石墨烯修饰的复合热界面材料的热性能研究

采用还原氧化石墨烯(RGO)和三维网状石墨烯(3DGN)修饰环氧树脂,制备高性能的复合热界面材料(TIM)。其中,3DGN为声子提供了快速输运网络,而RGO则显著增强石墨烯导热填料与环氧树脂基材界面处声子的传输能力。RGO表面官能团的类型对样品的热性能有显著影响,羧基是促进界面区域声子传输的最佳选择。优化RGO(20 wt%)和3DGN(10 wt%)的质量分数后,样品的导热系数达到6.5Wm-1K-1,是本征环氧树脂的32倍。此外,复合TIM的拉伸极限也达到280%。     

Analysis of coumarins by micro high-performance liquid chromatography-mass spectrometry with a particle beam interface

Coumarins are a large group of compounds that are naturally present in plant tissues and that exhibit a wide range of pharmacological properties. Analytical methods based on chromatographic techniques and conventional detectors are inadequate to accurately analyze coumarins in complex matrices such as plant extracts. In this article a new method based on a modified particle beam liquid chromatography-mass spectrometry interface is described. The method allows specific and accurate determination of several coumarins in biological matrices. An application regarding the analysis of 18 coumarins in the extract of Smyrnium perfoliatum L. is also reported.     

Determination of aflatoxins in peanut meal by LC/MS with a particle beam interface

Summary A new method for the analysis of aflatoxins in food extracts, based on liquid chromatography/mass spectrometry interfacing, is presented. The chromatographic separation was performed with a reversed phase packed capillary column coupled with a modified particle beam interface capable of handling microliter per minute flow rates. This system allows higher overall sensitivity and easier operation procedures. The method has proved to be particularly suitable for the analysis of the toxins in very complex matrices. The specificity of electron impact ionization allowed positive identification of the aflatoxins with an excellent response linearity for accurate quantitation.     

Electron capture ionization of explosives with a microflow rate particle beam interface

A method for the analysis of four widely used explosives based on reversed-phase liquid chromatography coupled to a quadrupole mass spectrometer is presented. A microflow rate particle beam interface was employed that offers simplified operation procedures and improved interfacing performance. A positive role played by the reduced size of the aerosol droplets generated by the microflow rate interface is outlined in this work. Greater vaporization efficiency and negligible thermal decomposition were observed for the selected compounds in the ion source of the mass spectrometer. Electron capture ionization allowed specific and sensitive determination of the analytes. Detection limits that ranged between 60 and 200 pg and had a signal-to-noise ratio of 5:1 were obtained in selected ion monitoring mode after column elution.     

Coupling a particle beam interface directly to a quadrupole ion trap mass spectrometer

A particle beam interface has been coupled to a quadrupole ion trap mass spectrometer. The system allows the collection of electron ionization mass spectra from analyte in solution. The interface incorporates a pneumatic nebulizer, a heated desolvation chamber, and a three-stage separator region. Additional helium, for improved performance, is added through stage 3. The particles formed in the interface are separated from solvent molecules and are transferred directly to the ion trap where they are expected to collide with the hot hyperbolic surface of the end cap. The end cap serves both as a heated target used to vaporize the particles and as an ion-trapping electrode. Mass analysis is achieved with the mass-selective instability scan supplemented with resonance ejection. Electron ionization spectra from 100 ng of caffeine [molecular weight (MW) = 1941; 1-naphthalenol methylcarbamate (carbaryl) (MW = 2011, 17α-hydroxyprogesterone (MW = 330), and reserpine (MW = 608) are shown using sampling by a segmented flow analysis. Some charge exchange is evident with methanol as well as self-chemical ionization at higher analyte levels. The interface shows a nonlinear caffeine calibration curve for analyte amounts below 30 ng and a more linear response at higher amounts. Caffeine was detected at 25 pmol (5 ng), with a signal-to-noise ratio of 50, 20-μL loop, full scan.     

Characterization of chromium-carbene complexes by high-performance liquid chromatography-mass spectrometry with particle beam interface

High-performance liquid chromatography (HPLC) coupled with mass spectrometric (MS) detection was used to separate and characterize a series of chromium -aminocarbene and alkoxycarbene complexes of the Fischer type, some of which were synthesized as new compounds. Chromium-carbene complexes are known to have interesting photochemical properties. The separation of all the compounds examined was performed under normal-phase conditions and a particle beam LC-MS interface was used. The acquisition of positive-ion and negative-ion chemical ionization mass spectra of the eluates was performed. The use of the LC-PB-MS system demonstrated the potential role of this technique in the elucidation of the structure of polar organometallic compounds, such as the carbene complexes of chromium examined.     

Liquid chromatographic-mass spectrometric determination of phenolic compounds using a capillary-scale particle beam interface.

