Capillary Separation Research Articles

A Capillary-Force-Assisted Transfer for Monolayer Transition-Metal-Dichalcogenide Crystals with High Utilization.

The capillary-force-assisted transfer has shown application potential for constructing two-dimensional (2D) electronic and optoelectronic devices for the advantage of free of spin coating the organic compound and etching the substrate. Currently, the transfer mechanism remains obscure. The capillary adhesion mechanism and capillary invasion separation mechanism were proposed independently and rarely discussed in a comprehensive manner. What is more, the integrity and utilization remain to be improved. Here, we developed the capillary-force-assisted transfer method with high utilization and integrity. Uniformity of water transport was improved by introducing water from the sidewall of the small polydimethylsiloxane (PDMS) stamp driven by capillary force. The transfer integrity rate increased, and the location of the complete samples became predictable. The transfer utilization increased as the limited water transportation minimized the impact on the surrounding WS2. The monolayer triangle WS2 crystals from adjacent areas on the sapphire substrate were transferred one after another. Besides, local mechanical exfoliation of the continuous WS2 thin films was demonstrated, implying that the capillary adhesion is strong enough to break the strong in-plane covalent bond and overcome the van der Waals force between WS2 and sapphire substrate. Finally, the water transport model between two surfaces with different hydrophobicity combinations was derived on the basis of the Young-Laplace equation. The analysis of water transport between different interfaces reveals how capillary adhesion and capillary invasion work together to achieve capillary force transfer. This study highlights the potential of the capillary-force-assisted transfer as an efficient technique for fabricating van der Waals structures based on two-dimensional atomic crystals, especially periodic structures.

Nanoflow Sheath Voltage-Free Interfacing of Capillary Electrophoresis and Mass Spectrometry for the Detection of Small Molecules.

Coupling capillary electrophoresis (CE) to mass spectrometry (MS) is a powerful strategy to leverage a high separation efficiency with structural identification. Traditional CE-MS interfacing relies upon voltage to drive this process. Additionally, sheathless interfacing requires that the electrophoresis generates a sufficient volumetric flow to sustain the ionization process. Vibrating sharp-edge spray ionization (VSSI) is a new method to interface capillary electrophoresis to mass analyzers. In contrast to traditional interfacing, VSSI is voltage-free, making it straightforward for CE and MS. New nanoflow sheath CE-VSSI-MS is introduced in this work to reduce the reliance on the separation flow rate to facilitate the transfer of analyte to the MS. The nanoflow sheath VSSI spray ionization functions from 400 to 900 nL/min. Using the new nanoflow sheath reported here, volumetric flow rate through the separation capillary is less critical, allowing the use of a small (i.e., 20 to 25 μm) inner diameter separation capillary and enabling the use of higher separation voltages and faster analysis. Moreover, the use of a nanoflow sheath enables greater flexibility in the separation conditions. The nanoflow sheath is operated using aqueous solutions in the background electrolyte and in the sheath, demonstrating the separation can be performed under normal and reversed polarity in the presence or absence of electroosmotic flow. This includes the use of a wider pH range as well. The versatility of nanoflow sheath CE-VSSI-MS is demonstrated by separating cationic, anionic, and zwitterionic molecules under a variety of separation conditions. The detection sensitivity observed with nanoflow sheath CE-VSSI-MS is comparable to that obtained with sheathless CE-VSSI-MS as well as CE-MS separations with electrospray ionization interfacing. A bare fused silica capillary is used to separate cationic β-blockers with a near-neutral background electrolyte at concentrations ranging from 1.0 nM to 1.0 μM. Under acidic conditions, 13 amino acids are separated with normal polarity at a concentration ranging from 0.25 to 5 μM. Finally, separations of anionic compounds are demonstrated using reversed polarity under conditions of suppressed electroosmotic flow through the use of a semipermanent surface coating. With a near-neutral separation electrolyte, anionic nonsteroidal anti-inflammatory drugs are detected over a concentration range of 0.1 to 5.0 μM.

