Pseudostationary Phase In Capillary Electrophoresis Research Articles

Deep eutectic solvents as a green alternative to organic solvents for β-cyclodextrin pseudo-stationary phase in capillary electrophoresis

β-cyclodextrin (β-CD), as an important pseudo-stationary phase (PSP) in capillary electrophoresis (CE), frequently confronts challenges stemming from its limited water solubility, particularly when high concentrations are required for resolving complex analytes. Traditionally, researchers often resort to the use of (toxic) organic solvents to enhance the solubility of β-CD, establishing non-aqueous capillary electrophoresis (NACE) for specific separations. However, such practices are hazardous to health and run counter to the principles of green analytical chemistry. In this study, we demonstrate a deep eutectic solvent (DES), Proline:Urea (PU), as a promising alternative to conventional organic solvents for β-CD-based CE separations. The DES exhibits a solubility of up to 30% for β-CD, a significant improvement compared to the 1.8% solubility in the aqueous phase. Utilizing this DES-type separation medium, we achieved simultaneous baseline separation of a complex analyte composed of eight structurally similar naphthoic acid derivatives. Furthermore, we conducted a systematic comparison of β-CD's performance in aqueous CE buffers, organic solvents, and DESs, highlighting the superiority of this novel and environmentally friendly CE separation medium.

A feasibility study on the use of hydrophobic eutectic solvents as pseudo-stationary phases in capillary electrophoresis for chiral separations

A critical challenge in using deep eutectic solvents (DESs) in capillary electrophoresis (CE) is to develop separation systems in which a DES can really work as a single entity. To achieve this, the authors recently demonstrated a novel strategy that takes advantage of the aqueous dispersibility of hydrophobic DESs (or more accurately hydrophobic eutectic solvents (HESs)). However, the previous work was limited only to the separation of achiral analytes, e.g., analogues, homologues, and isomers. The present study was designed as a follow-up study in order to explore the feasibility of employing HES-type pseudo-stationary phases (PSPs) in CE for chiral separations. By using carboxymethyl-β-cyclodextrin (CM-β-CD) as a model chiral selector, we provide the first evidence that there is a potential synergistic effect between HESs and traditional chiral selectors. Specifically, the combined use of HES (−)-menthol:octanoic acid and CM-β-CD allowed excellent enantioseparations of several basic drugs which were not able to be resolved in the single CM-β-CD system. The enantioresolutions were significantly improved while the migration times of the enantiomers were also shortened due to the hydrophobic mechanism of the HES-type PSP. Critical factors influencing the novel chiral CE system were systematically investigated. Since HESs are considered as “designer” solvents with highly tunable properties, this study demonstrates the potential of employing HESs (or HDES)-type PSPs in CE for chiral separations.

Enantiomeric separation of β-blockers and tryptophan using heparin as stationary and pseudostationary phases in capillary electrophoresis.

The separation methods of the enantiomers of two β-blockers and tryptophan were studied using capillary electrochromatography with heparin covalently as well as non-covalently, bonded onto the capillary inner wall as stationary phase and electrokinetic chromatography with heparin as pseudostationary phase. In the case of heparin, used as a stationary phase, the method was unable to resolve enantiomers in both cases β-blockers and tryptophan. On the other hand, when heparin was used as a pseudostationary phase, the resolution of the enantiomers was obtained only with 3-aminopropyltriethoxysilane which were immobilised onto the inner phase of the capillary. The results of this study let us infer that the electrostatic, hydrophobic, and steric interactions were involved in the separation mechanisms. The separation was achieved in less than 10minutes under the optimized conditions: 30mM phosphate buffer (pH2.5) with the adding of 15mg/mL of heparin at 15°C and 10kV. The usefulness of heparin as a chiral selector both in electrokinetic chromatography using 3-aminopropyltriethoxysilane attached to the capillary was demonstrated for the first time. The developed method was powerful, sensitive, and fast, and it could be considered an important alternative to conventional methods used for chiral separation.

Microwave assisted synthesis of metal-organic framework MIL-101 nanocrystals as sorbent and pseudostationary phase in capillary electrophoresis for the separation of anthraquinones in environmental water samples.

