Fabrication of monolithic capillary columns with inner diameter 50-530 µm employing a mixture of pentaerythritol tetraacrylate and polyhedral oligomeric silsesquioxane-methacrylate as crosslinkers.

Highly crosslinked monolithic capillary columns with inner diameters in the range of 50-530 µm were prepared by radical polymerization of pentaerythritol tetraacrylate, polyhedral oligomeric silsesquioxane-methacrylate, and n-octadecyl methacrylate in the presence of methanol, dodecyl alcohol, and polyethylene glycol lauryl ether. Columns were evaluated by inverse size-exclusion chromatography employing a set of polystyrene standards of narrow molecular-size distribution and by scanning electron microscopy. Chromatographic performance under reversed-phase conditions was also evaluated. The combination of two effective crosslinkers as pentaerythritol tetraacrylate and polyhedral oligomeric silsesquioxane-methacrylate in the polymerization mixture allows for the preparation of robust and efficient monolithic capillary columns within a fairly wide range of internal diameters.

Characterization and applications of a trioctyl(3/4-vinylbenzyl)phosphonium stationary phase for use in capillary liquid chromatography.

The morphology, composition, and selectivity of a silica-based monolithic stationary phase, grafted by a layer of trioctyl(3/4-vinylbenzyl)phosphonium chloride ([P888VBn]Cl), is presented. The results of elemental analysis confirmed that the prepared stationary phase contains 38.8 at.% of silicon, 60.2 at.% of carbon, and 1.0 at.% of phosphorus. Capillary columns (150 × 0.1 mm) for liquid chromatography were evaluated using alkylbenzenes, monosubstituted benzenes, polyaromatic compounds, substituted benzene regioisomers, and aromatic carboxylic and amino acids. The prepared ionic liquid (IL)-based stationary phase exhibits hydrophobic, hydrophilic, and electrostatic interactions, as confirmed by experiments on the evaluation of the effect of the mobile phase composition (content of acetonitrile and ammonium formate) on the isocratic chromatographic separation. Thus, the IL-based capillary column demonstrates a unique separation selectivity compared to Phenyl-, C8-, and C18-stationary phases, and high efficiency for an expanding number of structurally diverse compounds.

Compact optical detector utilizing light emitting diodes, 50 nL L-shaped silica capillary cell and CCD spectrometer for simultaneous multi-wavelength monitoring of absorbance and fluorescence in microcolumn liquid chromatography.

Optical detection setup utilizing light emitting diodes (LEDs), 50 nL L-shaped silica capillary detection cell (L-cell), and low-cost CCD spectrometer is described in this work. Experimental configuration can be equipped with two different LEDs for absorbance measurement and other two LEDs for fluorescence excitation. This setup is capable of simultaneous multi-wavelength monitoring of absorbance and fluorescence when light produced by the individual LEDs and light emitted by the fluorescent analytes is resolved in the spectrum outputted by the CCD spectrometer. Effective optical path of the 0.25 µm I. D. L-cell is 1 mm. Absorbance baseline noise is 1 mAU due to use of low-cost and relatively noisy CCD spectrometer and LED drivers. Nevertheless, the setup can detect adenosine 5'-monophosphate down to micromolar concentration. Performance of fluorescence monitoring allows detection of 5·10-10 M fluorescein when 23 mW 470 nm LED is used for excitation. The dynamic range of absorbance and fluorescence measurement is 867:1 and 1622:1, respectively. Separation of test mixture (alkylbenzenes and polyaromatic hydrocarbons) demonstrate the effective use of the detector for simultaneous absorbance and fluorescence detection with 0.2 × 150 mm packed capillary column. The benefits of the setup are relative simplicity, compact design and the fact that it can be operated without any optical filters, slits, and extremely precise positioning of the optical elements.

Nanolitre-scale cell based on L-shaped silica capillary and optical fibre for absorption photometric detection in capillary liquid chromatography.

Miniaturised photometric detection cell based on the 150 µm I. D. L-shaped fused silica capillary (L-cell) and silica optical fibre is discussed in this paper. Light enters the cell via right angle bend and transmitted light is collected by silica optical fibre inserted into the capillary outlet. Ray trace analysis reveals that L-cell geometry works as a waveguide and effectively eliminates parasitic light, which is guided by the wall of any fused silica capillary cell. Low total level of stray light is further documented by measurement of absorbance of acetone-water mixture at 266 nm with the observed upper limit of dynamic range 2.2 AU. 3-mm L-cell has a volume of 53 nL and exhibits effective optical path 2.7 mm. Cell variance determined in the flow rate range 1-10 µL min-1 indicates, that 3-mm L-cell is optimal for use with 200-300 µm I. D. capillary columns. Furthermore, isocratic separation of alkylbenzenes mixture at 0.5 µL min-1 demonstrates effective use of 0.5-mm L-cell (8.8 nL) and possible performance of longer L-cells with high efficiency 100 µm I. D. capillary column.

