The influence of carrier gas pressure on the retention of sorbates on monolithic capillary columns in gas chromatography

The influence of carrier gas pressure on the retention factor k′ of light hydrocarbons C₁–C₄ in a monolithic capillary column based on divinylbenzene was studied. It was shown that, for monolithic columns and nonideal carrier gases, the pressure dependence of lnk′ was nonlinear over a wide pressure range and could be described by the classic Everett equation. It was concluded that the competitive adsorption model failed to describe the experimental data correctly, especially for strongly retained sorbates and/or heavy carrier gases. Original Russian Text ? A.A. Korolev, V.E. Shiryaeva, T.P. Popova, A.V. Kozin, A.A. Kurganov, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 4, pp. 776–783.     

Preparation of hollow silica microspheres with controlled shell thickness in supercritical fluids

This article presents a novel route to prepare hollow silica microspheres with well-defined wall thickness by using cross-linked polystyrene (PS) microspheres as templates with the assistance of supercritical carbon dioxide (SC-CO₂). In this approach, the cross-linked PS templates can be firstly prepared via emulsifier-free polymerization method by using ethylene glycol dimethacrylate or divinylbenzene as cross-linkers. Then, the silica shell from the sol–gel process of tetraethyl orthosilicate (TEOS) which was penetrated into the PS template with the assistance of SC-CO₂ was obtained. Finally, the hollow silica spheres were generated after calcinations at 600 °C for 4 h. The shell thickness of the hollow silica spheres could be finely tuned not only by adjusting the TEOS/PS ratio, which is the most frequently used method, but also by changing the pressure and aging time of the SC-CO₂ treatment. Fourier transform infrared spectroscopy, transmission electron microscopy, and scanning electron microscope were used to characterize these hollow silica spheres.     

Polar monolithic capillary columns: Analysis of light hydrocarbons

The influence of the nature of the stationary phase and carrier gas (helium, hydrogen, nitrogen, carbon dioxide, or nitrous oxide) on the efficiency and separating ability of monolithic ethyleneglycol dimethacrylate (EDMA) polymer capillary columns was studied using a model mixture of light hydrocarbons C₁-C₄. The results were compared with the properties of silica gel and divinylbenzene (DVB) monolithic columns. For EDMA polymer monolithic columns, the effect of the carrier gas on the separating ability was markedly lower than for silica gel columns. A reduction in HETP observed in the series He > H₂ > N₂ > N₂O > CO₂ is also known for hollow capillary columns with polymer stationary phases, but the change in efficiency was ～20–30% in this case. Under the optimum conditions, HETP was minimum for the columns when CO₂ or N₂O was used.     

Chromatographic characteristics of monolithic capillary columns with different polarities

Relative polarity values and retention indices for a mixture of normal paraffins C₅–C₁₀ and alkylbezenes C₆–C₉ are determined for four types of monolithic capillary columns based on divinylbenzene, ethylene glycol dimethacrylate, pentaerythritol tetraacrylate, and pentaerythritol triacrylate. It is established that the retention of polar sorbates increases with a rise in stationary phase polarity, while the retention of nonpolar compounds grows with a reduction in stationary phase polarity.     

Investigation of monolithic capillary columns based on ethylene glycol dimethacrylate in gas chromatography

The influence of the conditions of the synthesis of polymer monoliths based on ethylene glycol dimethacrylate on the properties of monolithic capillary columns for gas chromatography was investigated. It was established that the time of polymerization, the temperature of synthesis, and the composition of the polymerization mixture have a strong effect on the dynamic and chromatographic properties of the columns. It was concluded that monolithic capillary columns synthesized under optimum conditions are characterized by a height equivalent to a theoretical plate of 30–40 μm, which is considerably less than that for traditional hollow capillary columns of the same diameter.     

High productivity chromatographic separations on monolithic capillary columns

The productivity of monolithic capillary columns based on silica gel and polymers of different polarities (divinylbenzene and ethyleneglycol dimethacrylate) is investigated using a model mixture of light hydrocarbons. It is shown that the productivity of a column is noticeably affected by the type of gas carrier. The highest productivity is observed when using carbon dioxide or dinitrogen monoxide as the gas carrier. The lowest productivity is observed when uisng hydrogen or helium.     

