Kazuki Nakanishi

Octadecylsilylated porous silica rods as separation media for reversed-phase liquid chromatography.

Continuous porous silica rods consisting of a mesoporous (pore size, 14 or 25 nm) silica skeleton of ∼1 μm size and through-pores of ∼1.7 μm were prepared and derivatized to C(18) phase by on-column reaction with octadecyldimethyl-(N,N-diethylamino)silane. The C(18) silica rods gave plate heights of 10-20 μm for aromatic hydrocarbons in 80% methanol and 20-40 μm for insulin in acetonitrile-water mixtures in the presence of trifluoroacetic acid. The performance of the silica rods was much better at a high flow rate than that of conventional columns packed with 5 μm C(18) silica particles having 12 and 30 nm pores, especially for high molecular weight species.

Pore Structure Control of Silica Gels Based on Phase Separation

In the alkoxy-derived sol-gel system, various macroporous morphologies can be obtained by inducing the phase separation parallel to the sol-gel transition. This principle of macroporous morphology control can be best applied to pure silica and silica-based multicomponent oxide systems. The earlier the phase separation takes place than the sol-gel transition, the larger the characteristic sizes of pores and gel skeletons become. The time resolved light scattering measurements revealed that the morphology formation process exhibits the features of spinodal decomposition and that the final gel morphology is determined by the competitive kinetics between the domain coarsening and the structure freezing by sol-gel transition. The mesopore structure of such macroporous gel skeletons could be easily tailored by the solvent exchange procedures. Silica gels with controlled macropores and mesopores were successfully applied as a material for the continuous rod type column for high performance liquid chromatography.

Monolithic silica columns for high-efficiency chromatographic separations

Studies on the structural and chromatographic properties of monolithic silica columns were reviewed. Monolithic silica columns prepared from tetraalkoxysilane by a sol-gel method showed high efficiency and high permeability on the basis of the small-sized silica skeletons, large-sized through-pores, and resulting through-pore size/skeleton size ratios much larger than those found in a particle-packed column.

A New Monolithic-Type HPLC Column For Fast Separations

Summary The application of a new silica-based, monolithic-type HPLC-column for fast separations is presented. The column is prepared according to a new sol-gel process, which is based on the hydrolysis and polycondensation of alkoxysilanes in the presence of water soluble polymers. The method leads to “rods” made of a single piece of porous silica with a defined pore structure, i. e. macro- and mesopores. The main feature of silica rod columns is a higher total porosity, about 15% higher than of conventional particulate HPLC columns. The resulting column pressure drop is therefore much lower, allowing operation at higher flow rates including flow gradients. Consequently, HPLC analysis can be performed much faster, as it is demonstrated by various applications.

Monolithic silica columns for HPLC, micro-HPLC, and CEC

Two types of monolithic silica columns derivatized to form an ODS phase, one prepared in a fused silica capillary (SR-FS) and the other prepared in a mold and clad with an engineering plastic (poly-ether-ether-ketone) (SR-PEEK), were evaluated. The column efficiency and pressure drop were compared with those of a column packed with 5-μm ODS-silica particles and of an ODS-silica monolith prepared in a mold and wrapped with PTFE tubing (SR-PTFE). SR-FS gave a lower pressure drop than a column packed with 5-μm particles by a factor of 20, and a plate height of 20 μm at a linear velocity below 1 mm/s. SR-PEEK showed higher flow-resistance than the other monolithic silica columns, but they still showed a minimum plate height of 8-10 μm and a lower pressure drop than popular commercial columns packed with 5-μm particles. The evaluation of SR-FS columns in a CEC mode showed much higher efficiency than in a pressure-driven mode.

