Chiral Separations - ACS Publications - American Chemical Society

Anal. Chem. 2008, 80, 4363–4372

Chiral Separations Timothy J. Ward* and Beth Ann Baker Millsaps College 1701 North State Street, Box 150306, Jackson, Mississippi 39210 Review Contents Reviews Capillary Electrophoresis (CE) High-Performance Liquid Chromatography (HPLC) Countercurrent Chromatography (CCC) Preparative Chiral Chromatography Ionic Liquids Chiral Stationary Phases (CSPs) Applications Miscellaneous High-Performance Liquid Chromatography Polysaccharide CSPs Macrocyclic Antibiotic CSPs Cyclodextrin CSPs and Mobile Phase Additives Ligand Exchange CSPs Protein-Based CSPs Miscellaneous Capillary Electrophoresis Cyclodextrins Micelles Ligand Exchange Miscellaneous Capillary Electrochromatography Gas Chromatography Supercritical Fluid Chromatography Miscellaneous Techniques Literature Cited

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This fundamental review article covers developments and applications in chiral separations from January 2006 to January 2008 and is restricted to the English language literature. Chiral separations continue to increase in their importance due to their broad range of applications in drug development in pharmaceutical industries, pesticide development in agricultural industries, and preparation of additives in the food industry, to name a few. The ability to understand separation techniques as well as chiral recognition models is useful to their application in both the analytical and preparative scale. The increase in the number of reviews published in the past few years as compared to previous years indicates a mastery of the techniques that are commonly used in chiral separations. While the field continues to progress in the development of new chiral phases and better mechanistic understanding, many of the techniques that are used remain essentially unchanged. However, while the basic techniques are very similar, the expansion of those techniques to include a broader range of applications is noteworthy. Noteworthy growth in the field of chiral separation techniques include the increased use of ionic liquids to achieve chiral separation and its extension to countercurrent chromatography. The use of ionic liquids appears to be widely applicable across * To whom correspondence should be addressed. Phone: (601) 974-1405. Fax: 601) 974-1401. E-mail: [email protected]. 10.1021/ac800662y CCC: $40.75  2008 American Chemical Society Published on Web 05/16/2008

various chromatographic fields, including HPLC, SFC, and GC. Also, publications covering preparative chiral separations have continued to increase over the past few years, indicating a broad acceptance and mastery of the techniques applied. REVIEWS Capillary Electrophoresis (CE). Capillary electrophoresis continues to enjoy widespread use with respect to the number of applications as well as development of novel techniques. CE is also widely applicable to pharmaceutical analysis, including analysis of chiral drugs as discussed by Suntornsuk (1) and Ali et al. (2). Robb (3) reviewed the application of physically adsorbed polymer coatings in chiral CE. Van Eeckhaut and Michotte (4) reviewed different classes of CE chiral selectors, describing mechanisms of separations, new chiral selectors, and pharmaceutical and biomedical applications. Guebitz and Schmid (5) reviewed developments in CZE, EKC, and CEC discussing electromigration techniques and new methodological developments, as well as chiral selectors and stationary phases specific to CEC. Developments in CEC were further reviewed by Preinerstorfer and Laemmerhofer (6) who summarized separation methods, discussing chiral mobile phase additives as well as various chiral stationary phases. Okanda and El Rassi (7) reviewed affinity nano-LC and affinity CEC performed in capillaries and microchips, investigating biospecific interactions. Chiral polymers and their separations, as well as the development of polymeric pseudostationary phases (PSPs) and their application for the combination of EKC with MS detection were reviewed by Christopher Palmer (8). Kahle and Foley (9) reviewed chiral EKC, discussing mechanisms and applications of pseudostationary phases and chiral selectors, with particular emphasis on chiral microemulsion EKC. Applications of chiral microemulsion EKC were further reviewed by McEvoy et al. (10) and Otsuka (11). High-Performance Liquid Chromatography (HPLC). A review of the separation of enantiomers by direct chromatographic separation using chiral stationary phases in HPLC was discussed by Mericko, Lehotay, and Cizmarik (12). Ali and Aboul-Enein Hassan (13, 14) reviewed the impact of immobilized polysaccharide CSPs on separations, discussing methods of immobilization, applications, enantioselectivies, efficiencies, and also compared their chiral recognition abilities to coated CSPs. Choi and Hyun (15) reviewed LC separations using chiral crown ethers as CSPs, discussing the effect of organic, acidic, and inorganic modifiers in both aqueous and nonaqueous mobile phases. Ali, Kumerer, and Aboul-Enein (16) reviewed various chiral selectors, discussing complex formation and interactions responsible for chiral resolution in both CE and HPLC. The relationships Analytical Chemistry, Vol. 80, No. 12, June 15, 2008