Immobilization of a phosphonium ionic liquid on a silica monolith for hydrophilic interaction chromatography.

A methodology for preparing phosphonium-based ionic liquid modified silica-based monolithic capillary columns is presented. The silica monolithic columns with dimensions of 150 × 0.1 mm were modified by a phosphonium-based ionic liquid (trioctyl(3/4-vinylbenzyl)phosphonium chloride) via 3-(trimethoxysilyl)propyl methacrylate. The prepared columns were evaluated under hydrophilic interaction liquid chromatography separation conditions, employing a sample mixture containing purine and pyrimidine bases and nucleosides. Detection was made by UV. The high efficiency of the original silica monolith was preserved even after modification, and it reached values in the range of 98,000-174,000 theoretical plates/m. The effects of the concentration of acetonitrile in the mobile phase, the presence of additives in the mobile phase, such as, acetic acid or ammonium acetate, and the pH of the mobile phase on the separation of some selected analytes were investigated. The prepared columns showed different separation selectivity compared to silica, phenyl and sulfobetaine stationary phases.

Supercritical water-treated fused silica capillaries in analytical separations: Status review.

Near- and supercritical water (SCW) has recently been shown to provide an unusual but effective tool to roughen the inner surface or manipulate the internal diameter of fused silica capillaries for analytical separation methods In this review, the to-date existing variants of instrumental arrangement for etching the fused silica capillaries with SCW are described, the currently accessible morphologies of SCW-etched capillaries are outlined, and both existing and prospective applications of the SCW-etched capillaries in analytical separations are briefly discussed. Relative merits of SCW and other agents to treat the inner surfaces of fused silica capillaries are also mentioned.

Bridged polysilsesquioxane-based wide-bore monolithic capillary columns for hydrophilic interaction chromatography.

The synthesis and characterization of large-bore silica-based monolithic capillary columns (0.32mm×150mm) are presented. Columns were prepared by acidic hydrolysis of a mixture containing tetramethoxysilane (TMOS) and 1,2-bis(trimethoxysilyl)ethane (BTME) in different molar ratios in the presence of polyethylene glycol and urea. The monoliths were modified by zwitterionic monomer [2-(methacryloyloxy)ethyl]-dimethyl-(3-sulfopropyl)-ammonium hydroxide via 3-(trimethoxysilyl)propyl methacrylate. Prepared stationary phases were evaluated by scanning electron microscopy and chromatographic separation of nucleobases and their derivatives in the HILIC mode. The best chromatographic results were obtained with the column prepared from the reaction mixture containing BTME and TMOS in a 1:4 molar ratio. The permeability of such column reached 1.68×10-14m2 and the efficiency, expressed as a height equivalent of the theoretical plate, did not exceed 10.5µm for the tested compounds. The columns were successfully applied to HILIC separation of native and labeled oligosaccharides and glycans released from bovine ribonuclease B and human immunoglobulin G.

Phosphatidylcholine covalently linked to a methacrylate-based monolith as a biomimetic stationary phase for capillary liquid chromatography.

In this study a strategy to immobilize phospholipids onto a polymer-based stationary phase is described. Methacrylate-based monoliths in capillary format (150×0.1mm) were modified by soybean phosphatidylcholine through 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide coupling to obtain stationary phases suitable to mimic cell surface membranes. The covalent coupling reaction involves the phosphate group in phospholipids; therefore, the described methodology is suitable for all types of phospholipids. Immobilization of soy bean phosphatidylcholine on the monolith was confirmed by attenuated total reflectance Fourier transform infrared spectroscopy and gas chromatography-mass spectrometry of the fatty alcohol profile, generated upon reductive cleavage of the fatty acyl side chains of the phospholipid on the monolith surface with lithium aluminium hydride. The prepared stationary phases were evaluated through studies on the retention of low-molar mass model analytes including neutral, acidic, and basic compounds. Liquid chromatographic studies confirmed predominant hydrophobic interactions between the analytes and the synthesized stationary phase; however, electrostatic interactions contributed to the retention as well. The synthesized columns showed high stability even with fully aqueous mobile phases such as Dulbecco's phosphate-buffered saline solution.