Semi‐micro reversed‐phase liquid chromatography for the separation of alkyl benzenes and proteins exploiting methacrylate‐ and polystyrene‐based monolithic columns

Monolithic columns were synthesized inside 1.02 mm internal diameter fused‐silica lined stainless‐steel tubing. Styrene and butyl, hexyl, lauryl, and glycidyl methacrylates were the functional monomers. Ethylene glycol dimethacrylate and divinylbenzene were the crosslinkers. The glycidyl methacrylate polymer was modified with gold nanoparticles and dodecanethiol (C₁₂). The separation of alkylbenzenes was investigated by isocratic elution in 60:40 v/v acetonitrile/water. The columns based on polystyrene‐co‐divinylbenzene and poly(glycidyl methacrylate)‐co‐ethylene glycol dimethacrylate modified with dodecanethiol did not provide any separation of alkyl benzenes. The poly(hexyl methacrylate)‐co‐ethylene glycol dimethacrylate and poly(lauryl methacrylate)‐co‐ethylene glycol dimethacrylate columns separated the alkyl benzenes with plate heights between 30 and 60 μm (50 μL min^?1 and 60°C). Similar efficiency was achieved in the poly(butyl methacrylate)‐co‐ethylene glycol dimethacrylate column, but only at 10 μL min^?1 (0.22 mm s^?1). Backpressures varied from 0.38 MPa in the hexyl methacrylate to 13.4 MPa in lauryl methacrylate columns (50 μL min^?1 and 60°C). Separation of proteins was achieved in all columns with different efficiencies. At 100 μL min^?1 and 60°C, the lauryl methacrylate columns provided the best separation, but their low permeability prevented high flow rates. Flow rates up to 500 μL min^?1 were possible in the styrene, butyl and hexyl methacrylate columns.     

Preparation and characterisation of silica monoliths using a triblock copolymer (F68) as porogen

Silica-based monoliths with co-continuous structure were successfully prepared through a sol–gel process in the presence of a poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (F68). The triblock copolymer was compared to the classical PEG, in the formation of silica monoliths and was demonstrated to lead to co-continuous structures in a wider composition range, presenting smaller through pores. Moreover, mesoporous structures templated at the sol–gel transition were assumed to occur at the surface of the silica skeleton while PEG exhibited no mesopore templating.     

Loading capacities of monolithic capillary columns in gas chromatography

The loading capacities of monolithic capillary columns based on silica gel and divinylbenzene are studied for two carrier gases, CO₂ and N₂. It is shown that the efficiency of the column is more sensitive to the overload of the column than the retention time of the sorbate is, especially for the CO₂ carrier gas. It is established that the loading capacity of a monolithic column based on silica gel decreases significantly in going from N₂ to CO₂. For columns based on divinylbenzene, the loading capacity is found to be virtually the same for both carrier gases. For monolithic columns, the loading capacity per one meter of column length is found to be 10 and more times higher than that for a standard open capillary column.     

HPLC analysis of synthetic polymers on short monolithic columns

Ultrashort monolithic columns (disks) were thoroughly studied as efficient stationary phases for precipitation–dissolution chromatography of synthetic polymers. Gradient elution mode was applied in all chromatographic runs. The mixtures of different flexible chain homopolymers, such as polystyrenes, poly(methyl methacrylates), and poly(tert‐butylmethacrylates) were separated according to their molecular weights on both commercial poly(styrene‐co‐divinylbenzene) disks (12 id × 3 mm and 5 × 5 mm) and lab‐made monolithic columns (4.6 id × 50 mm) filled with supports of different hydrophobicity. The experimental conditions were optimized to reach fast and highly efficient separation. It was observed that, similar to the separation of monoliths of other classes of (macro)molecules (proteins, DNA, oligonucleotides), the length of column did not affect the peak resolution. A comparison of the retention properties of the poly(styrene‐co‐divinylbenzene) disk‐shaped monoliths with those based on poly(lauryl methacrylate‐co‐ethylene dimethacrylate), poly(butyl methacrylate‐co‐ethylene dimethacrylate), and poly(glycidyl methacrylate‐co‐ethylene dimethacrylate) supports demonstrated the obvious effect of surface chemistry on the resolution factor. Additionally, the results of the discussed chromatographic mode on the fast determination of the molecular weights of homopolymers used in this study were compared to those established by SEC on columns packed with sorbent beads of a similar nature to the monoliths.     