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between molecular structure, thermodynamic effects, and results of chiral surface self-organization were discussed by Paci, Szleifer, and Ratner (17). Kuksis and Itabashi (18) reviewed the LC-MS analysis of chiral hydroxy fatty acids, eicosanoids, acylglycerols, and glycerophospholipids. Ali et al. (19) reviewed HPLC as a tool to determine enantiorecognition trends affecting the pharmacological selectivity of β-adrenergics and β-adrenolytics and to enhance the efficiency of β-adrenoceptor-targeted therapies. Countercurrent Chromatography (CCC). Pan and Lu (20) reviewed progress in countercurrent chromatography as a chiral separation technique, including discussion of CCC methods, applications, and efficient chiral selectors. CCC was also reviewed by Perez and Minguillon (21) who discussed the development of a chiral environment by addition of a chiral selector. Preparative Chiral Chromatography. Francotte (22) and Abel (23) reviewed preparative scale chiral separations; Francotte focused on preparative CSPs and polymeric phases and preparation strategies, while Abel focused on preparative chromatographic processes such as batch elution chromatography and SFC. Andersson (24) also reviewed preparative chiral chromatography, discussing enantiomeric resolution, chromatographic processes, and citing examples of enantiomeric separations. Ionic Liquids. Ionic liquids have gained popularity in recent years as chiral separation media for GC, LC, and CE. Shamsi and Danielson (25) published a comprehensive review, tracing the use and development of ionic liquids from the mid 1980s to early 2007, showing their use as stationary phases in GC and as mobile phase additives in LC and CE. Chiral Stationary Phases (CSPs). Chen et al. (26) and Ikai et al. (27) reviewed polysaccharide CSPs including ester and carbamate derivatives and their use in both analytical and preparative HPLC. Both groups discussed the need to immobilize polysaccharide derivatives onto silica gel due to solvent limitations and methods by which this immobilization can be achieved. Macrocyclic antibiotics as a class of chiral selectors were reviewed by Ilisz et al. (28) , including characterization of the physicochemical properties of the antibiotics and their applications to enantiomeric separations. Further discussion included the mechanism of separation, elution order, and its relation to absolute configuration of the stereoisomers. Macrocyclic antibiotics were further reviewed by Beesley and Lee (29) who discussed the functional groups responsible for chiral recognition and the mechanism of interaction. Muderawan et al. (30) reviewed 16 derivatized cyclodextrin CSPs, including their synthesis, preparation, and application to enantioseparation of various compounds. Molecularly imprinted polymers were also reviewed as a chiral stationary phase (31), including discussion of applications, separation techniques, and molecular sensing. Applications. Nguyen et al. (32) published a comprehensive review of the nomenclature, pharmacology, toxicology, pharmokinetics, metabolism, and mechanisms of chiral drugs, including discussion on techniques for chiral separation. The role of enantioselective analysis in drug development and chiral drug separation techniques and mechanisms were reviewed by Maier and Lindner (33). Guebitz and Schmid (34) reviewed chiral separations by TLC, GC, SFC, HPLC, CE, and CEC, discussing 4364

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their principles and their applications, specifically to the pharmaceutical industry, as well as new developments within the techniques. Beyond the pharmaceutical industry, enantiomeric analysis is used for authentication of foods and beverages (35). Beginning in the 1970s, with the development of cyclodextrin-based GC CSPs, large numbers of underivatized chiral compounds, such as those found in foods, could be separated. Achieving enantiomeric separation can authenticate foods with respect to source (natural or synthetic), geographic origin, processing and aging treatments, and formation mechanisms (chemical or enzymic). Enantiomeric analysis is also used as a tool for drug abuse detection. Hair analysis by GC-MS-NCI, HPLC-MS, and GC/MS/ MS to detect drugs and their enantiomers is reviewed by Srogi (36). Estimation of chronological age can also be accomplished by enantiomeric analysis. Yekkala et al. (37) used chiral GC and chiral HPLC to determine the percent racemization of aspartic acid from human dentin from teeth to determine the chronological age of cadavers. Miscellaneous. Other chiral separations techniques that were reviewed include SFC (38) and microchip separations (39). Sakai et al. (40) reviewed dielectrically controlled resolution (DCR), showing that a chiral selector can recognize both enantiomers of a molecule depending on the dielectric property of the solvent employed in the separation. ESI-MS was reviewed by Schug (41) detailing the advantages of ESI-MS over commonly used solution phase approaches, specifically concerning development of new chiral selectors for enantiomeric purification and pharmaceutical industry application. Abel and Juza (42) reviewed simulated moving bed applications to the pharmaceutical industry. Hutanu and Remcho (43) reviewed aptamers as sorbents for HPLC, microLC, and CEC. HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY Paci, Szelifer, and Ratner (44) examined the mechanism of chiral separations, noting the connection between energetics enhancing chiral separation and molecular geometry. They developed models based on the chirality and interaction of small molecules, discussing aggregation of the molecules to form micelles. For larger molecules, formation of heterochiral pairs is energetically more favored than micellar clusters due to more steric influence on the intermolecular interactions. Polysaccharide CSPs. Large numbers of separations continue to be performed using polysaccharide-based chiral stationary phases. Huybrechts et al. (45) developed a multimodal screening strategy of polysaccharide-based CSPs in order to enhance efficiency of selecting an appropriate CSP to perform a separation. Kasat et al. (46) examined the separation mechanism of polysaccharide CSPs, determining the impact of the molecular environment of the chiral cavities on enantioselectivity. The effect of chiral additives in preparation of cellulose-based CSPs was investigated by Chang et al. (47) , indicating that chiral recognition was increased with chiral additives. Separations performed using polysaccharide-based CSPs included the separation of 2-methylcyclohexanone thiosemicarbazone (48). The majority of polysaccharide-based CSPs employed were cellulose-based polysaccharide columns, including Chiralcel