Etching of glass microchips with supercritical water.

A novel method of etching channels in glass microchips with the most tunable solvent, water, was tested as an alternative to common hydrogen fluoride-containing etchants. The etching properties of water strongly depend on temperature and pressure, especially in the vicinity of the water critical point. The chips were etched at the subcritical, supercritical and critical temperature of water, and the resulting channel shape, width, depth and surface morphology were studied by scanning electron microscopy and 3D laser profilometry. Channels etched with the hot water were compared with the chips etched with standard hydrogen fluoride-containing solution. Depending on the water pressure and temperature, the silicate dissolved from the glass could be re-deposited on the channel surface. This interesting phenomenon is described together with the conditions necessary for its utilization. The results illustrate the versatility of pure water as a glass etching and surface morphing agent.

Separation of nucleobases, nucleosides, and nucleotides using two zwitterionic silica-based monolithic capillary columns coupled with tandem mass spectrometry.

The capability of employing synthesized zwitterionic silica-based monolithic capillary columns (140 mm × 0.1mm) for separation of highly polar and hydrophilic nucleobases, nucleosides, and nucleotides in hydrophilic interaction chromatography is reported. The suitability of the columns for on-line conjunction with electrospray tandem mass spectrometry was explored. Our results show that the grafted layer of zwitterionic monomer ([2-(methacryloyloxy)ethyl]-dimethyl-(3-sulfopropyl)-ammonium hydroxide or 2-methacryloyloxyethyl phosphorylcholine) on the silica monolithic surface significantly improved the separation selectivity and reproducibility, as compared to the bare silica monolith. The stepwise elution from 90% to 70% of acetonitrile enabled separation of a complex sample mixture containing 21 compounds with a total analysis time less than 40 min.

Silica-based monolithic capillary columns modified by liposomes for characterization of analyte-liposome interactions by capillary liquid chromatography.

This study introduces a silica-based monolith in a capillary format (0.1 mm × 100 mm) as a support for immobilization of liposomes and its characterization in immobilized liposome chromatography. Silica-based monolithic capillary columns prepared by acidic hydrolysis of tetramethoxysilane in the presence of polyethylene glycol and urea were modified by (3-aminopropyl)trimethoxysilane, whereby amino groups were introduced to the monolithic surface. These groups undergo reaction with glutaraldehyde to form an iminoaldehyde, allowing covalent binding of pre-formed liposomes containing primary amino groups. Two types of phospholipid vesicles were used for column modification; these were 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphatidyl choline with and without 1,2-diacyl-sn-glycero-3-phospho-L-serine. The prepared columns were evaluated under isocratic separation conditions employing 20mM phosphate buffer at pH 7.4 as a mobile phase and a set of unrelated drugs as model analytes. The liposome layer on the synthesized columns significantly changed the column selectivity compared to the aminopropylsilylated monolithic stationary phase. Monolithic columns modified by liposomes were stable under the separation conditions, which proved the applicability of the suggested preparation procedure for the synthesis of capillary columns dedicated to study analyte-liposome interactions. The column efficiency originating from the silica monolith was preserved and reached, e.g., more than 120,000 theoretical plates/m for caffeine as a solute.

Isoelectric focusing in continuously tapered fused silica capillary prepared by etching with supercritical water.

This communication indicates the potential of etching with sub- and/or supercritical water for reproducible preparation of fused-silica capillaries with tapered geometry suitable for capillary isoelectric focusing (CIEF) with electroosmotic displacement. The etching procedure provided a single-piece combination of the tapered separation space with a cylindrical connection of the detection window to the electrode vial. Selected proteins and colored pI markers were used as model analytes. A comparison with conventional cylindrical capillary under comparable applied voltage and analysis time was made, and the resultant peaks were compared in terms of peak resolution under optimized conditions. In CIEF carried out in a tapered capillary with the inlet cross-section three times larger than the cross-section at the detection window, three to four times higher resolutions of corresponding peak pairs were obtained. The method described opens the way to increase the number of separable compounds without resorting to excessively high voltage.

Solubilities of selected organic electronic materials in pressurized hot water and estimations of aqueous solubilities at 298.15 K.

