5, 143-51. (12) Hage, D. S. Electrophoresis 1997, 18, 2311-21. (13) Lloyd, D. K.; Aubry, A. F.; De Lorenzi, E. J. Chromatogr., A 1997, 792, 349-69. (14) Camilleri, P. Electrophoresis 1997, 18, 2322-30. (15) Riekkola, M. L.; Wiedmer, S. K.; Valko, I. E.; Siren, H. J. Chromatogr., A 1997, 792, 13-35. (16) Nishi, H. J. Chromatogr., A 1997, 792, 327-47. (17) Aboul-Enein, H. Y.; El-Awady, M. I.; Heard, C. M.; Nicholls, P. J. Biomed. Chromatogr. 1999, 13, 531-7. (18) Wolf, C.; Pirkle, W. H. LC-GC 1997, 15, 352, 354-61, 363. (19) Allenmark, S.; Schurig, V. J. Mater. Chem. 1997, 7, 1955-63. (20) Bicchi, C.; D’Amato, A.; Rubiolo, P. J. Chromatogr., A 1999, 843, 99-121. (21) Bojarski, J. Chem. Anal. 1997, 42, 139-85. (22) Deng, Y.; Maruyama, W.; Kawai, M.; Dostert, P.; Naoi, M. Prog. HPLC-HPCE 1997, 6, 301-38. (23) Wang, F.; Khaledi, M. G. J. Chromatogr., B.: Biomed. Sci. Appl. 1999, 73, 187-97. (24) Izumoto, S.; Nishi, H. Electrophoresis 1999, 20, 189-97. (25) Wedig, M.; Holzgrabe, U. Electrophoresis 1999, 20, 1555-60. (26) Skanchy, D. J.; Xie, G.-H.; Tait, R. J.; Luna, E.; Demarest, C.; Stobaugh, J. F. Electrophoresis 1999, 20, 2638-49. (27) Gratz, S. R.; Stalcup, A. M. Anal. Chem 1998, 70, 5166-71. (28) Owens, P. K.; Fell, A. F.; Coleman, M. W.; Berridge, J. C. Enantiomer 1999, 4, 79-90. (29) Tamisier-Karolak, S. L.; Stenger, M.-A.; Bommart, A. Electrophoresis 1999, 20, 2656-63. (30) O’Keefe, F.; Shamsi, S. A.; Darcy, R.; Schwinte, P.; Warner, I. M. Anal. Chem. 1997, 69, 4773-82. (31) Haynes, J. L. I.; Shamsi, S. A.; O’Keefe, F.; Darcey, R.; Warner, I. M. J. Chromatogr., A 1998, 803, 261-7. (32) Wang, F.; Khaledi, M. G. J. Chromatogr., A 1998, 817, 121-8. (33) Wang, F.; Khaledi, M. G. Electrophoresis 1998, 19, 2095-100. (34) Rudaz, S.; Veuthey, J. L.; Desiderio, C.; Fanali, S. Chromatographia 1999, 50, 369-72. (35) Rudaz, S.; Veuthey, J. L.; Desiderio, C.; Fanali, S. Electrophoresis 1998, 19, 2883-9. (36) Salvador, A.; Varesio, E.; Dreux, M.; Veuthey, J. L. Electrophoresis 1999, 20, 2670-9. (37) Otsuka, K.; Honda, S.; Kato, J.; Terabe, S.; Kimata, K.; Tanaka, N. J. Pharm. Biomed. Anal. 1998, 17, 1177-90. (38) Chankvetadze, B.; Burjanadze, N.; Bergenthal, D.; Blaschke, G. Electrophoresis 1999, 20, 2680-5. (39) Wistuba, D.; Czesla, H.; Roeder, M.; Schurig, V. J. Chromatogr., A 1998, 815, 183-8. (40) Wistuba, D.; Schurig, V. Electrophoresis 1999, 20, 2779-85. (41) Jakubetz, H.; Juza, M.; Schurig, V. Electrophoresis 1998, 19, 738-44. (42) Amini, A.; Javerfalk, E.; Bastami, S.; Westerlund, D. Electrophoresis 1999, 20, 204-11. (43) Tacker, M.; Glukhovskiy, P.; Cai, H.; Vigh, G. Electrophoresis 1999, 20, 2794-8. (44) Cai, H.; Vigh, G. J. Pharm. Biomed. Anal. 1998, 18, 615-21.