Study of the separation ability differences of three covalent organic frameworks as coated materials in capillary electrochromatography

Covalent organic frameworks (COFs) have great potential applications in chromatographic separation. So it is crucial to understand the relationship between the separation ability of COFs and their structures. Here we report a strategy to evaluate the separation ability of three 2D COFs and explore the relationship between separation ability and their molecular structures. The three 2D COFs (COF-LZU1, COF-42 and COF-LZU8) have one same building unit 1,3,5-triformylbenzene, while varied from the conjugated linking units and functional side-chains. They were used to construct coated capillary column for capillary electrochromatographic separation of same groups of phthalates. They exhibited different separation efficiencies. COF-42 and COF-LZU8 coated capillary columns provided good signal resolutions and high column efficiencies with high theoretical plate numbers. It is demonstrated that COFs with hydrazone unit and longer side-chains provided higher selectivity and resolutions for the phthalates separation. Molecular simulations and DFT calculations were further proceeded to explore the deep reason why the three COFs coated CEC displayed different separation ability based on the host−guest interactions on molecular level. This work highlights a new opportunity to select or design functional COFs and improve their efficiency in chromatographic separation based on host−guest chemistry.

In situ synthesis of a spherical covalent organic framework as a stationary phase for capillary electrochromatography

Covalent organic frameworks (COFs) are a novel type of crystalline porous organic polymer materials recently developed. It has several advantages in chromatographic separation field, such as high thermal stability, porosity, structural regularity, and large specific surface area. Here, a novel spherical COF 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 2,5-bis(2-propyn-1-yloxy)-1,4-benzenedicarboxaldehyde (BPTA) was developed as an electrochromatographic stationary phase for capillary electrochromatography separation. The COF TAPB-BPTA modified capillary column was fabricated via a facile in situ growth method at room temperature. The characterization results of scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD) confirmed that COF TAPB-BPTA were successfully modified onto the capillary inner surface. The electrochromatography separation performance of the COF TAPB-BPTA modified capillary was investigated. The prepared column demonstrated outstanding separation performance toward alkylbenzenes, phenols, and chlorobenzenes compounds. Furthermore, the baseline separations of non-steroidal anti-inflammatory drugs (NSAIDs) and parabens with good efficiency and high resolution were achieved. Also, the prepared column possessed satisfactory precision of the intra-day runs (n = 5), inter-day runs (n = 3), and parallel columns (n = 3), and the relative standard deviations (RSDs) of the retention times of tested alkylbenzenes were all less than 2.58%. Thus, this new COF-based stationary phase shows tremendous application potential in chromatographic separation field.

Surface wave and thermocapillary instabilities on flowing film under the sway of Hall viscosity

The present study investigates the influence of Hall/odd-viscosity on the surface wave and thermocapillary instabilities in a thin viscous film flowing down an inclined plane. A detailed analysis considering a generalized scaling for the velocity field to understand the effect of odd-viscosity in the hydrodynamic and thermocapillary modes of instabilities has been performed, and later the results are presented for a specific velocity scale. Starting with Orr–Sommerfeld (OS) linear stability analysis for all ranges of wavenumber and then constructing two low-dimensional models; namely, the Long wave expansion model (LWE) and the Weighted residual integral boundary layer model (WRIBL) under the assumption of small wavenumber are analysed for linear and nonlinear behaviour of the instabilities analytically and numerically. Two energy transferring mechanisms reinforce each other to the disturbance: hydrodynamic H-mode and thermocapillary S-mode are focused and found that the odd viscosity plays a stabilizing role for both the modes of instabilities. For small Reynolds number (Re) values, both LWE and WRIBL models give consistent results as that of the OS. However, for a large value of Re, WRIBL gives way better match with OS than LWE. Increasing odd viscosity flattens the streamlines in a moving frame of reference, confirming the odd viscosity’s stabilizing effect. However, being non-dissipative, odd viscosity has no significant influence on the temperature field. In a static frame, the intensity of capillary separation and flow reversal phenomena decreases with the increase of odd viscosity, confirming its stabilizing effect on the flow.