Increasing production and disposal of organic light-emitting diode (OLED) displays for smartphones and tablets may have impact on the environment depending on the aqueous solubility of the pertinent chemicals. Here, aqueous solubilities are presented for several compounds, mostly aromatic amines, used as hole transport materials in the OLED displays. Solute selection includes 1,4-bis(diphenylamino)benzene, tetra-N-phenylbenzidine, 4,4'-bis(N-carbazolyl)-1,1'-biphenyl, 1,3,5-tris(diphenylamino)benzene, and 9,10-bis(phenylethynyl)anthracene. The solubilities are those in pressurized hot water (PHW), i.e., measured at elevated temperature (up to 260 °C) and pressure. The semi-quantitative estimates of room-temperature solubilities of the solutes have been obtained from extrapolations of the solubilities in PHW. For the compounds studied, the estimated aqueous solubilities at room temperature do not exceed 2×10(-11) g of the solute per 1 kg of water. Aqueous solubilities of triphenylamine have also been measured and used to upgrade a recent group-contribution model of aqueous solubilities of organic nonelectrolytes with the parameters for the nitrogen atom in aromatic amines.

Near- and supercritical water as a diameter manipulation and surface roughening agent in fused silica capillaries.

The prospects of near- and supercritical water for treatment of the inner surfaces of fused silica capillaries have been tested employing an in-lab-assembled apparatus. Unlike all other agents used for the purpose, water cannot introduce any undesirable heteroatoms to the treated surface. Theoretical background for this work comes from the well-known fact that water near its critical point can solubilize silica. The results show that depending on the temperature, water flow rate, flow mode, and exposure time, high-temperature water has wide-ranging effects on both the surface roughness and the internal diameter profile along the length of the treated capillary. By judicious selection of the operating conditions, tapered capillaries of various profiles for applications in electromigration techniques can be prepared with relatively high reproducibility. The water-treated fused silica capillaries with uniform internal diameter appear to be useful for preparation of monolithic silica capillary columns.

Zwitterionic silica-based monolithic capillary columns for isocratic and gradient hydrophilic interaction liquid chromatography.

This study introduces zwitterionic monolithic capillary columns intended for isocratic and gradient HILIC separations. Silica-based monolithic capillary columns (100 µm × 150 mm) prepared by acidic hydrolysis of tetramethoxysilane in the presence of polyethylene glycol and urea were modified by a sulfoalkylbetaine zwitterion ([2-(methacryloyloxy)ethyl]-dimethyl-(3-sulfopropyl)-ammonium hydroxide) to HILIC stationary phase. The prepared columns were evaluated under the isocratic and gradient separation conditions using a standard mixture containing nucleic acid bases, nucleosides, and 2-deoxynucleosides. Mobile phases contained high concentration of acetonitrile (95-85%, v/v) and 5-50mM ammonium acetate or ammonium formate in the pH range of 3-6. The synthesized columns showed a long-term stability under the separation conditions while the high permeability and efficiency originating from dual structure of the silica monolith were preserved. The relative standard deviations (RSDs) for the retention times of tested solutes were lower than 2% under the isocratic conditions and lower than 3.5% under the gradient conditions.

Generalized linear solvation energy model applied to solute partition coefficients in ionic liquid-supercritical carbon dioxide systems.

Biphasic solvent systems composed of an ionic liquid (IL) and supercritical carbon dioxide (scCO(2)) have become frequented in synthesis, extractions and electrochemistry. In the design of related applications, information on interphase partitioning of the target organics is essential, and the infinite-dilution partition coefficients of the organic solutes in IL-scCO(2) systems can conveniently be obtained by supercritical fluid chromatography. The data base of experimental partition coefficients obtained previously in this laboratory has been employed to test a generalized predictive model for the solute partition coefficients. The model is an amended version of that described before by Hiraga et al. (J. Supercrit. Fluids, in press). Because of difficulty of the problem to be modeled, the model involves several different concepts - linear solvation energy relationships, density-dependent solvent power of scCO(2), regular solution theory, and the Flory-Huggins theory of athermal solutions. The model shows a moderate success in correlating the infinite-dilution solute partition coefficients (K-factors) in individual IL-scCO(2) systems at varying temperature and pressure. However, larger K-factor data sets involving multiple IL-scCO(2) systems appear to be beyond reach of the model, especially when the ILs involved pertain to different cation classes.

Silica-based monolithic capillary columns-Effect of preparation temperature on separation efficiency.