(45) Tsunoda, M.; Kato, M.; Fukushima, T.; Santa, T.; Homma, H.; Yanai, H.; Soga, T.; Imai, K. Biomed. Chromatogr. 1999, 13, 335-9. (46) Esquisabel, A.; Hernandez, R. M.; Gascon, A. R.; Igartua, M.; Calvo, B.; Pedraz, J. L. J. Pharm. Biomed. Anal. 1997, 16, 35766. (47) Zhu, X.; Lin, B.; Jakob, A.; Wuerthner, S.; Koppenhoefer, B. Electrophoresis 1999, 20, 1878-89. (48) Amini, A.; Merclin, N.; Bastami, S.; Westerlund, D. Electrophoresis 1999, 20, 180-8. (49) Mey, B.; Paulus, H.; Lamparter, E.; Blaschke, G. Chirality 1999, 11, 772-80. (50) Zaugg, S.; Caslavska, J.; Theurillat, R.; Thormann, W. J. Chromatogr., A 1999, 838, 237-49. (51) Pak, C.; Marriott, P. J.; Carpenter, P. D.; Amiet, R. G. J. Chromatogr., A 1998, 793, 357-64. (52) Ban, E.; Choi, S.; Lee, J. A.; Lho, D. S.; Yoo, Y. S. J. Chromatogr., A 1999, 853, 439-47. (53) Pesek, J. J.; Matyska, M. T.; Menezes, S. J. Chromatogr., A. 1999, 853, 151-8. (54) Yang, L. L.; Yuan, Z. B. Fresenius' J. Anal. Chem. 1999, 365, 541-4. (55) Ma, L. Y.; Han, J. H.; Wang, H.; Gu, J. L.; Fu, R. N. Electrophoresis 1999, 20, 1900-3. (56) Meyring, M.; Chankvetadze, B.; Blaschke, G. Electrophoresis 1999, 20, 2425-31. (57) Li, J.; Waldron, K. C. Electrophoresis 1999, 20, 171-9. (58) Stalcup, A. M.; Gahm, K. H.; Gratz, S. R.; Sutton, R. M. Anal. Chem. 1998, 70, 144-8. (59) Kaniansky, D.; Simunicova, E.; Olvecka, E.; Ferancova, A. Electrophoresis 1999, 20, 2786-93. (60) Azevedo, D. C.; Pais, L. S.; Rodrigues, A. E. J. Chromatogr., A 1999, 865, 187-200. (61) Liu, L.; Nussbaum, M. A. J. Pharm. Biomed. Anal. 1999, 19, 679-94. (62) Nussbaum, M. A. Electrophoresis 1999, 20, 2664-9. (63) Fillet, M.; Chankvetadze, B.; Crommen, J.; Blaschke, G. Electrophoresis 1999, 20, 2691-7. (64) Deng, Y.; Zhang, J.; Tsuda, T.; Yu, P. H.; Boulton, A. A.; Cassidy, R. M. Anal. Chem. 1998, 70, 4586-93. (65) Tan, Z. J.; Remcho, V. T. Electrophoresis 1998, 19, 2055-60. (66) Ekborg-Ott, H.; Zientara, G. A.; Schneiderheinze, J. M.; Gahm, K.; Armstrong, D. W. Electrophoresis 1999, 20, 2438-57. (67) Fanali, S.; Desiderio, C.; Schulte, G.; Heitmeier, S.; Strickmann, D.; Chankvedatze, B.; Blaschke, G. J. Chromatogr., A 1998, 800, 69-76. (68) Carter-Finch, A. S.; Smith, N. W. J. Chromatogr., A 1999, 848, 375-85. (69) Fanali, S.; Aturki, Z.; Desiderio, C.; Bossi, A.; Righetti, P. G. Electrophoresis 1998, 19, 1742-51. (70) Risley, D. S.; Trelli-Seifert, L.; McKenzie, Q. J. Electrophoresis 1999, 20, 2749-53. (71) Haddadian, F.; Billiot, E. J.; Shamsi, S. A.; Warner, I. M. J. Chromatogr., A. 1999, 858, 219-227. (72) Billiot, E.; Macossay, J.; Thibodeaux, S.; Shamsi, S. A.; Warner, I. M. Anal. Chem. 1998, 70, 