OD-RH, Chiralcel OD, and Chiralcel ODR and amylose-based columns such as Chiralpak AD and Chiralpak AD-H. Separations performed using Chiralcel OD-RH columns included separation of all-E-astaxanthin (49), and the resolution of [4]heterohelicenium dyes (50), which were also resolved using Chirobiotic TAG CSPs. Enantioseparation of 1-(R-aminobenzyl)2-naphthol and 1-(aminoalkyl)-2-naphthol analogues was accomplished using both Chiralcel OD-RH and Chiralcel OD-H columns (51). Chiralcel OD-H columns were also used to separate five structurally related amino acids (52). A Chiralcel OD column was used to separate the enantiomers of abacavir sulfate (53); a Chiralcel ODR column was used to separate the two enantiomers of the active metabolite of the antiepileptic drug oxcarbazepine (54); and the enantiomers of zaltoprofen were separated using a Chiralcel OJ-RH column (55). Other separations performed on cellulose based columns included the separation of 20 chiral pesticides using cellulose tris3,5-dimethyl carbamate (56) and the separation of racemate n-BocRolipram, including discussion of the kinetic and equilibrium parameters of the separation, using cellulose tris(3,5-dimethylphenylcarmabate) supported on silica (57). Separations performed using Chiralpak AD columns included enantiomeric separation of tadalafil and its three isomers (58) and separation and determination of toxicity of fosthiazate, a chiral orgahophosphorus pesticide (59). Chiralpak AD-H columns were used in chiral separations of a series of proline derivatives (60), a β-blocker drug nadolol (61), and repaglinide, an antidiabetic (62). Other amylose-based chiral stationary phases used for chiral separations included the use of a Chiralpak AS column to separate chiral pyrazoles (63), and separation and determination of enantiomeric purity of pemetrexed disodium in bulk drugs using an amylose based CSP (64). Various chiral separations were performed using both amylosebased and cellulose-based polysaccharide chiral stationary phases. Caccamese, Bianca, and Carter (65) compared amylose tris-(3,5dimethylphenylcarbamate) CSP and cellulose tris-(3,5-dimethylphenylcarbamate) CSP and their enantioselectivity toward an aromatic amine and aminoalcohols. Separation of trans-arylcyclopropanecarboxylic acids and their amide and nitrile derivatives was studied on amylose tris-(3,5-dimethylphenylcarbamade), cellulose tris-(3,5-dimethylphenylcarbamate), and cellulose tris-(4methylbenzoate), including discussions of enantiomer recognition in terms of functional group, electronic and steric effects of analyte substituents, structure of the CSP, and mobile phase composition (66). Tris(2-phenylpyridine) iridium(III) complexes were separated on amylose 3,5-dimethylphenylcarbamate, cellulose 3,5dimethylphenylcarbamate, and cellulose 4-methylbenzoate (67). Chiralpak AD, Chiralpak IA, and Chiralcel OD columns were used to separate 15 pyrrolidine-2-ones, specifically investigating the influence of mobile phase composition on the enantioselectivity (68). Chiralpak AD, Chiralpak IA, and Chiralcel OD columns were used to perform the enantiomeric separation of N-phthaloyl protected R-amino acids (69). Many new derivatized polysaccharide chiral stationary phases have been developed over the last couple of years. Ming et al. (70) developed a new bonded cellulose 3,5-dimethylphenylcarbamate CSP and investigated its enantioselectivity toward five