In this work, a CE method was developed to separate five anthraquinones: aloe-emodin, rhein, emodin, chrysophanol, and physcion. The CE method used a nano-sized metal organic framework MIL-101 (nMIL-101) as pseudostationary phase (PSP) and sorbent for dispersed particle extraction (DPE). The nMIL-101 was synthesized by microwave technique and was characterized by UV-vis, TEM, Zeta potential, X-ray diffraction spectrometry and micropore physisorption. In this method, anthraquinones were adsorbed by nMIL-101 of a fast kinetics within 10 min and then separated by CE. The CE conditions were optimized considering time, pH, buffer ionic strength, and nanoparticles concentration. The optimal CE condition is using 20 mM sodium borate buffer (pH 9.1) containing 15% methanol (v/v) and 400 mg/L nMIL-101 as additives within 8 min. The LODs varied from 24 to 57 μg/L, which were lower than those previously reported. Our method has been successfully applied to determine trace anthraquinones in environmental water samples.

Separation of metal ions via capillary electrophoresis using a pseudostationary phase microfunctionalized with carbon nanotubes

This study reports on the use of dispersed multi-walled carbon nanotubes (MWCNTs) as a novel pseudostationary phase in capillary electrophoresis for the separation and determination of mono- and divalent metal ions. The use of a background electrolyte containing 1 μg⋅mL−1 of dispersed MWCNTs results in good analytical linearity, in detection limits in the range from 18.5 to 124 ng⋅mL−1, and in limits of quantifications in the range from 61 to 409 ng⋅mL−1. The method was applied to the analysis of the ions K(I), Ba(II), Ca(II), Na(I), Mg(II), Co(II), Ni(II), Zn(II), Li(I) and Cd(II) in spiked honey, and mean recoveries were found to be between 80.0 and 106.7%.

Interaction between Antibacterial Peptide Apep10 and Escherichia coli Membrane Lipids Evaluated Using Liposome as Pseudo-Stationary Phase.

Liposomes constructed from Escherichia coli membrane lipids were used as a pseudo-stationary phase in capillary electrophoresis and immobilised liposome chromatography to evaluate the interaction between antibacterial peptide (ABP) Apep10 and bacterial membrane lipids. The peptide mobility decreased as the concentration of liposomes increased, providing evidence for the existence of this interaction. The binding constant between Apep10 and the Escherichia coli membranes lipid liposome was higher than that of Apep10 with a mixed phospholipids liposome at the same temperature. The capillary electrophoresis results indicate that the binding ability of Apep10 with a liposome was dependent on the liposome’s lipid compositions. Thermodynamic analysis by immobilised liposome chromatography indicated that hydrophobic and electrostatic effects contributed to the partitioning of Apep10 in the membrane lipids. The liposomes constructed from bacterial membrane lipid were more suitable as the model membranes used to study dynamic ABP/membrane interactions than those constructed from specific ratios of particular phospholipids, with its more biomimetic phospholipid composition and contents. This study provides an appropriate model for the evaluation of ABP-membrane interactions.

Separation and stacking of iodine species from seafood using surfactant-coated multiwalled carbon nanotubes as a pseudo-stationary phase in capillary electrophoresis

This article describes an on-line technique for hydrodynamic injections of long sample plugs with simultaneous stacking of iodine species in capillary electrophoresis (CE) with diode array detection. Surfactant-coated multi-walled carbon nanotubes (SC-MWNTs) were used as a pseudostationary phase for the separation of iodate, tetraiodothyronine,triiodothyronine, diiodothyronine, and diiodotyrosine. The effects of MWNTs, concentration of SC-MWNTs and salt, of buffer pH value, injection times were examined. Under the optimized conditions, i.e. detection at 230 nm, a separation voltage 25 kV, a borate running buffer of pH 7.5 and a SC-MWNT concentration of 9 μg · mL−1, the method gave relative standard deviations of the retention times and peak areas in intra-day assays (for n = 6) and for inter-day assays (for n = 3) of less than 4.49 and 5.80 %, respectively. The CE method was then applied to the analysis of the above iodine species in (spiked) kelp (kunbu) and porphyra, and recoveries ranged from 81.6 to 98.4 % with RSDs (n = 3) for extraction repeatability of <3.39 % in all cases.

Preparation of chitosan-modified silica nanoparticles and their applications in the separation of auxins by capillary electrophoresis.

