Improvement of chromatographic performances of in-situ synthesized hybrid C8 silica monoliths by reduction of structural radial heterogeneities

Several modifications of a previously described protocol are proposed to improve the performances of in-situ synthesized C₈ hybrid silica monoliths. Our attention was focused on reducing the sources of radial heterogeneity that may be responsible for the poor efficiencies observed in the hydrodynamic elution mode. It was demonstrated that a decrease in the temperature of the capillary during the filling step equally to that of the polymerization mixture (0 °C), associated with a decrease of the gelation temperature to 20 °C along with a new pre-treatment of the capillary's internal walls [with a mixture of tetraethoxysilane (TEOS)/EtOH (1/3, v/v)] allows (i) increasing the radial homogeneity of the monolith, thus further enhancing the performances in the nano-liquid chromatography (nano-LC) mode, (ii) improving the capillary to capillary reproducibility in terms of permeability and efficiencies. In fact, the average minimum plate height H_min was lowered from 24 to 14 μm and the capillary-to-capillary reproducibility of the synthesis was widely improved by factors two and three of reduction on the calculated standard deviation, respectively for both the efficiency in the nano-LC mode and the permeability. At last, the improved radial homogeneity and anchoring of the synthesized monoliths allowed increasing the inner diameter of the capillary (up to 150 μm) without any significant loss in efficiency. Finally, long term stability of the as-obtained monolithic stationary phases in terms of retention and efficiency was studied. In addition, the evaluation of their chromatographic behaviour was also achieved with the Tanaka test and the results were compared to those already published for commercial monoliths (Chromolith) as well as for particulate stationary phases.     

Spectroscopic studies of post-doped silica gel monoliths

This paper describes the interfacial molecular interactions of cobalt, neodymium, Rhodamine 6G and polymethyl methacrylate with silica gel-glass monoliths for samples prepared by the post doping method.Formerly at Chemistry Department, Brunel University, Uxbridge, Middlesex UB8 3PH, UK.Formerly Carole C. Perry.     

Thermal analysis of fluorine containing gels and precursors of high silica zeolites

By means of simultaneous DTA-, TG- and DTG-technique, the silicate gels and the MFI crystals obtained from these gels have been investigated. The gels have been prepared in presence and in absence of tetrapropylammonium cation (TPA⁺) and with Li⁺, Na⁺, NH ₄ ⁺ and K⁺ fluorides. In absence of TPA⁺ no thermal effects have been observed in Li⁺- and Na⁺-gels. The effects observed in the NH ₄ ⁺ -gel stem from a decomposition and release of inorganic phases: SiF₄, NH₄F, NH₃. The DTA/DTG effects in the TPA⁺ containing gels and in the MFI crystals of monodisperse size are attributed to the decomposition of TPA⁺ cation. It can be concluded from these effects that the interaction between the gels and the TPA⁺ cation is rather weak. The interaction between TPA⁺ and MFI crystals obtained in fluoride medium is stronger than the interaction with crystals obtained from alkaline media. Similar thermal effects are obtained after grinding the long crystals to those having a large distribution of crystal sizes.

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Zusammenfassung Mittels simultaner DTA-, TG- und DTG-Techniken wurden Kieselgele und die MFI-Kristalle dieser Gele untersucht. Die Gele wurden in und ohne Gegenwart von Tetrapropylammoniumkationen (TPA⁺) und mit Li⁺-, Na⁺-, NH ₄ ^– und Kaliumfluoriden hergestellt. Bei Abwesenheit von TPA⁺ werden für Li⁺- und Na⁺-Gele keine thermischen Effekte beobachtet. Der bei NH ^4– -Gelen beobachtete Effekt ergibt sich aus der Zersetzung und Abspaltung von anorganischen Phasen: SiF₄, NH₄F, NH₃. Die DTA/DTG-Effekte in TPA⁺-haltigen Gelen und in den MFI-Kristallen monodispersen Ausmaes werden der Zersetzung des TPA⁺-Kations zugeschrieben. Daraus kann man schlieen, da die Wechselwirkung zwischen den Gelen und dem TPA⁺-Kation eher schwach ist. Die Wechselwirkung zwischen TPA⁺ und MFI-Kristallen aus fluoridischen Medien ist strker als die Wechselwirkung mit Kristallen aus alkalischen Medien. Ähnliche thermische Effekts erhlt man, nachdem man die langen Kristalle zerkleinert und eine breite Kristallgrenverteilung erhlt.