Homochiral iron-based γ-cyclodextrin metal-organic framework for stereoisomer separation in the open tubular capillary electrochromatography

Metal organic frameworks (MOFs), as a novel separation mediums, have in recent years attracted wide consideration in capillary electrochromatography (CEC). However, the instability of frameworks limited the application of the MOF-based chiral separation capillary. Herein, Iron-based γ-cyclodextrin metal-organic framework (Fe-CD-MOF) with mesoporous was developed as the chiral stationary phase in open tubular capillary electrochromatography column (OT-CEC) for the stereoisomer separation of fourteen chiral drugs and one chiral alcohol. The capillary column with MOF coating exhibited the best performance (resolution value: 17.07) for anisodamine in any MOF-based capillary column that has never been reported. Moreover, the effect of pH, buffer concentration, and methanol content has been investigated. The novel coated capillary was also applied for the quantitative analysis of a real sample. The Fe-CD-MOF coated capillary showed outstanding repeatability and stability, with the satisfactory relative standard deviations (RSDs) for intra-day, inter-day, and column-to-column. Finally, the enantioseparation mechanism of the Fe-CD-MOF coated capillary was evaluated by adsorption kinetic experiments. In summary, this work indicated the novel and renewable Fe-CD-MOF was a potential chiral stationary phase in CEC chiral separation.

The minimum information required for a glycomics experiment (MIRAGE): reporting guidelines for capillary electrophoresis.

The Minimum Information Required for a Glycomics Experiment (MIRAGE) is an initiative to standardize the reporting of glycoanalytical methods and to assess their reproducibility. To date, the MIRAGE Commission has published several reporting guidelines that describe what information should be provided for sample preparation methods, mass spectrometry methods, liquid chromatography analysis, exoglycosidase digestions, glycan microarray methods, and nuclear magnetic resonance methods. Here, we present the first version of reporting guidelines for glyco(proteo)mics analysis by capillary electrophoresis (CE) for standardized and high-quality reporting of experimental conditions in the scientific literature. The guidelines cover all aspects of a glyco(proteo)mics CE experiment including sample preparation, CE operation mode (CZE, CGE, CEC, MEKC, cIEF, cITP), instrument configuration, capillary separation conditions, detection, data analysis, and experimental descriptors. These guidelines are linked to other MIRAGE guidelines and are freely available through the project website https://www.beilstein-institut.de/en/projects/mirage/guidelines/#ce_analysis (doi:10.3762/mirage.7).

Application of Sol-Gels Modified with Natural Plants Extracts as Stationary Phases in Open-Tubular Capillary Electrochromatography.

Ethanol extracts of three widely growing plants were added to silica sol–gel solutions, which were subsequently applied as wall surface modifiers in inner quartz capillaries. Modified capillaries were used for open-tubular capillary electrochromatographic separation of nucleotides and amino groups containing biological compounds (neurotransmitters, amino acids and oligopeptides). The experiments were performed at physiological pH 7.40, and eventual changes of effective mobilities were calculated. Specific compounds characteristic for each plant were tested as sol–gel additives as well, and thus-modified capillaries were used for the separations of the same analytes under identical conditions. The aim of this study was to find out possible interactions between physiological compounds and extracts of freely available plants anchorded in the sol-gel stationary phase in the flowing system. Even though the amount of the modifier in each capillary is very small, basic statistical evaluation showed some not negligible changes in effective mobility of tested analytes. These changes were bigger than ±5% for separations of nucleotides in capillaries with curcuma, Moringa or the mixture of synthetic additives as the sol-gel aditive, and for separations of amino compounds where these changes varying by additive, analyte by analyte.

Simultaneous determination of NO released inside and outside cells at the single-cell level using CE-LIF

A simple, fast and reliable method based on capillary electrophoresis (CE) coupled with laser-induced fluorescence (LIF) detection was developed for the simultaneous analysis of NO released both inside and outside cells at the single-cell level closer to physiological conditions. After pre-capillary dual-labeling derivatization with a group of cells, single cells were injected into the separation capillary and lysed. The subsequent separation and detection of NO derivatives were achieved within 4.0 min producing mass limits of detection of 3.0 and 10.7 amol for intra- and extracellular NO, respectively. The developed method was successfully applied for simultaneous measurement of NO released both inside and outside single RAW 264.7 macrophage cells.