1375-81. (73) Szoko, E.; Gyimesi, J.; Szakacs, Z.; Tarnai, M. Electrophoresis 1999, 20, 2754-60. (74) Verleysen, K.; Sabah, S.; Scriba, G.; Sandra, P. Chromatographia 1999, 49, 215-8. (75) Lammerhofer, M.; Lindner, W. J. Chromatogr., A. 1998, 829, 115-25. (76) Piette, V.; Lammerhofer, M.; Lindner, W.; Crommen, J. Chirality 1999, 11, 622-30. (77) Krause, K.; Girod, M.; Chankvetadze, B.; Blaschke, G. J. Chromatogr., A. 1999, 837, 51-63. (78) Krause, K.; Chankvetadze, B.; Okamoto, Y.; Blaschke, G. Electrophoresis 1999, 20, 2772-8. (79) Ju, M.; Rassi, Z. E. Electrophoresis 1999, 20, 2766-71. (80) Tsukamoto, T.; Ushio, T.; Haginaka, J. J. Chromatogr., A 1999, 864, 163-71. (81) Phinney, K. W.; Jinadu, L. A.; Sander, L. C. J. Chromatogr., A 1999, 857, 285-93. (82) De Lorenzi, E.; Massolini, G.; Quaglia, M.; Galbusera, C.; Caccialanza, G. Electrophoresis 1999, 20, 2739-48. (83) Jin, Y.; Stalcup, A. M. Electrophoresis 1998, 19, 2119-23. (84) Wang, X.; Lee, J. T.; Armstrong, D. W. Electrophoresis 1999, 20, 162-70. (85) Gotti, R.; Cavrini, V.; Andrisano, V.; Mascellani, G. J. Chromatogr., A 1998, 814, 205-11. (86) Vegvari, A.; Schmid, M. G.; Kilar, F.; Gubitz, G. Electrophoresis 1998, 19, 2109-12. (87) Deng, Q.; Kang, S.; Zhu, Q.; Xu, Z. Am. Lab (Shelton, Conn.) 1999, 31, 43-4, 46-7. (88) Bhushan, R.; Thiongo, G. T. J. Chromatogr., B 1998, 708, 3304. (89) Suedee, R.; Songkram, C.; Petmoreekul, A.; Snagkunakup, S.; Sankasa, S.; Kongyarit, N. J. Pharm. Biomed. Anal. 1999, 19, 519-27. (90) Wolf, C.; Pirkle, W. H. J. Chromatogr., A 1997, 785, 173-8. (91) Blackwell, J. A.; Stringham, R. W.; Xiang, D.; Waltermire, R. E. J. Chromatogr., A 1999, 852, 383-94. (92) Sun, Q.; Olesik, S. V. Anal. Chem. 1999, 71, 2139-45.
Analytical Chemistry, Vol. 72, No. 18, September 15, 2000
4527
(93) Schneider, M.; Ballschmiter, K. J. Chromatogr., A 1999, 852, 525-34. (94) Schmidt, R.; Wahl, H. G.; Haberle, H.; Dieterich, H. J.; Schurig, V. Chirality 1999, 11, 206-11. (95) Quattrini, F.; Biressi, G.; Juza, M.; Mazzotti, M.; Fuganti, C.; Morbidelli, M. J. Chromatogr., A 1999, 865, 201-10. (96) Hoff, B. H.; Anthonsen, T. Chirality 1999, 11, 760-7. (97) Wiberg, K.; Letcher, R.; Sandau, C.; Duffe, J.; Norstrom, R.; Haglund, P.; Bidleman, T. Anal. Chem. 1998, 70, 3845-52. (98) Krupcik, J.; Spanik, I.; Oswald, P.; Skacani, I.; Onuska, F. I.; Sandra, P. J. High Resolut. Chromatogr. 1998, 21, 197-9. (99) Armstrong, D. W.; Chang, L. W.; Chang, S. C.; Wang, X.; Ibrahim, H.; Reid, G. R., III; Beesley, T. E. J. Liq. Chromatogr. Relat. Technol. 1997, 20, 3279-95. (100) Chen, H.; Lu, X.-Y.; Gao, R.-Y.; Wang, Q.