Acids In Capillary Electrophoresis Research Articles

Evaluation of deep eutectic solvents chiral selectors based on lactobionic acid in capillary electrophoresis.

Recently, deep eutectic solvents (DESs) have attracted considerable interest in analytical chemistry. This work described the enantioseparations of twenty amino alcohol drugs with several DESs based on lactobionic acid (LA) as the sole chiral selector in capillary electrophoresis (CE) firstly. Compared to the single LA system and the ionic liquid/LA synergistic system, the DES system exhibited considerably improved separations. The influences of some key parameters on separations were investigated in detail. This work also experimentally demonstrated that the carboxyl group was indispensable in the process of chiral recognition. The mechanisms of the improvements of DESs on enantioseparations were studied via ultraviolet spectroscopy. Furthermore, the proposed method was used to determine the enantiomeric purity of propranolol hydrochloride successfully. This is the first time that chiral DESs were utilized as the sole chiral selectors in CE, and this strategy has opened up a new prospect for the use of DESs in enantioseparation.

Use of Tetrabutylammonium Bromide and l-Arginine-Based Deep Eutectic Mixture in Combination with Beta-Cyclodextrin for Chiral Discrimination of Amino Acids in Capillary Electrophoresis

Use of Tetrabutylammonium Bromide and l-Arginine-Based Deep Eutectic Mixture in Combination with Beta-Cyclodextrin for Chiral Discrimination of Amino Acids in Capillary Electrophoresis

Enhanced enantioselectivity of tartaric acid in capillary electrophoresis: From tartaric acid to tartaric acid-based ionic liquid

This work presents, for the first time, a systematic and step-by-step methodology on how to improve the enantioseparation performance of tartaric acid (TT) in capillary electrophoresis (CE) using ionic liquid (IL) technology. A series of chiral and achiral ILs were applied in different separation modes to evaluate their influence on TT's enantioselectivity. Unlike previous ILs-related chiral CE studies, which typically report positive synergistic effect, in this work, negative synergistic effect was observed between TT and most chiral ILs (including several amino acids ILs and a saccharide-based IL). Instead, achiral ILs (the precursors of chiral ILs, mainly tetraalkylammonium hydroxides) exhibited significantly better performance in system modifications when used in combination with TT. The influence of the configuration of TT and chiral ILs on enantioseparation was investigated to explore the synergistic mechanism. The result showed that the existence of the chiral part of the ILs may cause interference rather than facilitation in the selector-selectand interactions. On the basis of the above results, we further designed a novel TT-based IL-type chiral selector, di-tetramethylammonium-D-tartrate ((TMA)2-D-TT). It is able to achieve excellent enantioseparation of all model analytes without adding any other chiral or achiral additives. This study demonstrates that TTs, which have almost been abandoned in chiral CE, are still able to provide satisfactory enantioselectivities with the help of ILs. In some application modes, e.g., under the achiral ILs modification or using TT-based ILs, their chiral separation performance is comparable or even superior to the well-established and widely used TT ester-type chiral selectors

Highly Sensitive Analysis in Capillary Electrophoresis Using Large-volume Sample Stacking with an Electroosmotic Flow Pump Combined with Field-amplified Sample Injection.

To achieve highly sensitive analysis without labor-intensive experimental procedures in capillary electrophoresis (CE), large-volume sample stacking with an electroosmotic flow pump (LVSEP)-field-amplified sample injection (FASI) was combined with a dynamic coating technique. In this study, poly(vinyl pyrrolidone) (PVP) was employed for the dynamic coating additive. Since a standard fluorescent dye, fluorescein, was well concentrated in a conventional LVSEP, the PVP dynamically-coated capillaries can be also applied to the LVSEP-FASI analysis. In our home-made CE apparatus, however, current breakdown was often caused, especially at a longer electrokinetic injection time due to bubble formation. To avoid the interference of bubble formation, the distance between the tips of the electrode and the capillary in the vertical direction was changed from 0 to 2.5 cm under the magnetic stirring condition. This allowed for a long electrokinetic injection time of up to 20 min, resulting in a sensitive enhancement factor (SEF) of 34900 for fluorescein. The developed method was applied to the chiral analysis of amino acids in CE. As a result, leucine (Leu) was successfully separated in LVSEP-FASI with SEFs of 6420 and 4500 for the D- and L-Leu peaks, respectively.

