Enantiomeric separation by using nano-liquid chromatography with on-column focusing

The present study shows a new nano-liquid chromatographic method for beta-blocker enantiomers' separation. This method consists of using a capillary column packed with silica particles which were chemically modified with vancomycin. On-column focusing allowed to inject relatively high sample volumes (1500 nL) increasing method sensitivity. The studied racemic compounds, namely atenolol, propranolol, oxprenolol, and metoprolol were dissolved in methanol and injected for chromatographic separation. The effect of injected sample volume was studied in the range of 50-2100 nL. Peak height of the two alprenolol enantiomers increased linearly up to 1500 nL. This volume was injected for validation and sample analysis. Under optimal experimental conditions, LODs and LOQs (LOD and LOQ for each alprenolol enantiomers) were 9.0 and 15.6 ng/mL, respectively. Calibration curves were linear in the studied range (9-250 ng/mL). The optimized method was applied to the analysis of a human plasma sample spiked with racemic alprenolol.     

Enantiomeric separation of mirtazapine and its metabolites by nano-liquid chromatography with UV-absorption and mass spectrometric detection

Mirtazapine (MIR) and two of its main metabolites, namely, 8-hydroxymirtazapine and N-desmethylmirtazapine, were separated in totheir enantiomers by nanoLC in a laboratory-made fused-silica capillary column (75 microm ID) packed with a vancomycin-modified silica stationary phase. The simultaneous separation of the three couples of the studied enantiomers was achieved in less than 33 min, using an experimentally optimized mobile phase delivered in the isocratic mode. Optimization of the mobile-phase composition was achieved by testing the influence of the buffer pH and concentration, the water concentration, the organic modifier type and concentration, and on the retention and resolution of the analytes. The optimum mobile-phase composition contained 500 mM ammonium acetate pH 4.5/water/MeOH/MeCN, 1:14:40:45 v/v/v/v. Using a UV detector at 205 nm, the method was validated studying several experimental parameters such as LOD and LOQ, intraday and interday repeatability, and linearity. Good results were achieved: LOD and LOQ were in the range 5-15 and 10-40 microg/mL, respectively (the highest value was obtained for the DEMIR enantiomers); correlation coefficients, 0.9993-0.9999; the intraday and interday precision was acceptable (RSD < 2%) using an internal standard. The method was tested for the separation of the studied enantiomers in an extracted (solid-phase) serum sample spiked with standard racemic mixture of MIR and its two metabolites. Finally, the nanoLC system was connected to a mass spectrometer through a nanoelectrospray interface and the MS, MS2, and MS3 spectra were acquired showing the potential of the system used for characterization and identification of the separated analytes.     

Cyclodextrins as a chiral mobile phase additive in nano-liquid chromatography: comparison of reversed-phase silica monolithic and particulate capillary columns

Enantioseparations of racemic nonsteroidal anti-inflammatory drugs (naproxen, ibuprofen, ketoprofen, flurbiprofen, suprofen, indoprofen, cicloprofen, and carprofen) were performed by nano-liquid chromatography, employing achiral capillary columns and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TM-β-CD) or hydroxylpropyl-β-cyclodextrin (HP-β-CD) as a chiral mobile phase additive (CMPA). Working under the same experimental conditions (in terms of mobile phase and linear velocity), the performance of a RP-C18 monolithic column was compared with that of a RP-C18 packed column of the same dimensions (100 μm i.d. × 10 cm). Utilizing a mobile phase composed of 30% ACN (v/v) buffered with 50 mM sodium acetate at pH 3, and containing 30 mM TM-β-CD, the monolithic column provided faster analysis but lower resolution than the packed column. This behavior was ascribed to the high permeability of the monolithic column, as well as to its minor selectivity. HP-β-CD was chosen as an alternative to TM-β-CD. Employing the monolithic column, the effects of different parameters such as HP-β-CD concentration, mobile phase composition, and pH on the retention factor and the chiral resolution of the analytes were studied. For the most of the analytes, enantioresolution (which ranged from R _s = 1.80 for naproxen to R _s = 0.86 for flurbiprofen) was obtained with a mobile phase consisting of sodium acetate buffer (25 mM, pH 3), 10% MeOH, and 15 mM HP-β-CD. When the same experimental conditions were used with the packed column, no compound eluted within 1 h. Upon increasing the percentage of organic modifier to favor analyte elution, only suprofen eluted within 30 min, with an R _s value of 1.14 (20% MeOH). Replacing MeOH with ACN resulted in a loss of enantioresolution, except for naproxen (R _s = 0.89).     

