Enantiomeric separation of chlorophenoxy acid herbicides by nano liquid chromatography-UV detection on a vancomycin-based chiral stationary phase

Enantiomeric separation of mecoprop, dichlorprop, and fenoprop herbicides in their acid form, commonly used to control the growth of broad-leaved weeds, was carried out by nano-liquid chromatography (nano-LC) at a flow rate of 60 nL/min, using a packed capillary column with vancomycin-modified silica particles of 5 microm. The length of chiral stationary phase was 21 cm, while the total and effective lengths were 43 and 33cm, respectively. Inner diameter was 0.075 mm. Separated peaks were detected at 195 nm. Several mixtures of methanol, water, and 500 mM ammonium acetate buffer at different pH's were tested as mobile phase, and experimental parameters such as resolution (Rs), capacity factor (k), efficiency (N/m), and enantioselectivity factor (alpha) were measured under all the test conditions. Baseline enantiomeric separation was obtained for the three studied herbicides with alpha in the range 1.6-1.9, using as the mobile phase aqueous solutions containing 85% methanol, 5% of 500mM ammonium acetate pH4.5 buffer, and 10% water. Experimental results show that the vancomycin stationary phase presents a great enantiorecognition capability towards chlorophenoxy acid herbicides on using nano-LC.     

Online transient micellar phase concentration of anions using CTAB in CE

A transient micellar phase extractor using CTAB was described for the online sample concentration of various anionic analytes (drugs and herbicides) in CE. Stacking and separation was performed at neutral pH in coelectroosmotic flow in a hexadimethrine bromide coated fused‐silica capillary. A micellar plug (e.g. 10 mM CTAB) was injected prior to hydrodynamic injection of the analytes prepared in aqueous organic solvent (e.g. with 30% ACN). In the presence of an electric field, the micelles interacted with the anions inside the capillary. This was followed by selective analyte focusing via the mechanism of micelle to solvent stacking. The micelles acted as transient extractor because the stacking ends when the injected micelles completely migrated through the boundary between the sample and micellar plug. Fundamental studies were performed (effect of surfactant concentration, etc.) and the technique yielded 13‐ to 30‐fold improvements in peak height. A stacking CE method in conjunction with liquid–liquid extraction was also tested for the detection of the herbicides fenoprop and mecoprop in fortified drinking water at analyte concentration levels relevant to Australian Drinking Water Guidelines.     

Enantiomeric separation of chiral phenoxy acid herbicides by electrokinetic chromatography. Application to the determination of analyte-selector apparent binding constants for enantiomers

The enantiomeric resolution of chiral phenoxy acid herbicides was performed by electrokinetic chromatography using a cyclodextrin as chiral pseudophase (CD-EKC). A systematic evaluation of several neutral and charged cyclodextrins was made. Among the cyclodextrins tested, (2-hydroxy)propyl beta-cyclodextrin (HP-beta-CD) was found to be the most appropriate for the enantioseparation of phenoxy acids. The influence of some experimental conditions, such as nature and pH of the background electrolyte, chiral selector concentration, and temperature, on the enantiomeric separation of phenoxy acids was also studied. The use of a 50 mM electrolyte solution in ammonium formate at pH 5 and a temperature of 40 degrees C enabled the enantiomeric resolution of four of the six phenoxy acids investigated (2-phenoxypropionic acid, 2(3-chlorophenoxy)propionic acid, 2-(4-chlorophenoxy)propionic acid, and 2-(2,4-dichlorophenoxy)propionic acid) obtaining migration times ranging from 9 to 15 min. Mixtures of the two phenoxy acids not enantiomerically resolved (2-(4-chlorophenoxy)-2-methylpropionic acid and 2-(2,4,5-trichlorophenoxy)propionic acid) and up to three of the phenoxy acids enantiomerically resolved were separated in about 15 min. Finally, the apparent binding constants for each enantiomer-HP-beta-CD pair were calculated at two temperature values (20 and 40 degrees C).     