The Influence of the Natures of the Carrier Gas and the Stationary Phase on the Separating Properties of Monolithic Capillary Columns in Gas Adsorption Chromatography

A model mixture of light hydrocarbons was used to study the separation capacity of monolithic capillary columns based on divinylbenzene with five different carrier gases, including helium, hydrogen, nitrogen, carbon dioxide, and nitrous oxide. The results were correlated with the previously obtained data on monolithic columns based on silica gel. It was shown that the influence of the nature of the carrier gas was weaker than for silica gel columns; the polymeric columns studied behaved similarly to hollow capillary columns with polymeric stationary phases and exhibited an efficiency gain of 20–30% along the series He < H₂ < N₂ ～ N₂O < CO₂. Based on the minimum HETP (～15 μm) obtained for the investigated monolithic columns under optimum conditions with N₂O or CO₂ as a carrier gas, the conclusion was drawn that the creation of divinylbenzene-based monolithic capillary columns with a high specific efficiency was possible.     

Extending the array of crosslinkers suitable for the preparation of polymethacrylate-based monoliths

The in situ preparation of monolithic capillary columns comprising copolymers of butyl methacrylate with ethylene dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, and pentaerythritol tetraacrylate using thermal polymerization within 250 microm ID capillaries and their application for micro-HPLC separations of proteins has been studied. For all crosslinkers, optimization of the porogenic mixture consisting of 1-propanol and 1,4-butanediol yielded monoliths with pore sizes above 1 microm suitable for rapid separations at low back pressure. Very good separations were achieved for a protein mixture consisting of ribonuclease A, cytochrome c, myoglobin, and ovalbumin with all tested columns.     

Preparation of Monolithic Capillary Chromatographic Columns Using Supercritical Fluid as a Porogen Solvent

Monolithic polymeric beds were synthesized in fused silica capillaries using either trimethylolpropane trimethacrylate (TRIM) or a mixture of butyl methacrylate (BMA) with ethylene glycol dimethacrylate (EDMA) as monomers. Carbon dioxide at temperature and pressure conditions above its critical values was used as a porogen solvent. The purpose of using the supercritical carbon dioxide was to have the possibility of changing the solvation power (and thus the porosity of the resulting monolith) of the porogen by pressure and temperature changes instead of changing the porogen composition. The experiments were performed using a special setup consisting of a stainless steel high-pressure reactor to which the fused silica capillary was connected. The synthesized monoliths underwent liquid chromatographic evaluation. The polyTRIM capillary monoliths were characterized by different permeability, which depended on the pressure of the synthesis. BMA/EDMA columns were applied for separation of alkylbenzenes and a model mixture of proteins.     

Adsorption and kinetics of the oxidation of ethylene on PdCl2/SiO2

A new variant of the kinetic distributive method was used to study the kinetics of the oxidation of ethylene by PdCl₂ complexes supported on silica gel taking account of the equilibrium distribution of ethylene between the gas phase, silica gel, and metal complex reaction sites. Mono-, bi-, and polynuclear palladium(II) species are found to participate in the reaction. The Pd^II activity increases with increasing amount of nuclei in the species. L. M. Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine, 70 R. Lyuksemburg ul., Donetsk 83114, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 36, No. 4, pp. 243–246, July–August, 2000.     

Influence of diluent composition on the porous structure of methacrylate copolymers

The porous structure of copolymers obtained by suspension polymerization has been investigated. Three different copolymers were synthesized—styrene‐divinylbenzene, ethylene glycol dimethacrylate‐divinylbenzene, and 1,4‐phenylene dimethacrylate‐divinylbenzene. All the copolymers were porous. As a pore‐forming diluent, the mixture of toluene (good solvent) and n‐dodecane (nonsolvent) was used. The influence of the composition of two‐component diluent on the porous structure of the copolymers has been examined. Surface areas, pore volumes, pore size distributions, skeletal and apparent densities, and swellability coefficients were determined for the copolymers obtained in the presence of 0, 15, 50, 85, and 100% (v/v) toluene in the mixture with n‐dodecane. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3079–3085, 2002     