-S. Chin. J. Chem. 1999, 17, 664-9. (101) Chen, S. J. Chin. Chem. Soc. 1999, 46, 239-44. (102) Gorman, S. H. J. Chromatogr., B: Biomed. Sci. Appl. 1999, 730, 1-7. (103) Araki, T.; Kashiwamoto, Y.; Tsunoi, S.; Tanaka, M. J. Chromatogr., A 1999, 945, 455-61. (104) Toro, G.; Peter, A.; Gaucher, A.; Wakselman, M.; Mazaleyrat, J.-P.; Armstrong, D. W. J. Chromatogr., A 1998, 846, 83-91. (105) Peter, A.; Torok, G.; Armstrong, D. W. J. Chromatogr., A 1998, 793, 283-96. (106) Peter, A.; Torok, G.; Toth, G.; Nest, W. V. d.; Laus, G.; Tourwe, D.; Armstrong, D. W. Chromatographia 1998, 48, 53-8. (107) Peter, A.; Torok, G.; Armstrong, D. W.; Toth, G.; Tourwe, D. J. Chromatogr., A 1998, 828, 177-90. (108) Esquivel, J. B.; Sanchez, C.; Fazio, M. J. J. Liq. Chromatogr. Relat. Technol. 1998, 21, 777-91. (109) Tesarova, E.; Bosakova, Z.; Pacakova, V. J. Chromatogr., A 1999, 838, 121-9. (110) Lehotay, J.; Hrobonova, K.; Krupcik, J.; Cizmarik, J. Pharmazie 1998, 53, 863-5. (111) Fried, K. M.; Koch, P.; Wainer, I. W. Chirality 1998, 10, 48491. (112) Joyce, K. B.; Jones, A. E.; Scott, R. J.; Biddlecombe, R. A.; Pleasance, S. Rapid Commun. Mass Spectrom. 1998, 12, 1899910. (113) Tesarova, E.; Zaruba, K.; Flieger, M. J. Chromatogr., A 1999, 844, 137-47. (114) Kleidernigg, O. P.; Kappe, O. C. Tetrahedron: Asymmetry 1997, 8, 2057-67. (115) Aboul-Enein, H. Y.; Serignese, V. Chirality 1998, 10, 358-61. (116) D′Acquarica, I.; Gasparrini, F.; Misiti, D.; Villani, C.; Carotti, A.; Cellamare, S.; Muck, S. J. Chromatogr., A 1999, 857, 145-55. (117) Kosel, M.; Eap, C. B.; Amey, M.; Baumann, P. J. Chromatogr., B: Biomed. Sci. Appl. 1998, 719, 234-8. (118) Ekborg-Ott, K. H.; Liu, Y.; Armstrong, D. W. Chirality 1998, 10, 434-83. (119) Ekborg-Ott, K. H.; Wang, X.; Armstrong, D. W. Microchem. J. 1999, 62, 26-49. (120) Ekborg-Ott, K. H.; Kullman, J. P.; Wang, X.; Gahm, K.; He, L.; Armstrong, D. W. Chirality 1998, 10, 627-60. (121) Sharp, V. S.; Risley, D. S. Chirality 1999, 11, 75-81. (122) Okamoto, Y.; Yashima, E.; Yamamoto, C. Polym. Prepr. 1999, 40, 503. (123) Cass, Q. B.; Tiritan, M. E.; Calafatti, S. A.; Matlin, S. A. J. Liq. Chromatogr. 1999, 22, 3091-9. (124) Kuesters, E.; Nozulak, J. Chromatographia 1998, 47, 440-2. (125) Cass, Q. B.; Bassi, A. L.; Matlin, S. A. Chirality 1999, 11, 469. (126) Aboul-Enein, H. Y.; Bakr, S. A. J. Liq. Chromatogr. Relat. Technol. 1998, 21, 1137-45. (127) Ferretti, R.; Gallinella, B.; La Torre, F.; Turchetto, L. J. Chromatogr., B: Biomed. Sci. Appl. 1998, 710, 157-64. (128) Kim, H.; Likhari, P.; Lin, C. C.; Nomeir, A. A. J. Chromatogr., B: Biomed. Sci. Appl. 1999, 728, 133-41. (129) Vaccher, C.; Fourmaintraux, E.; Belloli, E.; Vaccher, M. P.; Bonte, J. P. Chromatographia 1998, 48, 790-6.