Automatic measurement of bentazone and phenoxy acid herbicides by HPLC with three different detectors

Summary An automated procedure is described for the simultaneous UV, fluorescence and electro chemical detection in series of bentazone and six phenoxy acid herbicides. A two step liquid-liquid extraction with dichloromethane giving an enrichment about a thousand fold has been used. Recovery rates after enrichment and detection limits of bentazone, 2,4 D, 2,4 DB, MCPA, MCPB, MECOPROP and DICHLORPROP in river water are given. It is concluded that the herbicides examined can be detected at levels between 20 ngl^–1 without the necessity of derivatisation.     

Fast enantiomeric separation with vancomycin as chiral additive by co-electroosmotic flow capillary electrophoresis: increase of the detection sensitivity by the partial filling technique

A fast and sensitive method is described by using vancomycin as a chiral additive for enantiomeric separation by capillary electrophoresis (CE). In order to overcome disadvantages associated with use of vancomycin as chiral additive in CE, several strategies including the dynamic coating technique, the co-electroosmotic flow technique, and the partial filling technique were employed sequentially in this method. Using the polycationic polymer hexadimethrine bromide (HDB) as a buffer additive, the capillary wall was dynamically coated with a thin film formed by the adsorbed HDB. Consequently, the adsorption of vancomycin onto the capillary wall was minimized via electrostatic repulsion between the coating of the capillary wall and the vancomycin molecule. In addition, the reversed electroosmotic flow (from cathode to anode) produced by the positively charged capillary wall migrates in the same direction of negatively charged analytes (co-electroosmotic flow electrophoresis). Thereby the electrophoretic mobility of negatively charged analytes were drastically accelerated leading to a short separation time of less than 3.4 min. The separation time was further reduced by the use of a short-end-injection technique. For example, the analysis time was achieved by as short as 55 s for a baseline separation of dansyl-alpha-amino-n-butyric acid. Concurrently, the partial filling technique was used to avoid the loss of detection sensitivity caused by the presence of vancomycin in the running buffer. The effect of several parameters, such as HDB concentration, buffer pH, plug length of the chiral selector, concentration of the chiral selector and applied voltage, on enantioselectivity were investigated toward optimization. Besides the advantage of a very short separation time, the method is characterized by high detection sensitivity, high selectivity, and high efficiency.     

Enatiomeric analysis of simendan by CE with beta-CD as chiral selector compared with CMPA-HPLC

The chiral separation of simendan enantiomers using capillary electrophoresis was studied with beta-cyclodextrin (beta-CD) as chiral selector. The influences of the concentration and pH of borate buffer solution, beta-CD concentration and methanol content in the background electrolyte were investigated. These factors were compared with those in an HPLC with beta-CD as chiral mobile phase additive (CMPA-HPLC). The quantification properties of the developed CE method were examined. A baseline separation of simendan enantiomers was achieved in the background electrolyte of 20 mmol/L borate buffer (pH 11.0) containing 12 mmol/L beta-CD-methanol (50:50 in volume ratio). The CE method is comparable with CMPA-HPLC in chiral resolution, although the optimal pH in CE (11.0) is much higher than that (6.0) in CMPA-HPLC. This chiral CE method is applicable to the quantitative ananlysis and enantiomeric excess value determination of L-simendan.     

Optimization of a capillary electrophoresis method to determine the chiral purity of a drug

Capillary electrophoresis (CE) using hydroxypropyl-β-cyclodextrin (HP-β-CD) in the separation buffer was investigated to determine the overall chiral purity of a drug containing a single stereogenic center. The effects of primary factors —pH, buffer components, buffer concentration, cyclodextrin concentration and sample amount (concentration and injection volume) — on the resolution of the enantiomers were investigated. Secondary factors such as the HP-β -CD source, lot and degree of substitution that were expected to affect the robustness of the assay were investigated also. The linearity, quantitation limit for the trace enantiomer and the precision of the measurements were determined. This study shows that understanding and optimizing the assay conditions leads to a chiral CE separation that is comparable to that obtained by chiral HPLC. However, chiral CE separations achieved with buffer additives have the advantages of shorter run times, higher numbers of theoretical plates (greater resolution), smaller amounts of chiral additive (less cost) and greater ruggedness (separation virtually independent of column properties unlike HPLC).     