Development of an affinity silica monolith containing alpha1-acid glycoprotein for chiral separations

An affinity monolith based on silica and containing immobilized alpha(1)-acid glycoprotein (AGP) was developed and evaluated in terms of its binding, efficiency and selectivity in chiral separations. The results were compared with data obtained for the same protein when used as a chiral stationary phase with HPLC-grade silica particles or monoliths based on a copolymer of glycidyl methacrylate (GMA) and ethylene dimethacrylate (EDMA). The surface coverage of AGP in the silica monolith was 18% higher than that obtained with silica particles and 61% higher than that measured for a GMA/EDMA monolith. The higher surface area of the silica monolith gave materials that contained 1.5- to 3.6-times more immobilized protein per unit volume when compared to silica particles or a GMA/EDMA monolith. The retention, efficiency and resolving power of the AGP silica monolith were evaluated by injecting two chiral analytes onto this column (i.e., R/S-warfarin and R/S-propranolol). In each case, the AGP silica monolith gave higher retention plus better resolution and efficiency than AGP columns containing silica particles or a GMA/EDMA monolith. The AGP silica monolith also gave lower back pressures and separation impedances than these other materials. It was concluded that silica monoliths can be valuable alternatives to silica particles or GMA/EDMA monoliths when used with AGP as a chiral stationary phase.     

Preparation and characterization of the chromatographic properties of ethylene glycol dimethacrylate/divinylbenzene polymeric microspheres

The preparation of porous ethylene glycol dimethacrylate/divinylbenzene polymeric microspheres is presented. These materials were investigated as column packings for high‐performance liquid chromatography. Their porous structures in both the dry and wet states were also studied. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3049–3058, 2005     

Polystyrene resins cross-linked with di- or tri(ethylene glycol) dimethacrylates as supports for solid-phase peptide synthesis

Polystyrene resins cross-linked with di(ethylene glycol) dimethacrylate (DEGDMA) and tri(ethylene glycol) dimethacrylate (TEGDMA), DEGDMA-PS and TEGDMA-PS, were synthesized by suspension copolymerization. Four functionalized resins, chloromethyl resin, 4-hydroxymethylphenoxymethyl resin (Wang resin), 4-methylbenzhydrylamine resin (MBHA resin) and 2-chlorotrityl chloride resin, were prepared from DEGDMA-PS and TEGDMA-PS. DEGDMA-PS and TEGDMA-PS showed high reactivity in the functionalization reactions in comparison with Merrifield resin (polystyrene cross-linked with divinylbenzene, DVB-PS). DEGDMA-PS-Wang resin and TEGDMA-PS-Wang resin were used as the solid-phase support for the synthesis of a difficult sequence, the fragment of acyl carrier protein 65-74. The yields of the crude peptide synthesized using DEGDMA-PS-Wang resin, TEGDMA-PS-Wang resin and DVB-PS-Wang resin were 92.3%, 91.6% and 78.8%, respectively. The purities of the crude peptides were 85.7%, 88.1% and 73.3%, respectively.     

Polymer monolithic capillary column fabricated by using monodisperse iron oxide nanocrystal template to enhance the electrochromatographic separation of small molecules

Monodisperse iron oxide nanocrystals and organic solvents were utilized as coporogens in monolithic poly(glycidyl methacrylate‐co‐ethylene glycol dimethacrylate) capillary columns to afford stationary phases with enhanced electrochromatographic performance of small molecules. While the conventional monoliths using organic solvents only as a porogen exhibited poor resolution (Rs) <1.0 and low efficiency of 40 000–60 000 plates/m, addition of a small amount of nanocrystals to the polymerization mixture provided increased resolution (Rs > 3.0) and high efficiency ranged from 60 000 to 100 000 plates/m at the same linear velocity of 0.856 mm/s. It was considered that the mesopores introduced by the nanocrystals played an important role in the improvement of the monolith performance. This new strategy expanded the application range of the hydrophobic monoliths in the separation of polar alkaloids and narcotics. The successful applications demonstrated that the glycidyl methacrylate based monoliths prepared by using nanocrystal template are a good alternative for enhanced separation efficiency of small molecules.