Cationic permethylated 6-monoamino-6-monodeoxy-β-cyclodextrin as chiral selector of dansylated amino acids in capillary electrophoresis

The resolution power of permethylated 6-monoamino-6-monodeoxy-βCD (PMMABCD) – a single isomer, cationic CD derivative – developed previously for chiral analyses in capillary electrophoresis was further studied here. Dansylated amino acids (Dns-AA) were chosen as amphoteric chiral model compounds. Changes in the resolutions of Dns-AAs by varying pH and selector concentrations were investigated and correlated with their structures and chemical properties (isoelectric point and lipophilicity). Maximal resolutions could be achieved at pH 6 or pH 4. The separations improved with increasing concentration of the selector. Baseline or substantially better resolution for 8 pairs of these Dns-AAs could be achieved. Low CD concentration was enough for the separation of the most apolar Dns-AAs. Chiral discrimination ability of PMMABCD was demonstrated by the separation of an artificial mixture of 8 Dns-AA pairs.

Novel methoxypropylimmidazolium β-cyclodextrin for improved enantioseparation of amino acids

A new single-isomer cationic cyclodextrin, mono-6A-[3-(3-methoxypropyl)imidazol-1-ium]-6A-β-cyclodextrin chloride, has been synthesized and successfully used for the chiral separation of dansyl amino acids in capillary electrophoresis. With methoxy functionality, the new cationic cyclodextrin exhibits significantly improved enantioselectivities. Excellent enantioseparations for amino acids are obtained in chiral selector concentration range between 2.5mM to 15mM at pH 6.0. Chiral resolution as high as 7.3 was achieved for Dns-Aca with 5mM chiral selector. Comparison study and theoretical calculation with Wren’s model attribute the enhanced enantioseparation to the stronger inclusion complexation between amino acids and cyclodextrin. The binding constants for dansyl amino acids and the cationic cyclodextrins are calculated to be 173–253M−1, while the optimum cyclodextrin concentrations were estimated to be 4.1–7.6mM.

Electromigration behavior of nucleic acids in capillary electrophoresis under pulsed-field conditions

We have presented a study focused on the migration pattern of double-stranded DNA (dsDNA) and RNA under pulsed field conditions. By calculating the dependence of nucleic acid mobility on its molecular size in a double logarithm plot, we found that (I) dsDNA molecules proceeded by a sigmoidal migration regime which was probably related to Ogston sieving, transition regime, and reptation model. Furthermore, the transition regime disappeared if DNA was resolved in a higher molecular mass HEC. (II) The migration pattern of RNA was relevant to the denaturant used for separation. When RNA was denatured by acetic acid, its mobility parabolically declined with its molecular size. The mobility was linearly decreased with the molecular size if urea was employed as denaturant. (III) RNA may migrate by Ogston, reptation without orientation mechanism when denatured by urea, whereas these two models were not suitable for RNA if denatured by acetic acid. Even though the electrophoretic conditions of PFCE were varied, the sigmoidal, linear, parabolic migration patterns could still be observed. (IV) Under certain modulation depth, the migration time (Tm) of acetic acid decreased with the increase of average separation voltage (Va), and when RNA denatured in 4.0M urea, Tm showed a linear correlation with Va. (V) The mobility of nucleic acids increased with the growth of artificial temperature in the capillary volume due to the decrease in the viscosity of the polymer. This is the first systematic and comparative research of high molecular mass nucleic acids in PFCE, which provides us deep insight into RNA and DNA migration behavior under pulsed electric field conditions.

New Avenue for Mid-UV-Range Detection of Underivatized Carbohydrates and Amino Acids in Capillary Electrophoresis

Capillary electrophoresis (CE) appeared as an interesting alternative to chromatographic methods for carbohydrate analysis, but it can be difficult to implement, because of the lack of easily ionizable functions and chromophore groups. Recently, a promising method was proposed by Rovio et al. for the CE separation under extremely high alkaline conditions of neutral carbohydrates under their alcoholate form and their direct UV detection [Rovio et al. Electrophoresis 2007, 28, 3129-3135; and Rovio et al. J. Chromatogr. A 2008, 1185, 139-144], which is claimed to be due to the absorption of enediolate at 270 nm. Even so, most of the detected compounds in Rovio's paper (for example, sucrose) cannot give such enediolate, lacking a carbonyl group. In this work, a deeper insight was paid to the understanding of detection mechanism. In effect, unusual detection phenomena were observed in comparing reducing and nonreducing carbohydrate behaviors, which pointed to the existence of photochemical reactions in the detection window. A more systematic study of the influence of many parameters (carbohydrate nature, electrolyte pH, residence time in the detection window, and capillary diameter) was undertaken. In addition to this, most of this work was performed under cathodic (reversed) electro-osmotic flow conditions (using Polybrene-modified capillaries), to obtain much faster separations than under Rovio's conditions. This study also opens up new avenues for the detection in mid-UV range of non-UV-absorbing compounds bearing reducing moieties, such as amino acids.