In recent years, nanoparticles have gained more attention when used in separation science. In this study, chitosan-modified silica nanoparticles were successfully synthesized and characterized by transmission electron microscopy, elemental analysis and zeta potential measurements, etc. When added into the running buffer solution as pseudo-stationary phase in capillary electrophoresis, the separation of four representative auxins, i.e., indole-3-acetic acid, indole butyric acid, 2,4-dichlorophenoxyacetic acid, 1-naphthaleneacetic acid, was carried out. Some important factors, such as the nanoparticles concentration, the pH and concentration of the running buffer solution, were also investigated on the separation. Under optimized experimental conditions, all the auxins investigated can be baseline separated within 5 min with higher column performance. The method established can also be used for quantitative analysis. The relative standard deviations obtained for indole-3-acetic acid, indole butyric acid, 2,4-dichlorophenoxyacetic acid, 1-naphthaleneacetic acid were in the range of 1.6-5.7% for peak area and 0.53-1.60% for migration time. The calibration curves obtained from the peaks areas for auxins were linear in the range of 0.1-80 mg/L with the correlation coefficients of 0.994-0.999. The limit of detection (S/N = 3) was 11-75 μg/L. The developed method was also successfully used for the determination of auxins in fruits and vegetables samples with good recoveries.

ChemInform Abstract: Application of Resorcinarene Derivatives in Chemical Separations

AbstractReview: 62 refs.

Gold nanomaterials based pseudostationary phases in capillary electrophoresis: a brand-new attempt at chondroitin sulfate isomers separation.

In this work, a CE method with bare gold nanorods (GNRs) based pseudostationary phase was developed and applied for the separation of chondroitin sulfate (CS) isomers, CS, and dermatan sulfate (DS). The separation efficiency was investigated by varying the experimental parameters such as concentration and pH of the BGE, separation voltage, internal diameter of capillary, different size, and morphology of gold nanomaterials. Results showed that different size and morphology of gold nanomaterials had different effects on the separation of CS and DS. The best separation of CS and DS was achieved in the BGE composed of aqueous 150 mmol/L (mM) ethylenediamine + 20 mM sodium dihydrogen phosphate + 30% v/v GNRs, pH 4.5, at the separation voltage of -10kV. Capillary was 59.2 cm in length (effective length 49 cm), 50μm id capillary thermostated at 25°C. CE with bare GNRs used as pseudostationary phase was shown to be a suitable technique for the separation of CS and DS mixtures with wider peaks. RSD of migration time and peak area of CS and DS were 0.13, 0.14 and 0.86, 1.07%, respectively.

Dendritic glycopolymers as dynamic and covalent coating in capillary electrophoresis: View on protein separation processes and detection of nanogram-scaled albumin in biological samples

Unique properties of dendritic polymers make them promising candidates for application as additives in various analytical methods. From this point, we investigated the potential use of maltose-modified hyperbranched poly(ethylene imine) (PEI-Mal) as a dynamically or covalently bound coating and a pseudostationary phase in capillary electrophoresis. The EOF mobilities were measured at different pH values (2.2, 8.5, and 10.2) using PEI-Mal in the background electrolyte (BGE) and desirable repeatability of the EOF with % RSD (n=50) ≤3.3 was obtained. The influence of pH, polymer concentration, and density of maltose shell on the separation properties of a model mixture of four proteins (albumin, lysozyme, myoglobin, insulin) were investigated. Applying PEI-Mal as a dynamic coating, improved separation of the protein mixture with a high repeatability was achieved. Applying PEI-Mal as a covalent coating for concentrating proteins in the large volume sample stacking (LVSS) combined with the field-enhanced sample injection (FESI), up to 1320-fold enhancement of sensitivity was achieved. The detection limit of 100–500ng/ml allowed successful analysis of albumin level both in blood and urine samples without additional preconcentration.