racemates. Chiralpak IB, cellulose tris(3,5-dimethylphenylcarbamate) immobilized onto silica gel, was developed and studied for separation of various racemates (71). Chankvetadze et al. (72) modified monolithic capillary columns containing native silica gel with 3,5-disubstituted phenylcarbamate derivatives of cellulose and amylose. Other new polysaccharide CSPs include amylose 3,5dimethylphenylcarbamate immobilized onto silica gel (Chiralpak IA) (73) and Kromasil CelluCoat, a silica-based CSP coated with tris-(3,5-dimethylphenyl)-carbamoyl cellulose (74). Chiral packing materials made of spherical beads of cellulose 3,5-dimethylphenylcarbamate with partial hydroxyl groups were prepared and tested in HPLC (75). Many studies using polysaccharide-based CSPs included applications to pharmacokinetic studies, such as the separation of warfarin enantiomers and its metabolite in human plasma using a column-switching HPLC method which was useful for therapeutic drug monitoring (76). Chiralpak AD-H columns were used in the enantioseparation and quantification of (+)- and (-)torcetrapib enantiomers in hamster plasma, with application to a pharmacokinetic study (77). Another pharmacokinetic application was the enantioselective analysis of citalopram and demethylcitalopram in human and rat plasma (78). Polysaccharide-based CSPs also have the potential to be used on a preparative scale, as shown by the preparative scale enantiomeric separation of different benzimidazoles, using Chiralpak AD and Chiralpak AS columns, as well as a Kromasil-CHITBB column (79). Macrocyclic Antibiotic CSPs. Macrocyclic antiobiotics, primarily vancomycin, teicoplanin, and ristocetin A, are also commonly used for chiral separations in HPLC. The Chirobotic V column, containing a vancomycin chiral stationary phase, was used in a method for the simultaneous detection of R- and S-propranolol and R- and S-hyoscamine in human plasma (80) and for the simultaneous detection of bufaralol enantiomers in plasma and pharmaceutical formulations (81). Enantioseparation of duloxetine and its R-enantiomer was achieved using a Chirobiotic V column and hydroxyproply-β-cyclodextrin as a chiral selector (82). A vancomycin crystalline degradation products chiral stationary phase was used in enantiomeric separations of haloxyfop-Me, fenoxaprop-P-Et, and indoxacarb, which are three agrochemical toxins (83). Chirobiotic T columns, teicoplanin-based chiral stationary phases, were used to separate enantiomers of seven arylsubstituted β-lactams (84) and in the development of an LC-MS method for the enantioselective detection of ifosfamide in human plasma and the N-dechloroethylated metabolites of ifosfamide (85). Teicoplanin aglycon chiral stationary phases (Chirobiotic TAG) columns were used to investigate the effect of temperature on retention and separation of chiral sulfoxides (86) and for the separation of urinary D- and L-lactic acid enantiomers by HPLC-MS/MS (87). Other work with teicoplanin chiral stationary phases included the investigation of the absorption behavior of L,D-threonine and L,D-methionine using various organic modifiers in the solvent (88). Work with multiple macrocyclic antiobiotic based chiral stationary phases included the enantioseparation of six β-2-agonists using Chirobiotic V, T, and R columns (89), the comparison of methylated teicoplanin aglycon, teicoplanin aglycon, and natural Analytical Chemistry, Vol. 80, No. 12, June 15, 2008

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teicoplanin CSPs to separate a variety of racemic analytes (90), and the comparison of Chirobiotic T, Chirobiotic m-TAG, Chirobiotic TAG, and Chirobiotic V in the enantioseparation of enantiomers of 1-methyl-2-piperidinoethylesters of 2-, 3-, and 4-alkoxyphenylcarbamic acid (91). Berkecz et al. (92) compared separation of β-lactam and β-amino acid stereoisomers on macrocyclic glycopeptide columns, including teicoplanin, vancomycin, and ristocetin A, and a new dimethylphenyl carbamate-derivatized β-cyclodextrin-based column. Other work with macrocyclic glycopeptide columns included the separation of enantiomers of 18 unnatural β-amino acids and comparison of teicoplanin, teicoplanin aglycon, vancomycin, and ristocetin A as chiral selectors (93). Petrusevska et al. (94) developed and characterized a new macrocyclic-CSP derivative which has eremomycin attached to silica particles. Cyclodextrin CSPs and Mobile Phase Additives. Work with cyclodextrins as chiral stationary phases included examination of the mechanism of cyclodextrin separation, finding that the greater the stability of the complex formed, the better the separation achieved (95). The thermodynamics and kinetics of chiral separations using β-cyclodextrin CSPs were examined with respect to the effects of mobile phase concentration (96) and the effects of temperature and pressure (97). Separations performed using cyclodextrin CSPs included separations of ruthenium(II) polypyridyl complexes (98), enantioseparation of tryptophan analogues (99), and enantioseparation of 1-(R-aminobenzyl)-2-naphthol and 2-(R-aminobenzyl)-1-naphthol analogues (100). Other work with cyclodextrin CSPs included the development of a method for the simultaneous detection of eslicarbazepine acetate, oxcarbazepine, S-licarbazepine, and Rlicarbazepine in human blood plasma (101). Cyclodextrin stationary phases that were developed, evaluated, and optimized included 2,6-dinitro-4-trifluromethylphenyl ether substituted CD bonded CSP (102), new dinitrophenyl (DNP) substituted β-cyclodextrin CSP (103), phenyl-carbamate-propylβ-CD (104), and a new β-CD bonded silica CSP called RAM-Chiral (105). Other work with cyclodextrins included adding cyclodextrins to the mobile phase as an economically advantageous way to achieve chiral separations (106). Ligand Exchange CSPs. Ligand-exchange liquid chromatography was used for the chiral separation of tryptophan enantiomers (107), the separation of three stereoisomers of octahydroindole2-carboxylic acid (108), enantioseparation of phenylaline analogues, (109), and enantioseparation of the Mannich ketone M9, a potential antifungal compound (110). Ligand-exchange chromatography was also used in the method development for the detection of ofloxacin enantiomers in human urine (111). New ligand-exchange columns were synthesized and classified, including a monolithic column (112), a S-benzyl-(R)-cysteine CSP (113),andmodularbidentateligantsbasedonaryl[2.2]paracyclophane (114). Hoffmann et al. (115) synthesized a chiral cation-exchange CSP based on de novo designed synthetic low molecular mass selectors (SOs) of which the functional unit for enantioselectivity contained a β-aminocyclohexanesulfonic acid moiety. Protein-Based CSPs. Proteins and protein derivatives were also used as chiral stationary phases in HPLC separations. Azelnidipine enantiomers in human plasma were separated on a chiral R-1-acid glycoprotein column (116). A chiral-AGP column 4366