Capillary Gas Chromatographic Separation Performances of a Tetraphenyl Porphyrin Stationary Phase

Tetraphenyl porphyrin (TPP) has enormous potential for use as gas chromatography stationary phases because it has a distinctive extended π–π conjugated coplanar structure and a range of interesting properties such as a good solubility in dichloromethane, high melting point, and good thermal stability. In this work, a TPP column was successfully prepared using a static method. The column was nonpolar and had a high efficiency. The chromatographic selectivity of the TPP column was assessed. The TPP column showed superiority retention and higher resolution for alicyclic, aromatic molecules through ring matching and π-π stacking interaction comparable to HP-5MS column. The unique mechanisms through which the TPP column retained polychlorinated biphenyls allowed the peak pair of 2,2ʹ,5-trichlorobiphenyl and 4,4ʹ-dichlorobiphenyl to be resolved better on the TPP column than the HP-5MS column. The TPP column was thermally stable even at 260°C for 2 h and gave results of a high degree of precision (run-to-run and column-to-column) with relative standard deviations <0.05% and <4.96%, respectively. The results indicated that porphyrin derivatives will be useful gas chromatography stationary phases.

Development of Novel Analysis and Characterization Methods Utilizing Reaction Dynamics in a Separation Capillary

Electrophoretic migration of an analyte in capillary electrophoresis (CE) reflects reaction dynamics of the analyte in solution. In affinity CE, an analyte of interest interacts with a modifier added in the separation buffer in fast equilibrium, and effective electrophoretic mobility of the analyte is contributed from its equilibrium species. Precise measurement of effective electrophoretic mobility allows analyzing the equilibrium. Analysis of equilibria under CE separation possesses several advantages against traditional analyses in homogeneous solution; coexisting substances including impurities and kinetically generated substances are resolved by CE from the equilibrium species of interest. Characteristics of the CE analysis have been applied to analyses of acid-base equilibria of degradable substances and ion-association equilibria in an aqueous solution. Since CE is operated in an open-tubular capillary, it is also suitable for the characterization of carbon nanoclusters such as graphene and carbon nanotube, and measurement of effective electrophoretic mobility helps characterization of nanoclusters. A novel analysis technique of capillary electrophoresis/dynamic frontal analysis (CE/DFA) has also been proposed for the analysis of such reactions as involving equilibria and kinetic reactions. In CE/DFA, kinetically generated product is continuously resolved from the equilibrium species, and a plateau signal would be detected when the reaction rate is constant. Michaelis-Menten constants have successfully been determined through the plateau height by CE/DFA. In this review, analysis and characterization methods utilizing reaction dynamics in a separation capillary are summarized.

Maximizing MS/MS Acquisition for Lipidomics Using Capillary Separation and Orbitrap Tribrid Mass Spectrometer.

Liquid chromatography-mass spectrometry (LC-MS) is a typical strategy for lipidomics analysis. Although capillary LC-MS is a common analytical technique for proteomics analysis, its application to lipidomics has been limited. In this study, we aim at improving lipid identifications achieved in a single LC-MS analysis by a 3-fold approach: capillary LC and nanoelectrospray for enhanced ionization, ion trap for higher sensitivity tandem MS, and parallelization of mass analyzers for increased speed of acquisition on an Orbitrap hybrid system. By applying the methods to a complex lipid mixture of human plasma, we identified and performed relative quantification on over 1500 lipids within a 60 min capillary LC-MS analysis.

