Ethanol Washing Effect on Textural Properties of the Sodium Silicate-Derived Silica Xerogel

This study deals with the use of ethanol as washing solvent in the preparation of the silica gels from sodium silicate in order to enhance the textural properties, especially surface area. We here examined the effect of ethanol-washing on surface area, micro- and mesopore volume, and average pore size. The silica xerogels prepared from sodium silicate solution exhibited an extremely high surface area of 1139 m2/g by washing their hydrogels with ethanol. Compared to water-washed xerogels, ethanol-washed xerogels showed higher surface areas, total pore volumes, and larger average pore sizes. Unlike the surface area of water-washed xerogel, that of the ethanol-washed xerogel was not affected by the silica concentration of initial solution. This study indicates that the textural properties of sodium silicate-derived xerogels are further enhanced by using ethanol as washing solvent.     

Infrared Emission Spectroscopic Study of the Dehydroxylation via Surface Silanol Groups of Synthetic and Natural Beidellite

The structural changes of synthetic and natural beidellites during dehydroxylation have been studied using infrared emission spectroscopy of the OH-stretching and bending regions. The OH-stretching region is characterized by two OH-stretching modes around 3600-3615 cm-1 and around 3650 cm-1. These bands strongly decrease in intensity upon dehydroxylation up to 600 degrees C for the natural beidellite and 700-750 degrees C for the synthetic ones. The differences in bandwidth, intensity, and dehydroxylation behavior are interpreted as due to differences in crystallinity with crystallinity increasing in the order natural beidellite < synthetic beidellite BSK3 < synthetic beidellite E498. Above 400 degrees C a new band attributed to silanol groups becomes visible in all samples due to transfer of the hydroxyls from the octahedral layer to the siloxane layer before they are lost. The broad band around 3300-3400 cm-1 is assigned to both H-bonding in H2O and H-bonding to Si-O-Al linkages. The presence of two different OH groups is also reflected in the OH-bending modes around 875-895 cm-1 and 915-925 cm-1 and in the OH-libration modes around 780 and 800-820 cm-1. These bands show a decrease in intensity upon heating and dehydroxylation of the clay structure. Here again the same order can be observed for the disappearance of the bands as for the OH-stretching region. Copyright 1999 Academic Press.     

High density Mn−Zn ferrites: Synthesis and characterization

Ultrafine and nearly spherical particles of Mn_0.8Zn_0.2Fe₂O₄ were prepared under mild hydrothermal conditions by precipitating from metal nitrates. Mössbauer spectrosopic and magnetization measurements revealed the superparamagnetic nature of these particles having crystallite sizes around 18 nm. These ferrites were found to sinter to almost theoretical density with little or no intergranular porosity.     

Barium Sodium Niobate Powders and Thin Films: Preparation and Ferroelectric Behavior

Powders and thin films of barium sodium niobate, Ba₄Na₂Nb₁₀O₃₀, of filled tungsten bronze type ferroelectric were processed by a sol-gel route using barium metal, niobium ethoxide and sodium methoxide as precursors. Polycrystalline powder resulted after heat treating the gel powder at or above 650°C. Thin films of Ba₄Na₂Nb₁₀O₃₀ showed no preferred orientation on Si(100), Pt/Si(100) and sapphire substrates. Hysteresis measurements at 1 kHz for the thin films annealed at 750°C, obtained using a prehydrolyzed precursor solution, and gave remanent polarization of 17.34 µC/cm² and coercive field of 62.5 kV/cm. Microstructural investigation of surface morphology of these films revealed grains about 0.3 µm in size. Prehydrolysis of the precursor solution was found to be necessary to achieve dense films with ferroelectric properties.     