手性流动相HPLC法拆分对羟基苯甘氨酸对映体的研究

将β-环糊精、羟丙基-β-环糊精作为手性流动相添加剂,系统地研究了D,L对羟基苯甘氨酸在RP-HPLC系统中的拆分。分别考察了手性流动相的种类,手性试剂β-环糊精的浓度,流动相的pH,修饰剂的种类及浓度,色谱柱温度等对拆分效果的影响,以-βCD为手性流动相添加剂,建立了-βCD手性流动相分离对羟基苯甘氨酸对映体的方法。结果表明:用ODS柱(250 mm×4.6 mmi.d.),以V(甲醇-β环糊精)∶V(pH 4.5磷酸盐缓冲液)=30∶70为流动相,流速0.2 mL/min,柱温25℃,检测波长为230 nm时对羟基苯甘氨酸对映体得到了良好的基线分离,分离度可达1.71。     

Enantiomeric resolution of dansyl phenylalanine and analogs by microcolumn liquid chromatography with γ-cyclodextrin as mobile phase additive

Dansyl derivatives of racemic phenylalanine and its analogs have been resolved by microcolumn liquid chromatography with γ-cyclodextrin as mobile phase additive. The enantioselectivity of the system was influenced by the concentrations of both γ-cyclodextrin and acetonitrile in the mobile phase. Enantiomers of phenylalanine, phenylglycine, and tyrosine were resolved by isocratic elution in a single chromatographic run.     

Enantiomeric separation of some demethylated analogues of clofibric acid by capillary zone electrophoresis and nano-liquid chromatography

The enantiomeric separation of some demethylated analogues of clofibric acid, namely 2-(6-chloro-benzothiazol-2-ylsulfanyl)-, 2-(6-methoxy-benzothiazol-2-ylsulfanyl)-, 2-(quinolin-2-yloxy)-, 2-(6-chloro-quinolin-2-yloxy)-, 2-(7-chloro-quinolin-4-yloxy)-propionic acid (compounds A-E, respectively), has been studied by CZE and nano-LC using for the first technique two beta-CD derivatives and vancomycin added to the BGE and vancomycin-modified silica particles for the second one, with the aim to find the optimum experimental conditions for the baseline resolution. The type and the concentration of the chiral selector added to the BGE, the buffer pH, the type of organic modifier and its concentration, the capillary temperature and the applied voltage played a very important role in the enantioresolution of the analysed compounds. The use of 6-monodeoxy-6-monoamino-beta-CD allowed to achieve baseline resolution of four of five clofibric acid derivatives in less than 10 min while heptakis-(2,3,6-tri-O-methyl)-beta-CD partially resolved the same compounds in their enantiomers. Employing vancomycin as the chiral selector in CZE, the counter-current partial filling method was chosen achieving baseline resolution of four analytes. All the studied compounds were enantioresolved employing a capillary column packed with vancomycin stationary phase by nano-LC, and the resolution was strongly influenced by the concentration of the organic modifier and by the pH of the mobile phase. The best results were achieved at pH 4.5 in presence of 60% of methanol (MeOH). However, longer analysis times were observed in the experiments carried out by nano-LC.     

Enantiomeric separation of alcohols and amines on a proline chiral stationary phase by gas chromatography

A new chiral stationary phase for gas chromatography was prepared by covalently attaching a diproline chiral selector that has proven to be effective in liquid chromatography to a methylhydrosiloxane-dimethylsiloxane copolymer. With this new chiral stationary phase for GC, racemic aromatic alcohols could be resolved without derivatization. Racemic aromatic and aliphatic amines could also be resolved after derivatization of the amino groups with trifluoroacetic anhydride or isopropyl isocyanate. On this stationary phase, the isopropyl isocyanate derivatives of amines showed higher enantioselectivity than the trifluoroacetic anhydride derivatives. In both the enantiomeric separations of alcohols and derivatized amines, the aromatic racemic analytes showed higher enantioselectivities than their aliphatic analogs. Some of the alpha-amino and alpha-hydroxy aromatic acids could also be separated after derivatization to N-trifluoroacetyl methyl esters for amino acids or O-trifluoroacetyl methyl esters for hydroxyl acids.     

Liquid chromatography determination of citalopram enantiomers using beta-cyclodextrin as a chiral mobile phase additive

A reliable and specific method for the determination of citalopram enantiomers was developed and validated. Chromatographic resolution of citalopram enantiomers was made on a Shim-pack (5 microm particle size) cyanopropyl column with beta-cyclodextrin (beta-CD) as an effective chiral mobile phase additive. The composition of the mobile phase was (90 + 10, v/v) aqueous 0.1% triethylammonium acetate buffer, pH 4.0 (adjusted with acetic acid), and acetonitrile, containing 12 mM beta-CD. The flow rate was 0.8 mL/min with ultraviolet detection at 240 nm. The effects of the mobile phase composition, concentration of beta-CD, and pH of the triethylammonium acetate buffer on peak shape and resolution of the enantiomers were investigated. The calibration graphs were linear (r = 0.9999, n = 8) in the range of 1-40 microg/mL for S(+) citalopram and R-(-) citalopram. The limit of detection values were 5.51 x 10(-3) and 4.35 x 10(-3) pg/mL, while the limit of quantification values were found to be 1.84 x 10(-2) and 1.45 x 10(-2) microg/mL for S-(+) citalopram and R-(-) citalopram, respectively.     