Back Cover: Single bubble in‐tube microextraction coupled with capillary electrophoresis

DOI: 10.1002/elps.202100216 The cover picture shows three modes of headspace liquid phase microextraction (HS-LPME) in-line coupled with CE. Headspace single drop microextraction (HS-SDME)-CE: When a single drop of the acceptor phase (blue) is formed at the inlet of a separation capillary filled with a run buffer (pale pink), the analytes evaporated from the sample donor solution (green) into the HS are extracted into the drop leaving less- or non-volatile matrix components in the sample. After extraction, a portion of the drop containing enriched analytes is injected into the capillary for CE analysis. Due to the large volume ratio between the sample donor solution and the acceptor drop, and the large surface-to-volume ratio of the drop, the sample enrichment factors obtained with SDME are quite high for a given extraction time, compared with other conventional LPME techniques. However, it is critical to maintain the drop stably attached to the capillary during extraction and thus it is rather difficult to carry out SDME for a long time especially under harsh conditions such as elevated temperature. Headspace in-tube microextraction (HS-ITME)-CE: Using a liquid plug in the capillary inlet as an acceptor phase instead of a hanging drop, the shortcomings of the hanging drop can be resolved. In HS-ITME-CE, the capillary inlet containing the acceptor plug is simply placed in the HS above a sample solution. Since the acceptor phase is well protected by the capillary, a long extraction at a temperature as high as 90°C can be performed with ease. Furthermore, since the enriched acceptor plug is already inside the capillary, the whole extract can be utilized for the subsequent CE analysis without additional injection step. Single bubble in-tube microextraction (SB-ITME)-CE: For a short extraction time, the extraction efficiency of HS-SDME or HS-ITME was not maximized because it takes some time for the analytes to fill the HS and reach saturated concentrations, especially for compounds of low volatility. By using a single bubble as a micro-HS formed at the inlet of a separation capillary immersed in the sample donor solution, the HS extraction can be expedited without waiting for the HS equilibration. Since the surface-to-volume ratio of the submicroliter bubble is much higher than that of a conventional HS of a few milliliters or more, the analyte concentrations in the bubble can be replenished much more rapidly for short extraction times. SB-ITME demonstrated higher sample enrichments than HS-ITME for a short HS extraction of chlorophenols in water.

Cationic electrophoretic focusing on inverse electromigration dispersion profile with ESI-MS detection. New capillary electrophoretic method for high-sensitivity analysis of 2-hydroxy-s-triazines in waters

Electrophoretic focusing on an inverse electromigration dispersion profile is a relatively new electrophoretic separation method for the analysis of ionisable analytes. Although it offers alternative selectivity, powerful concentration capability and high sensitivity, so far only several systems for the analysis of anionic compounds were reported. This work describes for the first time a functional electrolyte system for the separation of cationic analytes by this method. Based on computer simulations and calculations, a functional electrolyte system was designed, based on 1-ethylpiperazine and pivalic acid. It is suitable for the analysis of weak bases with pKa values between 4.5 and 6 and due to the volatility of both components, it can be operated in combination with ESI-MS detection. Its practical applicability is demonstrated on direct analysis of four 2-hydroxy-s-triazines in waters. Separation in an uncoated capillary with application of negative pressure at the inlet provides very satisfactory results with good reproducibility of peak areas, peak heights and detection times, and with LODs at the 0.5–1 nM (100 ngL−1) level. The robustness of the method is shown on analyses of real tap and river water samples used without any pretreatment.

3-D printed injection system for capillary electrophoresis.

Commercial systems for capillary electrophoresis are designed for the unattended analysis of several samples, and are usually large, complex, and expensive. We report a compact system for manual injection of a single sample in capillary electrophoresis, which is ideal for method development and for student training. The injector consists of two parts that are manufactured by three-dimensional printing (STL and STEP files are included as ESI). One part is immobile and holds an electrode for powering electrophoresis and a gas line for pressurized injection and pumping fluids through the capillary. The second part is removable and is used to hold washing solutions, separation electrolyte, or sample. Conventional machining is used to tap holes to hold the electrode, separation capillary, gas line, and safety interlock. The system is used for either pressure or electrokinetic sample injection, and can be used to pump fluids through the capillary for changing background electrolytes and reconditioning the capillary between runs. We coupled the injection system to our high-dynamic range laser-induced fluorescence detector and evaluated the system by performing capillary zone electrophoresis on solutions of fluorescein. Electrokinetic injection produced a linear response across five orders of magnitude dynamic range (slope of the log-log calibration curve was 1.02), concentration detection limits of 5 pM, and mass detection limits of 1 zmol. Pressure injection produced a linear response across at least four orders of magnitude (slope of the log-log calibration curve was 0.92), concentration detection limits of 2 pM, and mass detection limits of 10 zmol.