     

Growth and aging phenomena in silica gels

Growth and aging of silica aggregates are influenced both by temperature and by catalyzing fluorine ions as shown by SAXS and BET. It was found that both fluorine and increased temperature slightly increased the fractal dimension Df during aging, but the fluorine catalyzed system showed a lower BET surface area.To understand the effect of fluorine and increased temperature on the aggregates, 2D aggregations and SAXS simulations were carried out using two new programs GRASP and DALAI. In agreement with experiments it was found that binary RLCCA aggregates have a slightly higher Df value compared to DLCCA aggregates and that branch-flexibility during aging increases Df even further.     

Synthesis and spectroscopy of tpps-doped silica gels by the sol-gel process

Tetraphenylporphinetetrasulfonic acid (TPPS), which is well known as a photochemical hole-burning (PHB) dye, was incorporated in silica gels obtained by the sol-gel process from tetramethoxysilane (TMOS). The form of TPPS must be free base (H₂P), which exists in basic condition, to be active in PHB. To obtain transparent and higher density silica gels doped with free-base TPPS, two-step hydrolysis processes using solutions containing NaOH were developed, and the gels having bulk density of about 1.5 g/cm³ were synthesized. The form of TPPS in the gels was investigated by measuring the absorption and luminescence spectra, and it was found that in the silica gels almost all the TPPS retained free-base form at the molar ratio of NaOH/TMOS above 10^–3.     

Morphological analysis of physically reconstructed capillary hybrid silica monoliths and correlation with separation efficiency

We report an experimental study on the structural (especially radial) heterogeneity of eleven 100 μm i.d. capillary tetramethoxysilane-methyltrimethoxysilane hybrid silica monoliths with different pore and skeleton sizes, which were imaged by an optimized confocal laser scanning microscopy method. This method allows the optical sectioning of the monoliths, which is a prerequisite for quantitative morphological image analysis. Both radial porosity profiles and chord length distributions were calculated in the macropore domain for each column from at least 100 complete cross-sectional views along the column axis. The statistical approach visualized radial heterogeneities on different length scales in the monolithic structures. Chord length distributions followed a simplified k-gamma function, and a structural parameter obtained from this function is introduced to provide a scalar measure of column heterogeneity. It enables the comparison of monoliths with different pore sizes and helps to establish correlations between the microscopic properties of a column, eddy dispersion, and its separation efficiency.     

Electrochromatography in packed tubes using 1.5 to 50 μm silica gels and ODS bonded silica gels

Summary Electrochromatography (that is HPLC where the eluent is driven along the column by electro-osmosis using fields of up to 100 kV m⁻¹) promises plate efficiencies for HPLC which are comparable to those attained in capillary gas chromatography, but this requires that narrow-bore columns can be successfully packed with submicron particles. This paper demonstrates that we have now moved a considerable distance towards this goal. We show (1) that, following theory, there is no evidence of any reduction in electroosmotic velocity in columns packed with particles down to 1.5 μm diameter, (2) that reduced plate heights as low as unity are attainable for unretained solutes using both slurrypacked and drawn-packed columns 30 to 200 μm bore and up to 1 m long when packed with conventional 3 and 5 μm silica gels or with 1.5 μm impermeable silica spheres, (3) that columns driven electrically show higher plate efficiencies than identical columns driven by pressure, and (4) that 100,000 plate HPLC separations can be achieved in relatively short times of 30 minutes using in situ derivatised drawn packed capillaries containing 3 and 5 μm ODS-silica gels.     

Investigations into the chromatographic behaviour of silica gels produced from ester silicates

Summary Silica gels produced from ester silicates (ES-gels) are excellent chromatographic supports. In comparison with other silica gels the RP-materials obtained from them show little peak tailing even with polar, and, in particular, with basic compounds. Gels produced by various manufacturing processes have been classified by adsorption with methyl pyridinium chloride. ES-gels yield very low methyl pyridinium chloride values and small asymmetry parameters. The results indicate that there are strongly acidic, structurally-related surface centres which cause peak tailing on most commercial gels. It was shown that surface silanols on silica gels do not, in themselves, lead to peak tailing.     