A capillary-scale particle beam interface was used to detect 18 phenolic compounds in red wine samples. This technique allows reproducible, library searchable electron ionization spectra at only 1 microliter/min mobile phase flow-rate for a sensitive detection of the analytes in complex matrices. The method makes use of a narrow bore, reversed-phase packed capillary column for sample separation. Detection limits were in the low picogram range for most compounds. Sensitivity and response linearity were evaluated for eight phenolic acids, which are often encountered in red wines. The phenolic compound composition was outlined in two red wines obtained using different aging processes.     

Electron ionization in LC-MS: recent developments and applications of the direct-EI LC-MS interface

The purpose of this article is to underline the possibility of efficiently using electron ionization (EI) in liquid chromatography (LC) and mass spectrometry (MS). From a historical perspective, EI accompanied the first attempts in LC-MS but, owing to several technical shortcomings, it was soon outshined by soft, atmospheric pressure ionization (API) techniques. Nowadays, two modern approaches, supersonic molecular beam LC-MS and direct-EI LC-MS, offer a valid alterative to API, and preserve the advantages of EI also in LC-MS applications. These advantages can be summarized in three crucial aspects: automated library identification; identification of unknown compounds, owing to EI extensive fragment information; inertness to coeluted matrix interferences owing to very unlikely ion–ion and ion–molecule interactions in the EI gas-phase environment. The direct-EI LC-MS interface is a simple and efficient solution able to produce high-quality, interpretable EI spectra from a wide range of low molecular weight molecules of different polarity. Because of the low operative flow rates, this interface relies on a nano-LC technology that helps in reducing the impact of the mobile phase on the gas-phase environment of EI. This review provides an extensive discussion on the role of EI in LC-MS interfacing, and presents in detail several performance aspects of the direct-EI LC-MS interface, especially in terms of response, mass-spectral quality, and matrix effects. In addition, several key applications are also reported.     

Investigation of the effect of a dc glow discharge on the performance of a particle beam liquid chromatography/mass spectrometry interface

A low voltage (180-V) dc glow discharge device was inserted just below the pneumatic nebulizer in a particle beam interface of a high performance liquid chromatography/mass spectrometry system. The discharge in a helium atmosphere increased the signal produced by 12 test compounds by factors of about 2–6. The increases in signal were probably produced by an increase in the efficiency of solute transmission through the interface. The signal increases caused by the glow discharge were compared to somewhat larger increases caused by 0.01-M ammonium acetate in the mobile phase. The combination of glow discharge and ammonium acetate provided no meaningful advantage over the individual techniques. The mechanism of improved transport efficiency is not proven, but the neutralization of charged particles is a viable hypothesis.     

Development of electrochemical performances of carbon black obtained by the surface organosilicon-modified method

Carbon black (CB) was simply modified by tert-butylchlorodimethylsilane (TBDMCl) to form the–C–OSiR and–C–OOSiR structures on the surface of CB. The formation of–C–OSiR and–C–OOSiR structures on CB was verified by FTIR and XRD measurements. The modified carbon black (MCB) has relative high electrochemical performance, which was determined by electrochemical studies. The first charging capacity of MCB is at 343 mAh g^–1 and the conductivity of MCB is at 148.1 Sm^–1 which is similar to that of CB 155.2 Sm^–1. These results indicate that application of the obtained MCB will be promising for lithium anode materials. Meanwhile, these results can also suggest solving the overproduction problem of CB in China.     

Evidence for thermal decomposition contributions to the mass spectra of chlorinated phenoxyacid herbicides obtained by particle beam liquid chromatography mass spectrometry

The spectral quality of a group of chlorinated phenoxyacid herbicides has been shown to degrade under certain conditions upon introduction into the mass spectrometer by a particle beam interface. Experiments were performed to investigate these changes in spectra. Normalized ion chromatograms were generated for the herbicides, and the results showed a broadening of the profiles of some ions, indicating a longer residence time in the ion source. These ions were postulated as coming from the ionization of thermal degradation products from the herbicides. The generation of these ions was dependent on ion source temperature, analyte concentration, and, by implication, ion source cleanliness. Tandem mass spectrometry experiments were performed on these ions from the herbicides and ions from the corresonding phenols. The tandem mass spectra of the ions from the herbicides were similar to the tandem mass spectra of the ions from the phenols. Therefore, it appears that the particle beam mass spectra of the chlorinated phenoxyacid herbicides are composite spectra with contributions from the gas phase ionization of the parent herbidides and thermal decomposition products.     