A Monolithic Azacalix[4]Pyridine Column for High-Resolution and High-Efficiency Capillary Electrochromatographic Separation

A Monolithic Azacalix[4]Pyridine Column for High-Resolution and High-Efficiency Capillary Electrochromatographic Separation

Capillary Chromatographic Separation of Nanogram Protein Quantities on a Wide-Pore Superficially Porous Particle Column for Top-Down Proteomics

Capillary Chromatographic Separation of Nanogram Protein Quantities on a Wide-Pore Superficially Porous Particle Column for Top-Down Proteomics

Metal-Organic Framework-Based Enzyme Composites as Chiral Stationary Phase for High-Efficiency Capillary Electrochromatographic Enantioselective Separation

Metal-Organic Framework-Based Enzyme Composites as Chiral Stationary Phase for High-Efficiency Capillary Electrochromatographic Enantioselective Separation

Enzyme Metal-Organic Framework Composites as Chiral Stationary Phase for High-Efficiency Capillary Electrochromatographic Enantioselective Separation

Enzyme Metal-Organic Framework Composites as Chiral Stationary Phase for High-Efficiency Capillary Electrochromatographic Enantioselective Separation

Capillary electrophoresis-UV analysis using silica-layer coated capillary for separation of seven phenolic acids and caffeine and its application to tea analysis

This work presents an innovative silica-layer coated capillary with comparison study of the silica-layer coated capillary and the fused-silica capillary for the separation of seven phenolic acids viz. p-hydroxyphenylacetic acid (PHPA), p-coumaric acid (PCA), p-hydroxybenzoic acid (PHBA), caffeic acid (CFA), (3,4-dihydroxyphenyl)acetic acid (DHPA), gallic acid (GLA), and 2,3,4-trihydroxybenzoic acid (THBA), together with caffeine (CF), by capillary electro-chromatography (CEC) and micellar electrokinetic chromatography (MEKC), respectively. The running buffer was 25.0 mM borate at pH 9.0, with addition of 50.0 mM sodium dodecyl sulfate for the MEKC mode. The non-coated capillary could not separate all seven phenolic acids by CEC or MEKC. This was achieved using the coated capillary for both CEC and MEKC. The innovative coated capillary with CEC had plate number N ≥ 2.0 × 104 m−1 and resolution Rs ≥ 1.6 for all adjacent pairs of peaks. The capillary was also able to separate GLA and THBA which are structural isomers. Although MEKC mode provided comparable efficiency and selectivity, the reduced EOF of the coated capillary led to longer separation time. The linear calibration range of the seven phenolic acids and caffeine were different but the coefficients of determinations (r2) were all > 0.9965. The precisions of the relative migration times and peak area ratios of analyte to internal standard were 0.1–1.8% and 1.8–6.8%, respectively. There were no statistical differences in the efficiency of separation of the phenolic acids and caffeine for three coated capillaries. It was applied to the analysis of caffeine and phenolic acids in brewed tea using tyramine as the internal standard. The tea samples were diluted prior to analysis by CEC. The separation was less than 15 min. Caffeine, gallic acid and p-coumaric acid were detected and quantified. Caffeine and gallic acid contents were 10.8–15.0 and 2.6–4.8 mg g−1 dry tea leaves, respectively. p-Coumaric acid was detected in only one of the samples with a content of 0.4 mg g−1. Percent recoveries of spiked diluted samples were 90 ± 9 to 106 ± 13%, respectively.Article highlightsSilica-layer coated capillary is first reported for simultaneous separation of seven phenolic acids by non-MEKC analysis.Performance between coated, and non-coated capillaries with analysis by CEC and MEKC were compared.Plate number, resolution, capillary reproducibility, and electroosmotic flow mobility are investigated.Graphical abstract