One‐pot synthesis of a new high vinyl content hybrid silica monolith dedicated to nanoliquid chromatography

A new vinyltrimethoxysilane‐based hybrid silica monolith was developed and used as a reversed‐phase capillary column. The synthesis of this rich vinyl hybrid macroporous monolith, by cocondensation of vinyltrimethoxysilane with tetramethoxysilane, was investigated using an unconventional (formamide, nitric acid) porogen/catalyst system. A macroporous hybrid silica monolith with 80% in mass of vinyltrimethoxysilane in the feeding silane solution was obtained and compared to a more conventional low vinyl content hybrid monolith with only of 20% vinyltrimethoxysilane. Monoliths were characterized by scanning electron microscopy, ²⁹Si nuclear magnetic resonance spectroscopy and N₂ adsorption–desorption. About 80% of the vinyl precursor was incorporated in the final materials, leading to 15.9 and 61.5% of Si atoms bonded to vinyl groups for 20% vinyltrimethoxysilane and 80% vinyltrimethoxysilane, respectively. The 80% vinyltrimethoxysilane monolith presents a lower surface area than 20% vinyltrimethoxysilane (159 versus 551 m²/g), which is nevertheless compensated by a higher vinyl surface density. Chromatographic properties were evaluated in reversed‐phase mode. Plots of ln(k) versus percentage of organic modifier were used to assess the reversed‐phase mechanism. Its high content of organic groups leads to high retention properties. Column efficiencies of 170 000 plates/m were measured for this 80% vinyltrimethoxysilane hybrid silica monolith. Long capillary monolithic columns (90 cm) were successfully synthesized (N = 120 000).     

Reducing pressure and increasing peak capacity by incorporating an adaptable flow stream splitting platform to high-performance liquid chromatography columns: A study on C18 silica-based monoliths

In this study, an adaptable end-column platform was fitted to a commercially available monolith, which enabled the column to be fitted with a flow-splitting device. A variety of flow-splitting adapters could be incorporated into the platform, and in this study, a radial flow stream splitter was utilized. The advantage of the radial flow stream spitter was that it overcame issues relating bed density variations that could cause bands to distort in the radial cross-section of the column. Using propylbenzene as a test standard in isocratic elution mode, height equivalent to a theoretical plate curves were constructed across ten flow rates, and it was found that the column efficiency improved by as much as 73%. Furthermore, the dual outlet flow splitter enabled a very substantial reduction in column back pressure, with the decrease being consistently between 20 to 30% depending on the column length. Additionally, sensitivity increased by 45%, consistent with the observed increase in efficiency. The adaptable end-column platform could be retrofitted to almost any commercial column with the expectation of gaining efficiency, sensitivity, and reducing back pressure.     

多孔整体材料在固相微萃取中的应用研究进展

作为新型的样品前处理技术,固相微萃取由于具有操作简便、使用灵活、样品用量少、环境友好以及便于与分析仪器联用等优点而受到人们的广泛青睐。多孔整体材料具有通透性好、传质速度快、制备简单和易于改性等优点,目前被广泛用于包括样品前处理在内的诸多领域。文章结合作者的研究工作,对近几年整体材料在固相微萃取中的应用研究进行综述,并对其发展方向进行了展望。     

Smart nanocomposite polymer gels

The combination of polymers with nanomaterials displays novel and often enhanced properties compared to the traditional materials. They can open up possibilities for new technological applications. The magnetic polymer gel represents a new type of composites consisting of small magnetic particles, usually from the nanometer range to the micron range, dispersed in a highly elastic polymeric gel matrix. Combination of magnetic and elastic properties leads to a number of striking phenomena that are exhibited in response to impressed magnetic fields. Giant deformational effects, high elasticity, anisotropic properties, temporary reinforcement and quick response to magnetic field open new opportunities for using such materials for various applications.     

Porogens and porogen selection in the preparation of porous polymer monoliths

Porogens are key components required for the preparation of porous polymer monoliths for application in separation science. Porogens determine the stability, selectivity, and permeability of polymer monoliths. This review summarizes the role of porogens in the preparation of porous polymer monoliths with a focus on clear understanding of effect of porogens on morphological properties, porosity, surface area, mechanical stability, and permeability of monoliths, particularly targeting the field of separation science. This review also includes the use of different types of porogens with the focus on various approaches used to set criteria for their systematic selection, including porogen‐free techniques recently used for synthesis of porous monoliths. It discusses the current state‐of‐the‐art applications of porogens in column preparation as well as where the future developments in this field may be directed.     