Monolithic silica columns with various skeleton sizes and through-pore sizes for capillary liquid chromatography

Reduction of through-pore size and skeleton size of a monolithic silica column was attempted to provide high separation efficiency in a short time. Monolithic silica columns were prepared to have various sizes of skeletons (approximately 1-2 microm) and through-pores (approximately 2-8 microm) in a fused-silica capillary (50-200 microm I.D.). The columns were evaluated in HPLC after derivatization to C18 phase. It was possible to prepare monolithic silica structures in capillaries of up to 200 microm I.D. from a mixture of tetramethoxysilane and methyltrimethoxysilane. As expected, a monolithic silica column with smaller domain size showed higher column efficiency and higher pressure drop. High external porosity (> 80%) and large through-pores resulted in high permeability (K = 8 x 10(-14) -1.3 x 10(-12) m2) that was 2-30 times higher than that of a column packed with 5-mirom silica particles. The monolithic silica columns prepared in capillaries produced a plate height of about 8-12 microm with an 80% aqueous acetonitrile mobile phase at a linear velocity of 1 mm/s. Separation impedance, E, was found to be as low as 100 under optimum conditions, a value about an order of magnitude lower than reported for conventional columns packed with 5-microm particles. Although a column with smaller domain size generally resulted in higher separation impedance and the lower total performance, the monolithic silica columns showed performance beyond the limit of conventional particle-packed columns under pressure-driven conditions.

Phase separation in silica sol-gel system containing polyacrylic acid I. Gel formaation behavior and effect of solvent composition

Phase separations of gelling solutions have been investigated for acid-catalyzed, alkoxysilane-based systems containing polyacrylic acid (HPAA). Gels with micrometer-range interconnected porous morphologies were obtained in a composition range broader than that reported for the systems containing sodium polystyrene sulfonate. It is suggested that the data of a light-scattering experiment can be explained by the occurrence of spinodal phase separation which forms an interconnected morphology. The analogy between a chemical crosslinking reaction and a finite-rate cooling of a network-forming liquid is introduced, on which basis the effects of compositional parameters on the resultant gel morphology are explained.

Preparation of monolithic silica columns for high-performance liquid chromatography.

Preparation methods of monolithic silica columns for HPLC including the surface modification were reviewed. Chemical modification methods recently reported to obtain stationary phases for reversed-phase (RP), chiral, ion-exchange, and hydrophilic interaction chromatography (HILIC) separations were discussed. Recent results related to preparation methods of monolithic silica were also covered. The characteristics and properties of silica monoliths and some applications of monolithic silica columns for different analytical and bioanalytical fields will be commented.

Monolithic silica columns for high-efficiency separations by high-performance liquid chromatography.

Generation of a large number of theoretical plates was attempted by capillary HPLC. Monolithic silica columns having small skeletons (ca. 2 microm) and large through-pores (ca. 8 microm) were prepared by a sol-gel method in a fused-silica capillary (50 microm I.D.), and derivatized to C18 phase by on-column reaction. High external porosity (>80%) and large through-pores resulted in high permeability (K= 1.2 x 10(-2) m2). The monolithic silica column in the capillary produced a plate height of about 12 microm in 80% acetonitrile at a linear velocity of 1 mm/s. Separation impedance, E value, was found to be as low as 200, that was about an order of magnitude lower than reported values for conventional columns packed with 5 microm particles. Reproducibility of preparation within +/- 15% was obtained for column efficiency and for pressure drop. It was possible to generate 100,000 plates by using a 130-cm column at very low pressure (<7 kg/cm2). A considerable decrease in column efficiency was observed at high linear velocity, and for solutes with large retention factors due to the slow mobile-phase mass transfer in the large through-pores. The monolithic silica columns, however, showed performance beyond the limit of conventional particle-packed columns in HPLC under favorable conditions.

Process of formation of bone-like apatite layer on silica gel

It has been proposed that a hydrated silica plays an important role in forming a biologically active apatite layer on the surfaces of bioactive glasses and glass-ceramics in the body. Recent experiments have shown that a silica hydrogel actually induces apatite formation on its surface in a simulated body fluid (SBF). In the present study the process of apatite formation on silica gel was investigated by means of thin-film X-ray diffraction, Fourier-transformed infrared reflection spectroscopy and scanning electron microscopic observation of the surface of the silica gel, as well as the measurement of changes in the ion concentration of the fluid. It was found that the induction period for the apatite nucleation on the surface of the silica gel was about 6 days. Once the apatite nuclei were formed they grew, taking a spherulitic form by consuming the calcium and phosphate ions from the surrounding fluid. Each spherulite consisted of a lot of flake that clustered into a petal-like morphology. The flake was carbonate-containing hydroxyapatite of small-crystallites and/or defective structure. The Ca/P ratio of the apatite was estimated as 1.5–1.6. Thus, the apatite formed was able to induce secondary nucleation of the apatite.