was studied with respect to the effect of tertiary alcohol additives on its enantioselectivity (117). New protein CSPs include an antibody CSP prepared by immobilizing a monoclonal anti-D-amino acid antibody onto silica (118) and a bovine serum albuminmodified magnesia-zirconia stationary phase (119). Miscellaneous. Polymer-type CSPs were also developed and classified, including bonding chiral polyurethane on animated silica gel (120), derivatives of the copolymer (1R,2R)-(+)1,2- (121), a synthetic polymeric CSP based on N,N′[(1R,2R)-1,2-diphenyl1,2-ethanediyl]bis-2-propenamide monomer (122), and surfacegrafted, imprinted polymers on silica gel (123). Molecularly imprinted polymers were also used for the chiral separation of a series of C2-asymmetric Binaphtyl compounds (124). Brush-type chiral stationary phases were used for the enantioseparation of 1,1′-binaphthyl-2,2′-diol, considering the influence of mobile phase and temperature (125). They were also used in the determination of the enantiomeric purity of R-β-amino-β-(4methoxyphenyl) propionic acid (126) and a molecular dynamics study of the docking of chiral epoxides on Whelk-O1 chiral stationary phase (127). Novel brush-type CSPs were tested for the enantiomers of dihydropyrimidinone derivatives and compared with commercially available columns (128). Twenty-two racemates with different functional groups were used to test the enantioselectivity of a new brush-type CSP comprising π-acidic N-(3,5dinitrobenzoyl)-D-R-phenylglycine chiral units bound to the quinoxaline units of modified γ-aminopropyl silica gel by a 1,2diaminoethane spacer (129). Chiral stationary phases based on (+)-(18-crown-6)-2,3,11,12tetracarboxylic acid were used to resolve secondary amino alcohols (130) and N-(substituted benzoyl)-R-amino acid amides (131). New crown ether CSPs include a CSP covalently bound with a chiral pseudo-18-crown-6 ether having a phenolic hydroxyl group (132) and a doubly tethered CSP based on (+)-(18-crown6)-2,3,11,12-tetracarboxylic acid containing an amide linkage (133). New chiral stationary phases include a CSP based on the (R)1-phenyl-2(4-methylphenyl)ethylamine amide derivative of (S)valine and 2-chloro-3,5-dinitrobenzoic acid (134), a (R)-1-phenyl2(4-methylphenyl) ethylamine amide derivative of (S)-isoleucine (135), dendrimer-like CSPs derived from (1R,2R)-(+)1,2-diphenylethylenediamine and 1,3,5-benzenetricarbonyl trichloride (136), boromycin covalently bound to silica gel (137), azido cellulose phenylcarbamate immobilized onto aminized silica gel to give ureabonded CSPs (138), and cellulose dimethylphenylcarbamatebonded carbon-clad zirconia (139). Other new CSPs include quinine carbamate-based CSPs (140), chitosan modified with N-nicotinoyl-L-phenylalanine and 3,5-dimethylphenyl isocyanate (141), a penicillin G acylase-based CSP (142), a polymer of (S)-N-maleoyl-L-leucine allyl ester (143), cellulose tris(3,5-dimethylphenylcarbamate) coated on aminopropylsilica (144), and chemically bonded methylated and methylated/acetylated 6-O-tertbutyldimethyl-silylated β-cyclodextrin (145). New LC separation techniques and applications include a chiral separation based on single walled carbon nanotubes conjugated with bovine serum albumin (SWNTs-BSA) immobilized in a microchip channel (146), development of an HPLC-MS/MS technique, based on the inverted chirality columns approach, for identification and detection of the minor enantiomer in nonracemic mixtures (147), the development of an automated chiral separation