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(130) Campanero, M. A.; Calahorra, B.; Valle, M.; Troconiz, I. F.; Honorato, J. Chirality 1999, 11, 272-9. (131) Miller, L.; Orihuela, C.; Fronek, R.; Murphy, J. J. Chromatogr., A 1999, 865, 211-26. (132) Sadler, B. R.; Chae, K.; Ishaq, K. S.; Korach, K. S. J. Chromatogr., A 1998, 799, 117-24. (133) Peng, S. X.; Henson, C.; Wilson, L. J. J. Chromatogr., B: Biomed. Sci. Appl. 1999, 732, 31-7. (134) Ceccato, A.; Hubert, P.; de Tullio, P.; Liegeois, J. F.; Stachow, M.; Geczy, J.; Crommen, J. J. Pharm. Biomed. Anal. 1998, 17, 1071-9. (135) Aboul-Enein, H. Y.; Van Overbeke, A.; Vander Weken, G.; Baeyens, W.; Oda, H.; Deprez, P.; De Kruif, A. J. Pharm. Pharmacol. 1998, 50, 291-6. (136) Rustum, A. M.; Estrada, V. J. Chromatogr., B: Biomed. Sci. Appl. 1998, 705, 111-7. (137) Liu, J.; Stewart, J. T. J. Chromatogr., B: Biomed. Sci. Appl. 1997, 700, 175-82. (138) Van Dort, M. E. Chirality 1999, 11, 684-8. (139) Machida, Y.; Nishi, H.; Nakamura, K. J. Chromatogr., A 1999, 830, 311-20. (140) Ho-Hyun, M.; Sung-Jin, J.; Lee, W. J. Chromatogr., A 1998, 822, 155-61. (141) Kontos, Z.; Huszthy, P.; Bradshaw, J. S.; Izatt, R. M. Tetrahedron: Asymmetry 1999, 10, 2087-99. (142) Abu-Lafi, S.; Turujman, S. A. J. Chromatogr., A 1999, 855, 15770. (143) Kontrec, D.; Vinkovic, V.; Sunjic, V. Chirality 1999, 11, 72230. (144) Lewandowski, K.; Murer, P.; Svec, F.; Frechet, J. M. J. Combinatorial Chem. 1999, 1, 105-12. (145) Quaglia, M. G.; Desideri, N.; Bossu, E.; Sgro, R.; Conti, C. Chirality 1999, 11, 495-500. (146) Malyshev, O. R.; Vinogradov, M. G. J. Chromatogr., A 1999, 859, 143-51. (147) Fulde, K.; Frahm, A. W. J. Chromatography 1999, 858, 33-43. (148) Haque, A.; Stewart, J. T. J. Liq. Chromatogr. Relat. Technol. 1998, 21, 2675-87. (149) Brandsteterova, E.; Wainer, I. W. J. Chromatogr., B: Biomed. Sci. Appl. 1999, 732, 395-404. (150) Olsen, B. A.; Wirth, D. D.; Larew, J. S. J. Pharm. Biomed. Anal. 1998, 17, 623-30. (151) Felix, G.; Descorps, V. Chromatographia 1999, 49, 595-605. (152) Yamamoto, C.; Hayashi, T.; Okamoto, Y.; Kobayashi, S. Chem. Lett. 2000, 1, 12-3. (153) Sandberg, A.; Markides, K. E.; Heldin, E. J. Chromatogr., A 1998, 828, 149-56. (154) Olsovska, J.; Flieger, M.; Bachechi, F.; Messina, A.; Sinibaldi, M. Chirality 1999, 11, 291-300. (155) Dondi, M.; Flieger, M.; Olsovska, J.; Polcaro, C. M.; Sinibaldi, M. J. Chromatogr., A 1999, 859, 133-42. (156) Healy, L. O.; Mcenery, M. M.; Mccarthy, D. G.; Harris, S. J.; Glennon, J. D. Anal. Lett. 1998, 31, 1543-51. (157) Takeuchi, T.; Chu, J.; Miwa, T. Chromatographia 1998, 47, 18388. (158) Zarbl, E.; Lammerhofer, M.; Hammerschmidt, F.; Wuggenig, F.; Hanbauer, M.; Maier, N. M.; Sajovic, L.; Lindner, W. Anal. Chim. Acta 2000, 404, 169-77. (159) Hosoya, K.; Tanaka, N. ACS Symp. Ser. 1998, No. 703, 14358. (160) Meng, Z.; Zhou, L.; Wang, J.; Wang, Q.; Zhu, D. Biomed. Chromatogr. 1999, 13, 389-93. (161) Ma, Y.; Ito, Y.; Berthod, A. J. Liq. Chromatogr. Relat. Technol. 1999, 22, 2945-55. (162) Bazylak, G.; Aboul-Enein, H. Y. J. Liq. Chromatogr. 1999, 22, 1171-92.
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