Liquid chromatography tandem mass spectrometry method for the quantification of clonidine with LLOQ of 10 pg/mL in human plasma

A sensitive high-performance liquid chromatography-positive ion electrospray tandem mass spectrometry method was developed and validated for the quantification of clonidine in human plasma. Following liquid-liquid extraction, the analytes were separated using an isocratic mobile phase on a reverse-phase column and analyzed by MS/MS in the multiple reaction monitoring mode using the respective [M + H](+) ions, m/z 230 to 44 for clonidine and m/z 254 to 44 for the internal standard. The assay exhibited a linear dynamic range of 10-2000 pg/mL for clonidine in human plasma. The lower limit of quantification was 10 pg/mL with a relative standard deviation of less than 6.8%. Acceptable precision and accuracy were obtained for concentrations over the standard curve range. A run time of 2.5 min for each sample made it possible to analyze more than 250 human plasma samples per day. The validated method was successfully used to analyze human plasma samples for application in pharmacokinetic studies. Copyright (c) 2008 John Wiley & Sons, Ltd.     

Quantification of fexofenadine in human plasma by liquid chromatography coupled to electrospray tandem mass spectrometry using mosapride as internal standard

A rapid high-performance liquid chromatography/positive ion electrospray tandem mass spectrometry method was developed and validated for the quantification of fexofenadine in human plasma using mosapride as internal standard. Following solid-phase extraction, the analytes were separated using an isocratic mobile phase on a reverse-phase column and analyzed by MS/MS in the multiple reaction monitoring mode using the respective [M+H]+ ions, m/z 502/466 for fexofenadine and m/z 422/198 for the IS. The method exhibited a linear dynamic range of 1-500 ng/mL for fexofenadine in human plasma. The lower limit of quantification was 1 ng/mL with a relative standard deviation of less than 5% for fexofenadine. Acceptable precision and accuracy were obtained for concentrations over the standard curve range. The total chromatographic run time of 2 min for each sample made it possible to analyze more than 400 human plasma samples per day. The validated method has been successfully used to analyze human plasma samples for application in pharmacokinetic, bioavailability or bioequivalence studies.     

Solid-state NMR and computational chemistry study of mononucleotides adsorbed to alumina

Solid-state NMR spectroscopy and ab initio computational chemistry are used to determine the structure of the complex formed upon adsorption of the mononucleotide 2'-deoxyadenosine 5'-monophosphate (dAMP) to the surface of a mesoporous alumina. In this multi-technique approach, rotational-echo double-resonance NMR results reveal that the phosphate group of dAMP interacts predominantly with octahedrally coordinated aluminum species at the surface, and therefore, adsorption is modeled with both mono- and bidentate sorption of the nucleotide phosphate group with octahedral aluminum. 31P chemical shielding tensors are calculated from the structure of the lowest energy conformations, and these results are compared to tensor values extracted from analysis of spinning-sideband patterns in the experimental 31P cross-polarization magic-angle-spinning NMR spectrum. The chemical shift anisotropy and asymmetry parameter indicate that the binding is via a monodentate, inner-sphere complex.     

LC-ESI-MS/MS method for quantification of ambrisentan in plasma and application to rat pharmacokinetic study

A sensitive high‐performance liquid chromatography–positive ion electrospray tandem mass spectrometry method was developed and validated for the quantification of ambrisentan in plasma. The analyte and the internal standard (armodafinil) were extracted from plasma by acetonitrile precipitation and they were separated on a reversed‐phase C₁₈ column with a gradient program. The MS acquisition was performed with multiple reaction monitoring mode using the respective [M + H]⁺ ions, m/z 379–347 for ambrisentan and m/z 274–167 for the IS. The assay exhibited a linear dynamic range of 1–2000 ng/mL for ambrisentan in plasma. Acceptable precision (<10%) and accuracy (100 ± 8%) were obtained for concentrations over the standard curve range. The method was successfully applied to quantify ambrisentan concentrations in a rodent pharmacokinetic study after a single oral administration of ambrisentan at 2.5 mg/kg to rats. Following oral administration the maximum mean concentration in plasma (C_max; 1197 ± 179 ng/mL) was achieved at 1.0 ± 0.9 h (T_max), and the area under the curve (AUC) was 6013 ± 997 ng h/mL. Therefore, development of such a simple and sensitive method in rat plasma should translate into a method for ambrisentan in human plasma for clinical trials. Copyright © 2012 John Wiley & Sons, Ltd.