screening platform with isochronal-parallel analysis, speeding up the screening process and method development (148), and the use of multiparallel screening of CSPs and mobile phases for efficient method development on both a microscale and preparative scale (149, 150). CAPILLARY ELECTROPHORESIS Capillary electrophoresis (CE) continues to be used extensively in the field of chiral separations. Sazelova et al. (151) developed a method to control the electroosmotic flow (EOF) in CE by application of a radial electric field; their analysis included the effects of magnitude, orientation, and various ways of applying the electric field on the flow rate of EOF and on the speed, efficiency, and resolution of separations. Their study included the application of the device to chiral separations. Cyclodextrins. Cyclodextrins remain a popular chiral selector for use in capillary electrophoresis. Jiang et al. (152) used sulfated R-cyclodextrin, sulfated β-cyclodextrin, and carboxymethyl β-cyclodextrin to separate mixtures of amino acids, analyzing the effects of chiral selector concentration, addition of organic modifier, and pH on migration times and resolution. Sulfated β-cyclodextrins were also used as a chiral selector to separate phenylglycidatesinCE(153),phenothiazinesincapillaryzoneelectrophoresis (CZE) (154), melatoninergic ligands in electrokinetic chromatography (EKC) (155), aromatic amino acids in EKC (156), and four imidazole derivatives in EKC (157). Multiple isomer sulfated β-CD was used as the chiral selector for the enantioselective analysis of ketamine and its metabolites (158). Hydroxypropyl-β-cyclodextrin (HP-β-CD) was used as a chiral selector to determine enantiomeric excess of aromatic 1,2-diols using CZE (159) and to study the separation and degradation of imazaquin enantiomers in field soils (160). HP-β-CD was also used as a chiral selector in the enantiomeric separation of four diastereomers of guaiacyl glycerol by CE (161) and in the enantiomeric separation of amlodipine in blood serum (162). New cyclodextrin-based chiral selectors used included a β-CD derivatized erythromycin selector used in CE (163), a β-alaninebridged hemispherodextrin used in EKC (164), two derivatives of β-cyclodextrin, the ethylendiamine derivative in the primary position and the cysteamine-bridged hemispherodextrin used in EKC (165). Separations performed using cyclodextrins in CE included the separation of dansyl amino acids using mono-(3-methyl-imidazolium)-β-cyclodextrin chloride as a chiral selector (166). Cyclodextrins were used to separate cetirizine in CE (167), ultimately to analyze the amount of levocetirizine in tablets. Separations performed using cyclodextrins in CZE included resolution of 2,4dinitrophenyl labeled amino acid enantiomers using N-methylated amino-β-cyclodextrins as a chiral selector (168) and separation of β-lactams using sulfated R-cyclodextrin, sulfated β-cyclodextrin, and carboxymethyl β-cyclodextrin as chiral selectors (169). In EKC, native β-CD, (2-hydroxy)propyl-β-CD and heptakis-2,3,6-triO-methyl-β-CD were used to separate a group of six weak base azole compounds (170). In MEKC, hydroxypropyl-γ-cyclodextrin was used as a chiral selector for separation of propiconazole (171) and heptakis(2,3-di-O-methyl-6-O-sulfo)-β-CD and sulfated β-CD were used to separate chiral tropa alkaloids (172). Studies on the effects of other run buffer additives on cyclodextrin-mediated separations included urea and buffer sub-