The temperature effects during the sol-gel process and ageing of the silica-based monolith on the structure and separation efficiency of the capillary columns (100microm i.d., 150mm) for HPLC separations were studied. The tested columns were synthesized from a mixture of tetramethoxysilane, polyethylene glycol and urea under the acidic conditions. The temperature was varied from 40 degrees C to 44 degrees C and formation of bypass channels between the silica mold and the capillary wall was examined. The temperature of 43 degrees C was estimated as optimal for preparation of efficient silica capillary columns which were subsequently modified by octadecyldimethyl-N,N-diethylaminosilane or covered by poly(octadecyl methacrylate) and tested using standard mixture of alkylbenzenes under the isocratic conditions.

Polymethacrylate monolithic and hybrid particle-monolithic columns for reversed-phase and hydrophilic interaction capillary liquid chromatography.

We prepared hybrid particle-monolithic polymethacrylate columns for micro-HPLC by in situ polymerization in fused silica capillaries pre-packed with 3-5microm C(18) and aminopropyl silica bonded particles, using polymerization mixtures based on laurylmethacrylate-ethylene dimethacrylate (co)polymers for the reversed-phase (RP) mode and [2-(methacryloyloxy)ethyl]-dimethyl-(3-sulfopropyl) zwitterionic (co)polymers for the hydrophilic interaction (HILIC) mode. The hybrid particle-monolithic columns showed reduced porosity and hold-up volumes, approximately 2-2.5 times lower in comparison to the pure monolithic columns prepared in the whole volume of empty capillaries. The elution volumes of sample compounds are also generally lower in comparison to packed or pure monolithic columns. The efficiency and permeability of the hybrid columns are intermediate in between the properties of the reference pure monolithic and particle-packed columns. The chemistries of the embedded solid particles and of the interparticle monolithic moiety in the hybrid capillary columns contribute to the retention to various degrees, affecting the selectivity of separation. Some hybrid columns provided improved separations of proteins in comparison to the reference particle-packed columns in the reversed-phase mode. Zwitterionic hybrid particle-monolithic columns show dual mode retention HILIC/RP behaviour depending on the composition of the mobile phase and allow separations of polar compounds such as phenolic acids in the HILIC mode at lower concentrations of acetonitrile and, often in shorter analysis time in comparison to particle-packed and full-volume monolithic columns.

Distribution of organic solutes in biphasic 1-n-butyl-3-methylimidazolium methyl sulfate-supercritical CO(2) system.

Interphase distribution of organic nonelectrolytes can be important in applications of biphasic solvent systems composed of a room-temperature ionic liquid and supercritical carbon dioxide (scCO(2)). Here, open tubular capillary-column supercritical fluid chromatography (SFC) with 1-n-butyl-3-methylimidazolium methyl sulfate ([bmim][MeSO(4)]) as the stationary liquid and scCO(2) as the carrier fluid was employed to measure retention factors of organic solutes within 313-353 K and 8.5-23.2 MPa. Solute selection included 18 compounds of diverse volatilities and chemical functionalities. The retention factors were converted to infinite-dilution solute partition coefficients in the biphasic [bmim][MeSO(4)]-scCO(2) system. At a constant temperature, an increase in scCO(2) density produced distinct shifts in relative retention (= separation factor), thus providing some pressure-tunable selectivity. At a particular temperature and density of CO(2), solute partition coefficients can be correlated in terms of linear solvation energy relationships. Analysis of the relative retention data by regular solution theory resulted in approximate values of the solubility parameter of CO(2)-expanded [bmim][MeSO(4)].

Limiting partition coefficients of solutes in biphasic trihexyltetradecylphosphonium chloride ionic liquid-supercritical CO2 system: measurement and LSER-based correlation.

Limiting partition coefficients of a selection of low-to-medium volatility solutes between both phases in a biphasic trihexyltetradecylphosphonium chloride ([thtdp][Cl]) ionic liquid (IL)-supercritical carbon dioxide (scCO2) system were obtained by capillary column chromatography with [thtdp][Cl] as the stationary liquid and scCO2 as the carrier fluid. It is shown that supercritical fluid chromatography can be used to probe the partitioning behavior of solutes in biphasic IL-scCO2 systems even when the IL has nonzero solubility in scCO2. Relative partition coefficients of solutes at a particular temperature and density of CO2 can be correlated within Abraham's linear solvation energy relationships. Compared with our previous results in a [bmim][BF4]-scCO2 system, the solute partition coefficients in the [thtdp][Cl]-scCO2 system are more sensitive to the solute hydrogen bond acidity descriptor.