Multivariate Optimization Approach for Chiral Resolution of Chlorophenoxy Acid Herbicides Using Teicoplanin as Chiral Selector in Capillary LC

In this paper, the selectivity and resolution of enantiomeric separation by capillary liquid chromatography (cLC) of racemates of phenoxy acid herbicides are modelled. The compounds studied were 2-(±)-(2,4,5-trichlorophenoxy)propanoic acid (2,4,5-TP), 2-(±)-(2,4-dichlorophenoxy)propanoic acid (2,4-DP), 2-(±)-(4-chloro-2-methyl)phenoxypropanoic acid (MCPP) and 2-(±)-[4-(2,4-dichlorophenoxy)phenoxy]propanoic acid] (diclofop acid), using a capillary column packed with silica particles modified with teicoplanin as chiral selector. Several mixtures of methanol (MeOH), water and triethylamine acetate (TEAA) buffer at different pHs have been tested as mobile phases, and experimental parameters such as resolution (Rs), retention factor (k) and enantioselectivity factor (α) have been measured in all tested conditions. The chemometric methods used to explore and to model the data were principal component analysis (PCA), stepwise multiple linear regression (stepwise-MLR) and response surface analysis (RSA). The results show that it is possible to quantitatively predict the quality of enantiomeric separations of related compounds in a given chromatographic system.     

Rhodium‐catalyzed Asymmetric Hydrogenation of α‐Dehydroamino Ketones: A General Approach to Chiral α‐amino Ketones

Rhodium/DuanPhos‐catalyzed asymmetric hydrogenation of aliphatic α‐dehydroamino ketones has been achieved and afforded chiral α‐amino ketones in high yields and excellent enantioselectives (up to 99 % ee), which could be reduced further to chiral β‐amino alcohols by LiAlH(tBuO)₃ with good yields_. This protocol provides a readily accessible route for the synthesis of chiral α‐amino ketones and chiral β‐amino alcohols.     

Chiral counter-current chromatography of gemifloxacin guided by capillary electrophoresis using (+)-(18-crown-6)-tetracarboxylic acid as a chiral selector

(+)-(18-crown-6)-tetracarboxylic acid (18C6H4) has been known as a highly efficient chiral selector for resolving primary amine enantiomers in capillary electrophoresis (CE). We investigated the chiral separation of gemifloxacin using 18C6H4 in analytical counter-current chromatography (CCC). The separation conditions for CE, including the binding constant, pH, and run buffer constituents, provided a helpful guideline for chiral CCC. A successful separation of gemifloxacin enantiomers could be achieved using a two-phase solvent system composed of 1-butanol-ethyl-acetate-bis(2-hydroxyethyl)aminotris(hydroxymethyl)methane acetate buffer with a small amount of 18C6H4. The hydrophobicity of the solvent system and the 18C6H4 concentration were varied to optimize the chiral separation.     

Enantiomeric separation by capillary electrochromatography. II. Chiral separation of dansyl amino acids and phenoxy acid herbicides on sulfonated silica having surface-bound hydroxypropyl-beta-cyclodextrin

A chiral silica-based stationary phase having surface-bound hydroxypropyl-beta-cyclodextrin (HP-beta-CD) with a relatively strong electroosmotic flow (EOF) was introduced for enantioseparation by capillary electrochromatography (CEC). The stationary phase contained a hydrophilic sulfonated sublayer to which a chiral top layer of HP-beta-CD was immobilized. While the sulfonated sublayer was to provide a relatively strong EOF, the top HP-beta-CD was to confer the desired chiral recognition towards enantiomeric solutes. This HP-beta-CD sulfonated silica (CDSS) stationary phase proved useful for the rapid separation of anionic enantiomers such as dansyl amino acids and phenoxy acid herbicides. The effects of the organic modifier content, pH, and ionic strength of the mobile phase on enantioseparation were investigated. Under the optimized separation conditions, ten dansyl amino acids and six phenoxy acid herbicides were enantioseparated with a resolution greater than unity.     

Sensitive analyses of agricultural chemicals by capillary electrochromatography

Capillary electrochromatography using a monolithic matrix was employed to develop a rapid and highly efficient separation methodology for the analyses of mixtures of agrochemical importance. Using this method, ppm‐ppb detection limits for urea, carbamate, and phenoxy acid herbicides were achieved without a preconcentration technique. The detection limits were further decreased to low‐ppb levels for the same class of compounds using an on‐column preconcentration technique.     

