Total synthesis of (+)-azimine via diastereoselective aminopalladation

The aminopalladation of amino allylic alcohol using Cl₂Pd(MeCN)₂ in CH₂Cl₂ gave the 2,6-disubstituted piperidine with excellent diastereoselectivity. This compound was successfully converted into (+)-azimine (1) using cross-metathesis and Shiina macrolactonization.     

The effects of substituting Sn for Ni on the thermal and some other characteristics of NiTiSn shape memory alloys

Journal of Thermal Analysis and Calorimetry - In this study, the thermal stability and phase transformation of Ni50-xTi50Snx (x?=?0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 at%) SMAs have...     

Comprehensive two-dimensional liquid chromatography with on-line Fourier-transform-infrared-spectroscopy detection for the characterization of copolymers

The on-line coupling of comprehensive two-dimensional liquid chromatography (liquid chromatography x size-exclusion chromatography, LC x SEC) and infrared (IR) spectroscopy has been realized by means of an IR flow cell. The system has been assessed by the functional-group analysis of a series of styrene-methylacrylate (SMA) copolymers with varying styrene content. Ultraviolet (UV) detection was used as a detection technique to verify the detection with IR. The LC x SEC-IR functional-group contour plots (comprehensive chromatograms) obtained for styrene were in agreement with the contour plots constructed from the UV signal. In addition, contour plots can be obtained from non-UV-active groups. One such plot, for the carbonyl-stretching vibration of methylacrylate (MA), is shown. Selective detection of MA proved possible using flow cell IR detection. The combination of the contour plots for styrene and MA allowed a full characterization of the copolymer and it was revealed that the present series of SMA copolymers exhibited homogeneous chemical-composition distributions (CCDs). In addition, commercially available fast-SEC columns have been assessed in this study with respect to their potential to serve as second-dimension separation columns.     

Characterization of thermal diffusion of polystyrene in binary mixtures of THF/dioxane and THF/cyclohexane

The thermal diffusion coefficient (D_τ) was determined for three polystyrene standards of different molecular masses in binary mixtures of tetrahydrofuran/dioxane and tetrahydrofuran/cyclohexane of various compositions. The D_τ values were obtained by combining retention data from thermal field-flow fractionation measurements with diffusion data from dynamic light scattering experiments. In agreement with earlier work of Schimpf and Giddings, the thermal diffusion coefficient was found to be virtually independent of the molecular mass of the polymers. In the binary mixtures of tetrahydrofuran and dioxane, both good solvents for polystyrene, the D_τ value was approximately equal to the average of the D_τ values in the pure solvents, weighted according to the mole fractions of the solvents in the mixture. However, for polystyrene in binary mixtures of tetrahydrofuran and cyclohexane this linear behavior of the thermal diffusion phenomenon was not observed. The addition of cyclohexane to tetrahydrofuran has initially only a minor effect on the molecular and thermal diffusion coefficients of the polystyrene standards. Because cyclohexane is a theta solvent for polystyrene, the preferential solvation of polystyrene by tetrahydrofuran could be an explanation for these results. © 1996 John Wiley & Sons, Inc.     

Practical experience with organic solvent flow field-flow fractionation

Summary Several types of membrane have been tested for use in organic solvent flow field-flow fractionation in an asymmetric channel. The practical problems most commonly encountered were leakage of air and solvent through the support layer on which the membranes are cast, and unequal swelling of the membrane and the support layer in the organic solvent, leading to ridging of the membrane in the channel. Three types of membrane were found suitable for the separation of polystyrene standards with tetrahydrofuran as solvent. The best results were obtained with a fluoropolymer membrane. Fair agreement was found between theory and practice for the dependence of retention times on the relative molecular mass of the standards and on the flow regime. Use of scanning electron microscopy revealed that for a number of the membrane materials some pores were much larger than expected on the basis of the indicated molecular weight cut-off. Whereas these materials could not be used for the fractionation of soluble polymers, they could be applied with some success to the separation of solid latex and silica particles. A PTFE membrane could be used for the separation of latexes and silica particles suspended in acetonitrile as carrier liquid. In general, however, the retention times of these particles were shorter than theoretically predicted.     

Surface Aggregation of Candida albicans on Glass in the Absence and Presence of Adhering Streptococcus gordonii in a Parallel-Plate Flow Chamber: A Surface Thermodynamical Analysis Based on Acid-Base Interactions

Adhesive interactions between yeasts and bacteria are important in the maintenance of infectious mixed biofilms on natural and biomaterial surfaces in the human body. In this study, the extended DLVO (Derjaguin-Landau-Verwey-Overbeek) approach has been applied to explain adhesive interactions between C. albicans ATCC 10261 and S. gordonii NCTC 7869 adhering on glass. Contact angles with different liquids and the zeta potentials of both the yeasts and bacteria were determined and their adhesive interactions were measured in a parallel-plate flow chamber.Streptococci were first allowed to adhere to the bottom glass plate of the flow chamber to different seeding densities, and subsequently deposition of yeasts was monitored with an image analysis system, yielding the degree of initial surface aggregation of the adhering yeasts and their spatial arrangement in a stationary end point. Irrespective of growth temperature, the yeast cells appeared uncharged in TNMC buffer, but yeasts grown at 37 degrees C were intrinsically more hydrophilic and had an increased electron-donating character than cells grown at 30 degrees C. All yeasts showed surface aggregation due to attractive Lifshitz-van der Waals forces. In addition, acid-base interactions between yeasts, yeasts and the glass substratum, and yeasts and the streptococci were attractive for yeasts grown at 30 degrees C, but yeasts grown at 37 degrees C only had favorable acid-base interactions with the bacteria, explaining the positive relationship between the surface coverage of the glass by streptococci and the surface aggregation of the yeasts. Copyright 1999 Academic Press.     