Dispersive solid-phase microextraction and capillary electrophoresis separation of food colorants in beverages using diamino moiety functionalized silica nanoparticles as both extractant and pseudostationary phase

In this work, a new method for the determination of food colorants in beverage samples is developed, using diamino moiety functionalized silica nanoparticles (dASNPs) as both adsorbents in dispersive solid-phase microextraction (dSPME) and pseudostationary phases (PSPs) in capillary electrophoresis (CE) separation. dASNPs were firstly used as adsorbents for the preconcentration of four colorants by the dSPME process. After that, colorants were efficiently separated by CE using 30mM phosphate buffer (pH 6.0) containing 2mM β-CD and 0.9mg/mL dASNPs as additives. All factors influencing dSPME and CE separations were optimized in detail. The investigated analytes showed good linearities with correlation coefficients (R2) higher than 0.9932. The limits of detection for the four food colorants were between 0.030 and 0.36mg/L, which are lower than those reported previously. The established method was also used to analyze four colorants in beverage samples with recoveries ranging from 82.7% to 114.6%. To the best of our knowledge, this is the first time to use NPs both as extractants in dSPME and pseudostationary phases in CE for the analytical purpose.

ChemInform Abstract: Recent Applications of Carbon Nanotube Sorbents in Analytical Chemistry

AbstractReview: 273 refs.

Separation of total lipids on human lipoproteins using surfactant‐coated multiwalled carbon nanotubes as pseudostationary phase in capillary electrophoresis

Surfactant-coated multiwalled carbon nanotubes (MWNTs) were used as pseudostationary phase (PSP) in CE to investigate the total lipids of high-density lipoproteins and low-density lipoproteins. To optimize the CE conditions, several experimental factors including carbon nanotube concentration, bile salt concentration, sodium phosphate (PB) concentration, organic modifier concentration and buffer pH value have been examined. In addition, the CE capillary temperature and applied voltage have also been examined. The optimal separation buffer selected was a mixture of 3.2 mg/L MWNT, 50 mM bile salt, 10 mM PB, 20% 1-propanol, pH 9.5. The optimal capillary temperature and applied voltage selected were 50°C and 20 kV, respectively. Phosphatidyl choline (PC) has been used as a model analyte and investigated by the optimal CE method. The linear range for PC was 0.1-3 mg/mL with a correlation coefficient of 0.9934, and the concentration LOD was 0.055 mg/mL. The optimal CE method has been used to characterize the total lipids of high-density lipoprotein and low-density lipoprotein. At absorbance 200 nm, one major peak and two or three minor peaks showed for the total lipids of lipoproteins within 13 minutes. Resolutions of the total lipids were enhanced using surfactant-coated MWNTs as PSPs in the CE separation buffer. However, resolutions of the total lipids were not enhanced using surfactant-coated single-walled carbon nanotubes as PSPs in the CE separation buffer.

Application of resorcinarene derivatives in chemical separations

The applications of resorcinarene derivatives in modern separation techniques including high performance liquid chromatography (HPLC), gas chromatography (GC), capillary electrophoresis (CE), ion chromatrography (IC), and liquid membranes are reviewed in this paper. Resorcinarenes are macrocyclic compounds which can be modified with various substituents on upper and lower rims to provide specific functionality and selectivity. The derivatives can be adsorbed or covalently bound to the stationary phase of HPLC, GC, and IC, or used as pseudo-stationary phase in CE to separate organic or inorganic species. They have been tested as carriers to selectively transfer species from source phase to receiving phase in bulk liquid membranes, supported liquid membranes, and polymer inclusion membranes. These broad applications of resorcinarene-based macrocyclic ligands indicate that they are a promising class of compounds that are worthwhile to be explored in both synthesis and separation applications.

ChemInform Abstract: Carbon Nanotubes: Solid‐phase Extraction

AbstractReview: 181 refs.

Application of Protein-Liposome Conjugate as a Pseudo-Stationary Phase in Capillary Electrophoresis

Liposomes have very similar structure to cell plasma membranes. Using liposomes as stationary phase in liquid chromatography (LC) or micellar electrokinetic chromatography (MEKC) has been demonstrated to be a good, dynamic method for the study of the interaction between cell membranes and important biomolecules. There has been no report on integrating plasma membrane proteins with phospholipids as pseudo-stationary phase in MEKC. In this paper, a novel mode of capillary electrophoresis (CE) is developed, that is, protein-liposome conjugate. This protein-liposome biomimetic membrane is demonstrated for the first time to be applicable as pseudo-stationary phase in MEKC. The protein is able to significantly improve chromatographic performance and stability. The experimental phenomena are further confirmed in terms of specific capacity factors and free binding energy. This new CE mode is used to investigate the interaction between dopamine transporter and dopamine-nomifensine.