stances (173), ionic liquids (174), and organic solvents (175). Riddle et al. (176) performed a comparative study of the separation of fluorescently labeled amino acids in CE using various additives, including β- and γ-cyclodextrins and a dual surfactant system of SDS and sodium taurodeoxycholate. A dual cyclodextrin system was used in the development of a method to detect enantiomeric purity of Efaroxan by CE (177). Applications of CE using cyclodextrins as chiral selectors included pharmokinetic studies on ibuprofen and its chiral metabolites using heptakis 2,3,6-tri-O-methyl-β-CD with reference to genetic polymorphism (178). Sodium cholate (SC) with either SDS or γ-CD were used in enantiomeric separations of neutral organophosphorus pesticides by CE (179). Kofink et al. (180) performed the enantioseparation of catechin and epicatechin in plant food using (2-hydroxypropyl)-γ-cyclodextrin as the chiral selector in CE. Micelles. In EKC, microemulsions and micelles continue to be used to perform chiral separations. Kahle and Foley (181) discussed the effects of using two chiral species, the chiral surfactant dodecoxycarbonylvaline, and the chiral cosurfactant S-2hexanol, rather than just a single chiral species, in a single pseudostationary phase to separate enantiomers. They repeated their study of a dual-chirality microemulsion system using six stereochemical combinations of dodecoxycarbonylvaline, racemic 2-hexanol, and diethyl tartrate as pseudostationary phases for the separation of chiral pharmaceutical compounds (182, 183). Other surfactants used as pseudostationary phases to perform enantiomeric separations included vesicle-forming single-tailed amino acid derivatized surfactants sodium N-[4-n-dodecyloxybenzoyl]-L-leucinate and sodium N-[4-n-dodecyloxybenzoyl]-L-isoleucinate (184), four diastereomers of poly(sodium N-undecanoyl leucylvalinate) (185), and a polymeric chiral surfactant, poly(sodium N-undecenoyl-L,L-leucyl-valinate) (186). Hou et al. also worked with poly(sodium N-undecenoxycarbonyl-L-leucinate) as a polymeric chiral surfactant to separate ephedrine alkaloids, discussing both the method development (187) and application in dietary supplements (188). Threeprom (189) used (S)-(+)-2-octanol as a chiral oil core for used in MEKC separation of ephedrine, norephedrine, synephrine, and propanolol. Ligand Exchange. Ligand exchange was also used in CE to perform chiral separations. Borate was used as a central ion of the chiral selector and (S)-1,3,6-trisulfonate as a chiral selector ligand to separate 8-aminonaphthalene-1,3,6-trisulfonate derivatized monosaccharides (190). Separation of aromatic amino acids was achieved using zinc(II) as the central ion with L- or D-lysine as the chiral ligand (191) and L-arginine and other amino acid ligands (192). Copper(II) complexes of L-taratric acid or L-threonine were chiral selectors of synpathomimetics and β-blockers by ligandexchange CE (193). Miscellaneous. Other chiral selectors employed in CE included macrocyclic antibiotics, such as vancomycin, which were used both as a mobile phase additive and bound as a CSP to separate NSAIDS (194), bovine serum albumin (195) to separate ephedrine isomers, microbial cyclooligosaccharides to separate chiral flavanones (196), crown ethers (197) with applications to the pharmaceutical industry, human serum albumin (198, 199), with applications to the pharmacokinetics of drugs. Analytical Chemistry, Vol. 80, No. 12, June 15, 2008

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Other chiral selectors employed in EKC included single-walled and multiwalled carbon nanotubes (200, 201). CAPILLARY ELECTROCHROMATOGRAPHY Developments in capillary electrochromatography (CEC) generally focused on capillary preparation and development of new CSPs. Various types of packed capillaries were developed, including a packed capillary with temporary quartz wool frit, a packed capillary with immobilized frit, and an immobilized packedcapillary from (S)-N-(3,5-dinitro-benzoyl)leucine-N-phenyl-N-alkylamide derived CSPs (202), and a packed capillary consisting of particle-loaded monoliths containing a polymethacrylamine backbone to separate amino acids by capillary-LC and CEC (203). Phospholipid-bovine serum albumin coating for chiral CEC was also developed (204), as was a β-CD derivatized BSA chiral selector (205). New CSPs used in CEC included a vancomycinimmobilized silica monolith (206) and avidin immobilized onto the inner surface of fused silica capillaries (207). Separations performed using CEC included separation of β-receptor blockers by both pressurized CEC and capillary liquid chromatography (CLC) (208), the antidepressant mirtazapine and its main metabolites (209), and two chiral compounds, tetrahydropalmatine and Troeger’s base, by molecularly imprinted monoliths (210). CEC was also used for the study of the stereoselective degredation of the herbicide 2-aryloxipropionic acid dichloroprop (DCPP), using a porous monolithic column prepared by in situ copolymerization of glycidyl methacrylate, methyl methacrylate, and ethylene glycol dimethacrylate (211). GAS CHROMATOGRAPHY Various chiral stationary phases with differing applications have been developed for use in GC over the review period. Six chiral selectors of S-(-)-t-Leu-cycloproplyamide, S-(-)-Leu-cyclopentylamide, S-(-)-t-Leu-cyclohexylamide, S-(-)-t-Leu-cycloheptylamide, S-(-)-t-Leu-cyclooctylamide, and S-(-)-t-Leu-cyclodecylamide were prepared and tethered through amide linkages to polydimethylsiloxane functionalized with 2,2,2-trifluroethyl ester groups with application to age estimation based on the amount of aspartic acid racemization in human dentins (212). L-Valine diamide and permethylated β-cyclodextrin were attached to polysiloxane creating a chiral stationary phase that separates R-amino acids, chiral alcohols, ketones, and hydrocarbons (213). Other novel chiral stationary phases included linear dextrins (acyclodextrins) (214), the cellulose derivatives cellulose triacetate, cellulose triphenylcarbamate, and cellulose tris-(3,5-dimethylphenylcarbamate) (215), and Chirasil-Val-C11 doped with octakis-(3-O-butanoyl-2,6-di-O-npentyl)-γ-cyclodextrin (216). Other work with stationary phases included the development and study of a mixed stationary phase containing permethylatedβ-CD and perpentylated-β-CD in fused silica capillary GC (217), study of permethylated β- and γ-cyclodextrin columns for enantioseparation of chiral chloropesticides (218), the use of a threephase model using a methylated cyclodextrin/polysiloxane stationary phase in chiral gas-liquid chromatography (219), and the use of solid phase ion exchangers to separate racemic mixtures of amino acid enantiomers (220). A method for the enantioseparation and detection of 2,2-dimethylcyclopropanecarboxamide and its acid was developed using γ-cyclodextrin based chiral column BGB-175 (221). 4368