High performance liquid chromatography of raw sugars and polyols using bonded silica gels

Summary Bonded silica columns have been evaluated for their ability to separate carbohydrates and polyols. Mobile phases consisting of dichloromethane/methanol produced the best separations in comparison with the acetonitrile/water mixtures commonly used with amino columns. Of all the bonded phases tested, LiChrospher Diol silica provided the best separations, and selectivities were not very different from those obtained on the most popular system using an amino bonded phase and acetonitrile/water as eluent. In addition, diol columns with a dichloromethane/methanol eluent offer excellent stability with no Schiffs base formation of reducing sugars. Using an evaporative light scattering detector, low limit detection is obtainable (20 ng of glucose from a column) and gradient elution is quite feasible.     

Formation of gels and gel-glasses in the VO2(V2O5)-SiO2 systems

Blue-coloured gels have been prepared in the VO₂-SiO₂ system up to 80 mol% VO₂ by sol-gel technology using TEOS and aqueous solutions of VOSO₄·5H₂O. It is established by means of VIS and ESR spectra that at low temperatures VO²⁺ complexes are formed. An oxidation of V⁴⁺ has taken place with increasing temperature, and V₂O₅ and cristobalite have been separated. Silica gel glasses stable up to 800°C have been obtained from gels containing 1–3 mol% VO₂.     

Hierarchically porous silver monoliths from colloidal bicontinuous interfacially jammed emulsion gels

Silver monoliths with interconnected hierarchical pore networks and three-dimensional (3D) bicontinuous morphology are synthesized from a colloidal bicontinuous interfacially jammed emulsion gel (bijel) via reduction of silver ions within a nanoporous cross-linked polymer template. The pore sizes may be tuned independently and range from tens of nanometers to over a hundred micrometers. The method is straightforward as well as flexible and can pave the way to a host of hierarchical materials for current technologies.     

Dynamic covalent gels assembled from small molecules: from discrete gelators to dynamic covalent polymers

Dynamic covalent chemistry has emerged recently to be a powerful tool to construct functional materials. This article reviews the progress in the research and development of dynamic covalent chemistry in gels assembled from small molecules. First dynamic covalent reactions used in gels are reviewed to understand the dynamic covalent bonding. Afterwards the catalogues of dynamic covalent gels are reviewed according to the nature of gelators and the interactions between gelators. Dynamic covalent bonding can be involved to form low molecular weight gelators. Low molecular weight molecules with multiple functional groups react to form dynamic covalent cross-linked polymers and act as gelators. Two catalogues of gels show different properties arising from their different structures. This review aims to illustrate the structure-property relationships of these dynamic covalent gels.     

Hierarchically macro/mesoporous silica monoliths constructed with interconnecting micrometer-sized unit rods

Under typical dilute reactant compositions (3 ~ 5 wt% of surfactant template concentration) and conventional hydrothermal conditions for mesoporous materials synthesis, successful preparation of hierarchically macro/mesoporous silica monoliths was reported in this paper. The resultant materials were characterized by a series of techniques including powder X-ray diffraction, N₂ adsorption–desorption, SEM, TEM/EDS, and Hg porosimetry. A new kind of stable and hierarchically porous pure silica monoliths was confirmed, which are featured with highly ordered mesoporous structures, rod-shaped unit particles, large specific surface area of 492 m²/g, continuous macropores of about 4.0 μm in size and high macropore volume of about 13.1 cm³/g. Moreover, using the resultant silica monoliths as hard templates, carbon monoliths have been successfully replicated, which inherit the structural characters of parent silica materials. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mechanistic investigation of hydrolysis reactions of dithioacetal derivatives grafted on silica gels

Silica gels believed to be grafted with dithioacetal derivatives were recently used for Hg(II) extraction, and were found to selectively remove 94-100% of Hg²⁺ ions from metal ion mixtures. The current studies with one derivative suggest that the functional group in the Hg(II) removal is the mercapto (SH) ligand. The dithioacetal group in (ClCOCH₂S)₂CHPh (2) was hydrolyzed during its reaction with amine (O)₃Si(CH₂)₃NH₂ (3) grafted on silica gel to give the mercapto ligand (O)₃Si(CH₂)₃NHCOCH₂SH (4). The silica gel grafted with the mercapto ligand 4 selectively removed Hg²⁺ ions with reported high capacities.