Synthesis, Characterization, and Application of Chiral Ionic Liquids and Their Polymers in Micellar Electrokinetic Chromatography

Two amino acid-derived (leucinol and N-methylpyrrolidinol) chiral ionic liquids are synthesized and characterized in both monomeric and polymeric forms. Leucinol-based chiral cationic surfactant is a room-temperature ionic liquid, and pyrrolidinol-based chiral cationic surfactant melts at 30-35 degrees C to form an ionic liquid (IL). The monomeric and polymeric ILs are thoroughly characterized to determine critical micelle concentration, aggregation number, polarity, optical rotation, and partial specific volume. Herein, we present the first enantioseparation using chiral IL as a pseudostationary phase in capillary electrophoresis. Chiral separation of two acidic analytes, (+/-)-alpha-bromophenylacetic acid and (+/-)-2-(2-chlorophenoxy)propanoic acid (+/-)-(2-PPA) can be achieved with both monomers and polymers of undecenoxycarbonyl-L-pryrrolidinol bromide (L-UCPB) and undecenoxycarbonyl-L-leucinol bromide (L-UCLB) at 25 mM surfactant concentration using phosphate buffer at pH 7.50. The chiral recognition seems to be facilitated by the extent of interaction of the acidic analytes with the cationic headgroup of chiral selectors. Polysodium N-undecenoxycarbonyl-L-leucine sulfate (poly-L-SUCLS) and polysodium N-undecenoxycarbonyl-L-leucinate (poly-L-SUCL) were compared at high and low pH for the enantioseparation of (+/-)-(2-PPA). At pH 7.5, poly-L-SUCLS, poly-L-SUCL, and (+/-)-(2-PPA) are negatively charged resulting in no enantioseparation. However, chiral separation was observed for (+/-)-(2-PPA) using poly-L-SUCLS at low pH (pH 2.00) at which the analyte is neutral. The comparison of chiral separation of anionic and cationic surfactants demonstrates that the electrostatic interaction between the acidic analyte and cationic micelle plays a profound role in enantioseparation.

Use of liposomes as a dispersed pseudo-stationary phase in capillary electrophoresis of basic proteins

Use of liposomes as a dispersed pseudo-stationary phase in capillary electrophoresis of basic proteins

Use of liposomes as a dispersed pseudo-stationary phase in capillary electrophoresis of basic proteins

The suitability of liposome capillary electrophoresis for separating basic proteins at different pH values in the acidic domain was evaluated preparing liposomes consisting of large unilamellar vesicles of the phospholipid 1-palmitoyl-2-oleyl- sn-glycero-3-phosphocholine (POPC), whose hydrodynamic diameter and size distribution were evaluated by dynamic light scattering measurements. The study was conducted evaluating different approaches of performing liposome capillary electrophoresis of proteins, using liposomes dispersed in the electrolyte solution of different composition and pH values ranging from pH 6.2 to 4.0. The liposomes were employed as a pseudo-stationary phase dispersed in the electrolyte solution, which was introduced into the capillary as a plug of volume equivalent to that of the capillary tube, whereas liposome-free electrolyte solutions were contained into the electrolyte vessels during electrophoresis. The study was performed with either bare fused-silica capillaries or capillaries previously treated with POPC that was employed as a dynamic coating agent. Such treatment was performed rinsing the capillary tube with a proper volume of electrolyte solution containing POPC and resulted in a significant reduction of the electroosmotic flow, indicative of the adsorptive behavior of liposomes onto the internal surface of bare fused-silica capillaries. The usefulness of using liposome dynamically coated capillaries for separating basic proteins in absence of the dispersed liposome pseudo-stationary phase filled into the capillary prior to injecting the protein sample was evaluated too. The results show that the presence of liposomes reduced drastically the untoward interactions between basic proteins and the capillary wall, also in capillary not subjected to the dynamic coating process with POPC. In addition, the use of liposomes as a pseudo-stationary phase dispersed in the electrolyte solution enhanced the separation of basic proteins also in POPC dynamically coated capillaries.