Efficiency Enhancement in the Capillary Electrophoretic Separation of Aromatic Acids with Organo-Silica Nanoparticles as Pseudo-Stationary Phase

Nanoparticles modified with aminopropyl group were prepared by a single-step electrospray process and used as a pseudo-stationary phase for the separation of anions, (e.g., organic acids in capillary electrophoresis). The nanoparticles form a uniform suspension which is stable in a buffer without adding other stabilizer. The amino containing nanoparticles have significant impact on the electro-osmotic flow (EOF) of the system, which, in turn, affects the overall separation of other analytes in this system. For example, five organic acids, which were not all separable in zone electrophoresis, were fully resolved in the presence of the nanoparticles in a wide range of pH. Various experimental parameters that are related to the electrophoresis including EOF, pH, organic modifier, voltage, concentration of the running buffer and concentration of the nanoparticles have been investigated. The repeatability of the migration time and peak area in this system are better than 5% for both inter- and intra-day evaluation.

Separation of enantiomers of N-derivatives of amino acids by capillary electrophoresis using macrocyclic antibiotics

The potential for the application of macrocyclic antibiotic eremomycin as a chiral selector for the separation of enantiomers of N-derivatives of amino acids in capillary electrophoresis was estimated. The influence of several factors (the composition and pH of the running electrolyte, the concentration of the chiral selector, capillary geometry, the value of the applied voltage) on the migration of the derivatives of amino acids and enantioselectivity in the presence of eremomycin was studied in order to choose the separation conditions. Using the example of the dansyl derivatives of amino acids the enantioselectivities of vancomycin and eremomycin in capillary electrophoresis were compared.

Direct analysis of polyols using 3-nitrophenylboronic acid in capillary electrophoresis: thermodynamic and electrokinetic principles of molecular recognition

The design of boronic acid sensors for photometric detection of carbohydrates has relied on exploiting differences in the thermodynamic stability of complex formation for molecular recognition. Herein, we introduce a direct method for analysis of sugar alcohols using 3-nitrophenylboronic acid (NPBA) as an electrokinetic probe in capillary electrophoresis (CE). Dynamic complexation of neutral polyols by NPBA during electromigration allows for their simultaneous resolution and UV detection based on formation of an anionic ternary boronate ester complex in phosphate buffer. Unlike conventional boronic acid sensors, thermodynamic and electrokinetic processes in CE allow for improved selectivity for the resolution of sugar alcohol stereoisomers having different vicinal polyol chain lengths even in cases when binding affinity is similar due to differences in their complex mobility. Three complementary approaches were investigated to compare the thermodynamics of polyol chelation with NPBA, namely direct binding assays by CE, UV absorbance spectroscopy and an indirect pK (a) depression method. Overall, CE offers a convenient platform for characterization of reversible arylboronic acid interactions in free solution while allowing for direct analysis of complex mixtures of neutral/UV-transparent polyols without complicated sample handling.

Determination of the enantiomers of α‐hydroxy‐ and α‐amino acids in capillary electrophoresis with contactless conductivity detection

The enantiomers of the anions of five alpha-hydroxy acids, namely lactic acid, alpha-hydroxybutyric acid, 2-hydroxycaproic acid, 2-hydroxyoctanoic acid and 2-hydroxydecanoic acid, as well as the two alpha-amino acids aspartic acid and glutamic acid, were baseline separated and detected by CE with contactless conductivity detection. Vancomycin was employed as chiral selector and could be used with conductivity detection without having to resort to a partial filling protocol as needed when this reagent is used with UV absorbance measurements. The procedure was successfully applied to the determination of the lactic acid enantiomers in samples of milk and yogurt. Linearity was achieved in the concentration range of 10-500 micromol/L with good correlation coefficients (0.9993 and 0.9990 for L- and D-lactic acid, respectively). The LODs (3 S/N) for L- and D-lactic acid were determined as 2.8 and 2.4 micromol/L, respectively.