手性环境污染物的毛细管电泳分离技术进展

围绕毛细管电泳(CE)技术近10年来在分离手性环境污染物方面的应用进行了介绍。对CE手性分离技术的特点做了简要概括,归纳了目前用于CE手性分离的手性选择剂。对CE技术在分离除草剂、杀虫剂、杀真菌剂以及多氯联苯(PCBs)等手性环境污染物方面的应用进行了综述,并对CE在手性环境污染物分离中的应用提出新的研究方向。     

Enantioselective stable isotope analysis (ESIA) of polar herbicides

Assessing the environmental fate of chiral micropollutants such as herbicides is challenging. The complexity of aquatic systems often makes it difficult to obtain hydraulic mass balances, which is a prerequisite when assessing degradation based on concentration data. Elegant alternatives are concentration-independent approaches like compound-specific isotope analysis or enantiospecific concentration analysis. Both detect degradation-induced changes from ratios of molecular species, either isotopologues or enantiomers. A combination of both—enantioselective stable isotope analysis (ESIA)—provides information on ¹³C/¹²C ratios for each enantiomer separately. Recently, Badea et al. demonstrated for the first time ESIA for the insecticide α-hexachlorocyclohexane. The present study enlarges the applicability of ESIA to polar herbicides such as phenoxy acids: 4-CPP ((RS)-2-(4-chlorophenoxy)-propionic acid), mecoprop (2-(4-chloro-2-methylphenoxy)-propionic acid), and dichlorprop (2-(2,4-dichlorophenoxy)-propionic acid). Enantioselective gas chromatography–isotope ratio mass spectrometry was accomplished with derivatization prior to analysis. Precise carbon isotope analysis (2σ?≤?0.5‰) was obtained with ≥7 ng C on column. Microbial degradation of dichlorprop, 2-(2,4-dichlorophenoxy)-propionic acid by Delftia acidovorans MC1 showed pronounced enantiomer fractionation, but no isotope fractionation. In contrast, Badea et al. observed isotope fractionation, but no enantiomeric fractionation. Hence, the two lines of evidence appear to complement each other. They may provide enhanced insight when combined as ESIA.     

Enantiomeric and isomeric separation of herbicides using cyclodextrin-modified capillary zone electrophoresis

Cyclodextrin-modified capillary zone electrophoresis (CD-CZE) was applied successfully to the enantiomeric and isomeric separation of three herbicides (imazaquin, diclofop and imazamethabenz). Commercially available cyclodextrins were evaluated for separation of the enantiomers and isomers of the three herbicides having varied molecular structures. The enantiomers of imazaquin and diclofop, and the isomers of imazamethabenz could be resolved with a resolution of ≥1.5. The resolution was found to depend on pH of the run buffer, cyclodextrin type and cyclodextrin concentration. By employing mixed cyclodextrins in the running buffer, the three herbicides were simultaneously separated in a single run. In addition, rapid (less than 3 min) enantiomeric separation is demonstrated using imazaquin as a model herbicide. The reported capillary electrophoresis (CE) methods are simple, rapid, efficient and reproducible and our results demonstrate that CE provides a powerful analytical tool for enantiomeric and isomeric separation of herbicides.     

Enantioseparation of citalopram analogues with sulfated β‐cyclodextrin by capillary electrophoresis

Capillary electrophoresis methods were developed for the enantiomeric separation of 27 citalopram analogues. Sulfated β‐cyclodextrin was the most broadly selective and useful chiral selector. The separations of most of the citalopram analogue compounds reported in this work have not been reported previously. Excellent enantiomeric separations were obtained for 26 out of 27 compounds, and most of the separations were achieved within 10 min. The effects of chemical parameters such as chiral selector types, buffer types, chiral selector and buffer concentrations, buffer pH and organic modifiers on the separation were investigated. The influence of analyte structure on separation also was examined and discussed.     