高效液相色谱手性流动相法拆分酮基布洛芬对映体

以Lichrospher C18为分析柱, 将β-环糊精、2,6-二甲基-β-环糊精、2,3,6-三甲基-β-环糊精分别作为手性流动相添加剂, 系统地研究了R,S-酮基布络芬对映体在HPLC系统中的拆分. 建立了以2,3,6-三甲基-β-环糊精为手性流动相添加剂分离R,S-酮基布络芬对映体方法.     

手性流动相添加剂法拆分比索洛尔对映体

以羧甲基-β-环糊精作为手性流动相添加剂,建立了高效液相色谱拆分比索洛尔对映体的方法。系统考察了手性添加剂的种类及浓度、流动相中甲醇的含量、pH值和流速等因素对拆分的影响。色谱分离条件:流动相甲醇∶乙腈∶水为20∶67∶13(V/V/V),羧甲基-β-环糊精浓度为0.5g.L-1,pH值为4.07(以三乙胺-冰乙酸调节),流速为0.3mL.min-1,检测波长223nm,对映体得到基线分离,分离度为1.89。     

高效液相色谱手性流动相添加剂法拆分氯噻酮对映体

应用反向高效液相色谱,以羟丙基-β-环糊精(HP--βCD)作为手性流动相添加剂,拆分了氯噻酮(Chlorthalidone)对映体。对主要的影响因素如羟丙基-β-环糊精浓度、流动相pH值、三乙胺(TEA)、甲醇、柱温、流速等进行了系统研究,建立了羟丙基-β-环糊精流动相添加剂法拆分氯噻酮对映体的方法。使用HarbonLichrospher-C18色谱柱(5μm,150 mm×4.6 mm),流动相为V(甲醇)∶V(水相)=20∶80(水相含30 mmol/L HP--βCD0、.1 mol/L Na2HPO4、体积分数2%的TEA、pH5),流速为0.8 mL/min,室温下拆分,氯噻酮对映体可得到良好分离。     

直链淀粉手性固定相拆分西那卡塞对映体

建立了一种快速、准确的高效液相色谱分析方法。使用Chiralpak AD-H手性色谱柱,检测波长224nm,考察了流动相中极性调节剂的体积分数、流动相中三乙胺的体积分数、柱温及流速对西那卡塞对映体拆分的影响。确立了最佳拆分条件:流动相为正己烷-异丙醇-三乙胺(90∶10∶0.1,V/V/V);流速为1.0 m L·min-1;柱温为30℃。在此优化实验条件下,得到西那卡塞对映体的出峰顺序为:先S体,后R体。所建立的方法简单准确,重复性好,可用于西那卡塞的质量研究和控制。     

纤维素手性固定相拆分阿替洛尔对映体

建立了阿替洛尔对映体的高效液相色谱分析方法。使用Chiralpak OD-H手性色谱柱,检测波长275nm。确定了最佳拆分条件:流动相为V(正己烷)∶V(异丙醇)∶V(三乙胺)=80∶20∶0.4;流速1.0 mL/min,柱温30℃。在该实验条件下,得到阿替洛尔对映体的出峰顺序为:先R体,后S体。所建立的方法简单准确,重复性好,可用于阿替洛尔的质量研究和控制。     

Enantiomeric separation of a group of chiral dihydropyridines by electrokinetic chromatography

Electrokinetic chromatography (EKC) was employed to achieve the enantiomeric separation of a group of chiral 1,4-dihydropyridines (DHPs) with pharmacological activity. Micelles of bile salts alone or mixed with neutral cyclodextrins, micelles of sodium dodecyl sulfate (SDS) mixed with neutral cyclodextrins, and anionic cyclodextrin derivatives, i.e., carboxymethyl-gamma-cyclodextrin (CM-gamma-CD), carboxymethyl-beta-cyclodextrin (CM-beta-CD), and succinylated beta-cyclodextrin (Succ-beta-CD), were employed as pseudostationary phases. The enantiomeric separation ability of these chiral selectors with respect to DHPs was studied in different experimental conditions. CM-beta-CD was shown to be the best chiral selector to perform the enantiomeric separation of DHPs by EKC. Next, the influence of the CM-beta-CD concentration, the pH and nature of the buffer, the temperature, and the applied voltage on the enantiomeric resolution of DHPs was studied. The use of a 50 mM ammonium acetate buffer, pH 6.7, 25 mM in CM-beta-CD together with an applied voltage of 15 or 20 kV, and a temperature of 15 degrees C enabled the individual enantiomeric separation of twelve DHPs, each one into its two enantiomers, and their separation in multicomponent mixtures of up to six DHPs into all their enantiomers.     

Fluorescence enhancement of dansylamino acids by bovine serum albumin as mobile phase additive in microcolumn liquid chromatography

Summary The effects of acetonitrile and bovine serum albumin (BSA) concentrations on the signal intensity and retention behavior of dansylamino acids have been examined by using -cyclodextrin-bonded silica gel as the stationary phase in microcolumn liquid chromatography. Fluorescence intensities of dansylamino acids were enhanced by BSA as a mobile phase additive, e.g., detection limits of dansyl derivatives of L-Ala and L-Phe were improved by a factor of 12–18.