Current Development and Challenges of Mixed Matrix Membranes for CO2/CH4 Separation

In the early stage of membrane technology development in gas separation, utilization of polymeric membranes has gained attention due to their robustness and ease of fabrication. However, the performance of polymeric membranes is limited by the trade-off between permeability and selectivity. Meanwhile, inorganic membrane is capable to exhibit great enhancement in separation performance but unfortunately its fabrication process is hard and costly. Thus, development of mixed matrix membranes (MMMs) by incorporating inorganic fillers into the polymer matrix has become a potential alternative to overcome the limitations of polymeric and inorganic membranes in gas separation. Nevertheless, fabrication of defect-free MMMs with improved separation performance and without compromising the mechanical and thermal stability is extremely difficult and challenging. In the current review paper, various types of inorganic fillers for MMMs fabrication and recent reported efforts to tailor the underlying problems on MMMs fabrication were discussed. The future outlook to advance the performance of MMMs in gas separation especially for CO₂/CH₄ separation was highlighted.     

Independent assessment of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) sample preparation quality: A novel statistical approach for quality scoring

Preparation of samples according to an optimized method is crucial for accurate determination of polymer sample characteristics by Matrix-Assisted Laser Desorption Ionization (MALDI) analysis. Sample preparation conditions such as matrix choice, cationization agent, deposition technique or even the deposition volume should be chosen to suit the sample of interest. Many sample preparation protocols have been developed and employed, yet finding the optimal sample preparation protocol remains a challenge. Because an objective comparison between the results of diverse protocols is not possible, “gut-feeling” or “good enough” is often decisive in the search for an optimum. This implies that sub-optimal protocols are used, leading to a loss of mass spectral information quality. To address this problem a novel analytical strategy based on MALDI imaging and statistical data processing was developed in which eight parameters were formulated to objectively quantify the quality of sample deposition and optimal MALDI matrix composition and finally sum up to an overall quality score of the sample deposition. These parameters can be established in a fully automated way using commercially available mass spectrometry imaging instruments without any hardware adjustments. With the newly developed analytical strategy the highest quality MALDI spots were selected, resulting in more reproducible and more valuable spectra for PEG in a variety of matrices. Moreover, our method enables an objective comparison of sample preparation protocols for any analyte and opens up new fields of investigation by presenting MALDI performance data in a clear and concise way.     

水热法制备近红外发光NaLa(WO4)2∶Ln(Ln=Er, Nd)球形粒子的研究

利用水热法制备了NaLa（WO4）2∶Ln（Ln=Er,Nd）球形粉末,分别用XRD,TEM和发光光谱等对粉末的物相、形貌、发光性质进行研究。XRD和SEM结果表明：制得的NaLa（WO4）2∶Ln（Ln=Er,Nd）粉末为白钨矿晶型,分散性良好,粒度均匀,呈规则的球形,粒径为1μm左右。发光光谱的测试表明：样品的最佳掺杂浓度为1.0%;随着煅烧温度的增加,样品发光强度逐渐增大;700℃下煅烧时最佳煅烧时间是4 h。     

Method validation for the analysis of 169 pesticides in soya grain, without clean up, by liquid chromatography-tandem mass spectrometry using positive and negative electrospray ionization

Part of a comprehensive study on the comparison of different extraction methods, GC-MS(/MS) and LC-MS/MS detection methods and modes, for the analysis of soya samples is described in this paper. The validation of an acetone-based extraction method for analysis of 169 pesticides in soya, using LC-MS/MS positive and negative electrospray ionisation (ESI) mode, is reported. Samples (5 g) were soaked with 10 g water and subsequently extracted with 100 mL of a mixture of acetone, dichloromethane and light petroleum (1:1:1), in the presence of 15 g anhydrous sodium sulphate. After centrifugation, aliquots of the extract were evaporated and reconstituted in 1.0 mL of methanol, before direct injection of the final extract (corresponding with 0.05 g soya mL(-1)) into the LC-MS/MS system. Linearity, r(2) of calibration curves, instrument limit of detection/quantitation (LOD/LOQ) and matrix effect were evaluated, based on seven concentrations measured in 6-fold. Good linearity (at least r(2)> or =0.99) of the calibration curves was obtained over the range from 0.1 or 0.25 to 10.0 ng mL(-1), corresponding with pesticide concentrations in soya bean extract of 2 or 5-200 microg kg(-1). Instrument LOD values generally were 0.1 or 0.25 ng mL(-1). Matrix effects were negligible for approximately 90% of the pesticides. The accuracy, precision and method LOQ were determined via recovery experiments, spiking soya at 10, 50, 100 microg kg(-1), six replicates per level. In both ESI modes, method LOQ values were mostly 10 or 50 microg kg(-1) and more than 70% of pesticides analysed by each mode met the acceptability criteria of recovery (70-120%) and RSD (< or =20%), at one or more of the three levels studied. A fast, easy and efficient method with acceptable performance was achieved for a difficult matrix as soya, without cleanup.