Levkin et al. (222) investigated the inversion in elution order of enantiomers caused by changes in temperature. Reiner et al. (223) studied the accurate detection of small deviations from racemic compositions using R-amino acids and a variety of CSPs. Applications of work with chiral GC included pharmaceutical applications such as the screening enzymes for efficiency in bioconversions, specifically hydrolysis of chiral esters, in drug discovery (224), separation of organophosphorous pesticides using Chirasil-Val and CP-Chirasil-Dex CB columns (225), and separation of stereoisomers of dipeptides on the CSPs ChirasilL-Val and Lipodex-E (226). Other chiral GC applications included analysis of enantiomers in hair to detect MDMA and other amphetamine-type stimulants (227), development of a method to analyze cyclosarin enantiomers in biological blood samples (228), and enantioseparation of methylsulfonyl PCB and DDE metabolites in human adipose tissues, seal blubber, and pelican muscle (229). Chiral GC was also used on a semipreparative level for the separation of filbertone enantiomers (230). SUPERCRITICAL FLUID CHROMATOGRAPHY The use of SFC as a chiral separation technique for the pharmaceutical industry has increased over the past few years. Wang (231) discusses the use of chiral SFC in the pharmaceutical industry, citing developments in SFC, such as method screening, hardware refinement, and system integration, that have led to an increased number of applications for the technique. Helmy et al. (232) also discuss the use of chiral SFC in pharmaceutical analysis, focusing on improving the sensitivity of chiral SFC and instrument modifications that allow for increased sensitivity. SFC was used for the synthesis of a chiral preclinical pharmaceutical candidate on a preparative scale (233), and SFC was also used to separate chiral sulfoxides on a Chiralpak AD column (234). Numerous comparisons of chiral separations using SFC to separations using other techniques were performed. Wang et al. (235) compared separations of urinary metabolites of nobiletin on chiral SFC to its separations on HPLC. Matthijs et al. (236) compared chiral separations in normal- and reversed-phase HPLC to chiral separations in SFC. SFC was also coupled to simulated moving columns (SMC), using short chiral columns, allowing for better resolution with enantiomeric separations (237). Polysaccharide derivatives were commonly used as chiral stationary phases in separations performed by SFC. The amylosebased CSP Chiralpak AD was used to separate four antimycotic azole drugs (238), and Chiralpak AD and AS columns were used to separate halogen and methoxy substituted phenylglycidols (239). Cellulose tris(3,5-dimethylphenylcarbamate) coated on silica support (Chiralcel-OD) was used to separate enantiomers of 1-phenyl-1-propanol (240). Chiralcel OD columns were also used in the SFC chiral separation of chiral Nutlin-3 (241). Four polymeric CSPs were synthesized for use in SFC by Han et al. (242). Welch et al. (243) used a screening tool developed by modifying a SFC with high-pressure column selection valves to resolve multicomponent chiral mixtures. MISCELLANEOUS TECHNIQUES Simulated moving beds are still used to perform chiral separations and have specifically been used to separate chiral chemicals (244). Zhang et al. (245) developed an eremomycin-

chiral stationary phase for use in SMB to separate methionine. Crawford et al. (246) discussed steady state recycling (SSR), a technique similar to SMB which allows for efficient separations of chiral compounds on a kilogram scale. Increased efficiency in SSR can be accomplished by using polarimetry to optimize the method. Thin layer chromatography (TLC) is another chiral separation technique, used by Taha (247) to separate R(+)- and S(-)ropivacaine in both the bulk powder and pharmaceutical dosage form, using cyclodextrins as the chiral selector. Other work with TLC included investigating the impact of four mobile phases on the separation of ibuprofen and naproxen using L-arginine as a chiral selector (248). Other chiral separations techniques include chiral ion mobility spectrometry, which separates ions based on their interactions with a chiral gas (249), biphasic recognition chiral extraction which is used to resolve chiral aromatic acids (250), countercurrent chromatography used to resolve DL-R-methylbenzylamine with L-(+)-tartaric acid as the chiral selector (251), and capillary isotachophoresis to resolve alkylamine antihistamines in pharmaceutical preparations using a charged cyclodextrin as a chiral selector (252). ACKNOWLEDGMENT Thanks are extended to Karen Ward for her generous assistance and editing with this review. Her input and patience was needed and appreciated. Timothy J. Ward, professor of chemistry and associate dean of sciences at Millsaps College (Jackson, MS 39210), received his B.S. degree from the University of Florida and his Ph.D. from Texas Tech University. Dr. Ward served as chair of the International Symposium on Chirality, July 2007 in San Diego. 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 archaeological analysis. Beth Ann Baker received her B.S. in chemistry summa cum laude from Millsaps College and currently is pursuing an M.D. at the University of Pittsburgh.

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