Prediction of Electrophoretic Mobilities of Organic Acids Using Artificial Neural Networks with Three Different Training Functions

The quantitative structure–mobility relationship study has been done to develop the correlation between the electrophoretic mobility of a number of common organic acids in capillary electrophoresis and their molecular structures. Molecular descriptors calculated from structure, were used to represent molecular structures. A subset of the calculated descriptors was used in model development. Multiple linear regression and artificial neural networks are utilized to construct the linear and non-linear prediction models. This paper focuses on investigating the role of weight update function in artificial neural networks. Therefore, artificial neural networks with three different weight update functions including Levenberg–Marquardt algorithm, gradient descent with variable learning rate back propagation and resilient back propagation were trained. Finally, obtained results using three different artificial neural networks have been compared.

Chiral separation of dansyl amino acids in capillary electrophoresis using mono‐(3‐methyl‐imidazolium)‐β‐cyclodextrin chloride as a selector

Journal of Separation ScienceVolume 30, Issue 11 p. 1806-1806 ErratumFree Access Chiral separation of dansyl amino acids in capillary electrophoresis using mono-(3-methyl-imidazolium)-β-cyclodextrin chloride as a selector This article corrects the following: Chiral separation of dansyl amino acids in capillary electrophoresis using mono-(3-methyl-imidazolium)-β-cyclodextrin chloride as selector Weihua Tang, Teng Teng Ong, Siu-Choon Ng, Volume 30Issue 9Journal of Separation Science pages: 1343-1349 First Published online: May 9, 2007 Weihua Tang, Weihua Tang Department of Chemistry, National University of Singapore, SingaporeSearch for more papers by this authorTeng Teng Ong, Teng Teng Ong Division of Chemical and Bimolecular Engineering, College of Engineering, Nanyang Technological University, Singapore. Fax: +65-6794-9220Search for more papers by this authorSiu-Choon Ng, Siu-Choon Ng [email protected] Search for more papers by this author Weihua Tang, Weihua Tang Department of Chemistry, National University of Singapore, SingaporeSearch for more papers by this authorTeng Teng Ong, Teng Teng Ong Division of Chemical and Bimolecular Engineering, College of Engineering, Nanyang Technological University, Singapore. Fax: +65-6794-9220Search for more papers by this authorSiu-Choon Ng, Siu-Choon Ng [email protected] Search for more papers by this author First published: 05 July 2007 https://doi.org/10.1002/jssc.200790042Citations: 2AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article.Citing Literature Volume30, Issue11Special Issue: ReviewsJuly 2007Pages 1806-1806 RelatedInformation

Chiral separation of dansyl amino acids in capillary electrophoresis using mono‐(3‐methyl‐imidazolium)‐β‐cyclodextrin chloride as selector

Enantioseparations of fourteen dansyl amino acids were achieved by using a positively-charged single-isomer beta-cyclodextrin, mono-(3-methyl-imidazolium)-beta-cyclodextrin chloride, as a chiral selector. Separation parameters such as buffer pH, selector concentration, separation temperature, and organic modifier were investigated for the enantioseparation in order to achieve the maximum possible resolution. Chiral separation of dansyl amino acids was found to be highly dependent on pH since the degree of protonation of these amino acids can alter the strength of electrostatic interaction and/or inclusion complexation between each enantiomer and chiral selector. In general, the chiral resolution of dansyl amino acids was enhanced at higher pH, which indicates that the carboxylate group on the analytes may interact with the imidazolium group of cationic cyclodextrin. For most analytes, a distinct maximum in enantioresolution was obtained at pH 8.0. Moreover, the chiral separation can be further improved by careful tuning of the separation parameters such as higher selector concentration (e.g. 10 mM), lower temperature, and addition of methanol. Enantioseparation of a standard mixture of these dansyl amino acids was further achieved in a single run within 30 min.