CE coupling with end-column electrochemiluminescence detection for chiral separation of disopyramide

CE with electrochemiluminescence (ECL) detection technique was successfully applied for the chiral separation of a kind of class IA antiarrhythmic racemic drug. To the best of our knowledge, this is the first report of ECL detection used in chiral CE. To get better detection sensitivity and good enantioresolution at the same time, the conditions of capillary inlet and outlet buffer were systematically optimized. Unlike the traditional chiral separation method, the buffers we used in the capillary inlet and outlet differed from each other in terms of buffer pH, ionic strength, type of BGE as well as buffer composition. Under the optimum conditions, baseline enantioseparation and highly sensitive detection of the enantiomers were achieved. Wide linear relationship of each enantiomer was achieved in the range of 5 x 10(-7) to 2 x 10(-5) mol/L with relative coefficients of 0.996 and 0.997, respectively. The detection limits were estimated to be 8 x 10(-8) and 1.0 x 10(-7) mol/L (S/N = 3) for the enantiomers, respectively. In addition, a successful application of this new method to the chiral separation of the racemic drug in spiked plasma samples confirmed the validity and applicability of the chiral CE-ECL method.     

Separation and determination of 2,4-D, dicamba and 2,4,5-T in tobacco by nonaqueous capillary electrophoresis

A practical method for residue analysis of 2,4-D, dicamba and 2,4,5-T in baked tobacco leaves has been developed using nonaqueous CE (NACE). The herbicide residues of 2,4-D, dicamba and 2,4,5-T in tobaccos were extracted by ultrasonication with ethyl acetate, followed by a cleanup procedure with gel permeation chromatography. The separation of 2,4-D, dicamba and 2,4,5-T by NACE was optimized based on orthogonal experiment design with four factors at three levels. The optimal NACE condition was established with the running buffer of 40.0 mmol/L ammonium acetate in 90% CH3CN (apparent pH 10.2), and the applied voltage of -25 kV over a capillary of 50 microm id x 46 cm (37.5 cm to the detector window), which gave a baseline separation of 2,4-D, dicamba and 2,4,5-T within 15 min. The LOD were ca. 0.4-0.6 microg/mL for the three herbicides, whereas the overall recovery ranged from 80.8 to 84.1%. The proposed method has been successfully applied to measure 300 real tobacco samples, and the residue profiles of the three herbicides in tobacco samples were obtained and evaluated.     

Graphene oxide coated capillary for chiral separation by CE

This article describes a new application of graphene oxide (GO) in CE based on the coating of fused silica capillary for chiral separation. The coated capillary was characterized by SEM, energy dispersive X‐ray spectroscopy, and Raman spectra. The results indicated that the capillary was coated with GO. Chiral separations were carried out in the GO‐coated capillary for the ephedrine–pseudoephedrine (E‐PE) isomers and β‐methylphenethylamine (β‐Me‐PEA) isomers at the optimal buffer conditions without any chiral selector by CE. The precision and reproducibility of GO‐coated capillary were investigated, and the RSDs of migration time (n = 6) for the E‐PE isomers were 1.35–1.41%, and 0.97–3.50% for β‐Me‐PEA isomers, respectively. The LOD for E‐PE isomers and β‐Me‐PEA isomers was 3 μM and 18 μM, respectively.     

Enantiomeric separation of R,S‐tolterodine and R,S‐methoxytolterodine with negatively charged cyclodextrins by capillary electrophoresis

The methods for separation of R,S‐tolterodine and R,S‐methoxytolterodine enantiomers using sulfated α‐, β‐CD and phosphated‐γ‐CD by CE in acidic BGE based on Tris/phosphate pH 2.5 buffer were developed. Sulfated α‐ and β‐CD allow anodic detection while phosphated‐γ‐CD allows only cathodic detection of the separated enantiomers. The influence of chiral selector (CS)'s concentration as well as the influence of composition and concentration of BGE on resolutions were studied. Reversal migration order of tolterodine and methoxytolterodine enantiomers was observed, when sulfated‐α‐ and sulfated‐β‐CD were used. The developed methods with all three studied CSs, were validated and compared. All proposed methods enable determination of 0.2% of S‐tolterodine as an optical impurity in pills, however the method with phosphated‐γ‐CD provided lower detection limit, better repeatability of peak areas and migration times, and also lower consumption of CS. Developed method employing phosphated‐γ‐CD that was applied for the determination of optical purity of R‐tolterodine in commercial pills.