Enhancing Effect of Phenylboronic Acid Compounds and Their Interactions with the Diol Groups of Saccharides in a Capillary Electrophoresis-Chemiluminescence Detection System

In our previous study, we proposed molecular recognition of mono- and disaccharides making use of the interaction between their diol groups and p-iodophenylboronic acid in capillary electrophoresis with a chemiluminescence detection system. Here, to extend our knowledge of molecular recognition, we first examined the enhancing effects of four phenylboronic acid compounds other than p-iodophenylboronic acid i.e., 4-biphenylboronic acid, 4-octyloxyphenyl-boronic acid, 3-octyloxyphenylboronic acid, and 4-dodecyloxyphenylboronic acid, for luminol-hydrogen peroxide-horseradish peroxidase reaction in the capillary electrophoresis-chemiluminescence detection system. Only 4-biphenylboronic acid showed an enhancing effect similar to that of p-iodophenylboronic acid; the effect was determined over the range of 0.5-10 microM in this system. Second, we estimated the apparent stability constants between the diol groups of saccharides (1-methyl-D-glucoside, D-saccharose, and D-fructose) and the boronic acid moieties of the two enhancers, p-iodophenylboronic acid and 4-biphenylboronic acid. The apparent binding constants obtained here provided insight to confirm the principle of molecular recognition for the saccharides examined here.

Migration behavior of isoluminol isothiocyanate-labeled α-amino acids in capillary electrophoresis with an absorption/chemiluminescence dual detection system

In our previous study, we developed capillary electrophoresis with an ultraviolet absorption/chemiluminescence dual detection system. Here, to demonstrate one of the possible applications of the capable system as well as confirm the advantage, migration behavior of isoluminol isothiocyanate-labeled α-amino acids was examined in the capillary electrophoresis with a dual detection system. The labeled samples were first analyzed by absorption detection with an on-capillary, followed by chemiluminescence detection with an end-capillary. The system easily, rapidly, and simultaneously produced useful information concerning chemiluminescence quenching and amino group-labeling due to the presence of both absorption and chemiluminescence detections.

Molecular recognition of mono- and disaccharides through interaction with p-iodophenylboronic acid in capillary electrophoresis with a chemiluminescence detection system

Molecular recognition of mono- and disaccharides was performed making use of the interaction between their diol groups and p-iodophenylboronic acid in capillary electrophoresis (CE) with a chemiluminescence (CL) detection system. p-Iodophenylboronic acid acted as an enhancer for luminol-horseradish peroxidase-hydrogen peroxide CL reaction. p-Iodophenylboronic acid was injected as a sample into the present system to give a CL peak on the electropherogram. The CL intensities were examined using running buffers including mono- and disaccharides. The CL intensities with 1-methyl- d-glucoside, d-saccharose, d-maltose, d-glucose, and d-fructose decreased in this order. The decrease in CL intensity was based on the formation by p-iodophenylboronic acid of cyclic esters with mono- and disaccharides, particularly with those including cis-diol groups. That is, the decrease in CL intensity affected the specific complexation between p-iodophenylboronic acids and saccharides, leading to the molecular recognition of saccharides. We also report separation of a mixture of p-iodophenol and p-iodophenylboronic acid as well as estimation of the apparent binding constant between p-iodophenylboronic acid and saccharides taking advantage of their molecular recognition behavior.

Separation of enantiomers in capillary electrophoresis with contactless conductivity detection

Contactless conductivity detection is successfully demonstrated for the enantiomeric separation of basic drugs and amino acids in capillary electrophoresis (CE). Derivatization of the compounds or the addition of a visualization agent as for indirect optical detection schemes were not needed. Non-charged chiral selectors were employed, hydroxypropylated cyclodextrin (CD) for the more lipophilic basic drugs and 18-crown-6-tetracarboxylic acid (18C6H 4) for the amino acids. Acidic buffer solutions based on lactic or citric acid were used. The detection limits were determined as 0.3 μM for pseudoephedrine as an example of a basic drug and were in the range from 2.5 to 20 μM for the amino acids.

Development of migration models for acids in capillary electrophoresis using heuristic and radial basis function neural network methods

A quantitative structure-mobility relationship (QSMR) was developed for the absolute mobilities of a diverse set of 277 organic and inorganic acids in capillary electrophoresis based on the descriptors calculated from the structure alone. The heuristic method (HM) and the radial basis function neural networks (RBFNN) were utilized to construct the linear and nonlinear prediction models, respectively. The prediction results were in agreement with the experimental values. The HM model gave a root-mean-square (RMS) error of 3.66 electrophoretic mobility units for the training set, 4.67 for the test set, and 3.88 for the whole data set, while the RBFNN gave an RMS error of 2.49, 3.19, and 2.65, respectively. The heuristic linear model could give some insights into the factors that are likely to govern the mobilities of the compounds, however, the prediction results of the RBFNN model seem to be better than that of the HM.