Electric forces induced by a charged colloid particle attached to the water-nonpolar fluid interface

Here, we solve the problem about the electric field of a charged dielectric particle, which is adsorbed at the water-nonpolar fluid (oil, air) boundary. The solution of this problem is a necessary step for the theoretical prediction of the electrodipping force acting on such particle, as well as of the electrostatic repulsion and capillary attraction between two adsorbed particles. In accordance with the experimental observations, we consider the important case when the surface charges are located at the particle-nonpolar fluid boundary. To solve the electrostatic problem, the Mehler-Fock integral transform is applied. In the special case when the dielectric constants of the particle and the nonpolar fluid are equal, the solution is obtained in a closed analytical form. In the general case of different dielectric constants, the problem is reduced to the numerical solution of an integral equation, which is carried out by iterations. The long-range asymptotics of the solution indicates that two similar particles repel each other as dipoles, whose dipole moments are related to the particle radius, contact angle, dielectric constant and surface charge density. The investigated short-range asymptotics ensures accurate calculation of the electrodipping force. For a fast and convenient application of the obtained results, the derived physical dependencies are tabulated as functions of the contact angle and the dielectric constants.     

Effect of surface tension and surface elasticity of a fluid-fluid interface on the motion of a particle immersed near the interface

The motion of a particle immersed in a fluid near a fluid-fluid interface is studied on the basis of the linearized Navier-Stokes equations. The motion is influenced by surface tension, dilatational surface elasticity modulus, and surface shear modulus, as well as by gravity. The backflow at the location of the particle after a sudden impulse has some universal features that are the same as for a rigid wall with stick boundary conditions. At short times the flow depends only on the mass densities of the two fluids. The nature of the short-time flow is calculated from potential flow theory. At a somewhat later time the particle shows a pronounced rebound. The maximum value of the rebound and the time at which the maximum occurs depend on the elastic properties of the interface.     

Use of a particle beam interface combined with mass spectrometry/negative chemical ionization to determine polar herbicide residues in soil by liquid chromatography

The use of mass spectrometry/negative chemical ionization (MS/NCI), in combination with selected-ion monitoring, allows sensitive and selective determination of polar and thermally unstable herbicide residues by liquid chromatography coupled to a particle beam interface. The method has been applied to the analysis of soil samples for herbicide residues, using various procedures for their extraction and employing matrix-standard calibration to avoid quantitative errors due to transfer of matter through the interface. Data for the fragment ions found in the MS/NCI procedure and chromatograms from analyses of real samples are also presented.     

Reexamination of the heat capacities obtained by flow microcalorimetry. Recommendation for the use of a chemical standard

A series of measurements with aqueous electrolyte and nonelectrolyte solutions indicates that there is a small systematic difference between the heat capacities per unit volume determined with a Picker flow microcalorimeter and the original prototype. Through various tests and comparisons, it is, concluded that the commercial instrument gives results closer to the true values. Most of the previous data obtained in our laboratory have been corrected and expressed relative to aqueous NaCl at 25°C taken as a standard.On leave of absence from the Free University of Amsterdam, Amsterdam, The Netherlands     

An improved bonded-polydimethylsiloxane solid-phase microextraction fiber obtained by a sol-gel/silica particle blend

A novel procedure for solid-phase microextraction fiber preparation is presented, which combines the use of a rigid titanium alloy wire as a substrate with a blend of PDMS sol–gel mixture/silica particles, as a way of increasing both the mechanical robustness and the extracting capability of the sol–gel fibers. The 30 μm average thick fibers displayed an improvement in the extraction capacity as compared to the previous sol–gel PDMS fibers, due to a greater load of stable covalently bonded sol–gel PDMS. The observed extraction capacity was comparable to that of 100 μm non-bonded PDMS fiber, having in this case the advantages of the superior robustness and stability conferred, respectively, by the unbreakable substrate and the sol–gel intrinsic characteristics. Repeatability (n = 3) ranged 1–8% while fiber production reproducibility (n = 3) ranged 15–25%. The presence of the silica particles was found to have no direct influence on the kinetics and mechanism of the extraction process, thus being possible to consider the new procedure as a refinement of the previous ones. The applicability potential of the devised fiber was illustrated with the analysis of gasoline under the context of arson samples.     

Translational motion of a spherical particle near a planar liquid-fluid interface

The translational electrophoretic motion of a colloidal spherical particle parallel to a planar liquid-fluid interface is analyzed by using the reciprocal theorem developed by Yariv and Brenner [E. Yariv, H. Brenner, J. Fluid Mech. 484 (2003) 85]. Based on the thin electric double layers assumption, analytical solutions of the forces acting on the particle are obtained, and the influence of the liquid-fluid interface on the electrophoretic velocity of the particle is studied. It is found that the speed of the particle's electrokinetic motion will increase as the separation distance between the particle and the interface decreases. This enhancement of electrophoretic mobility becomes more significant when the viscosity of the fluid phase becomes larger.     

Determination of chloramphenicol in muscle using a particle beam interface for combining liquid chromatography with negative-ion chemical ionization mass spectrometry.

A simple and rapid liquid chromatographic-mass spectrometric analysis of chloramphenicol in calf muscle is presented. A particle beam interface was used, with negative-ion chemical ionization mass spectrometry using methane as the reagent gas. The method specificity was tested for three related compounds, dehydrochloramphenicol, nitrosochloramphenicol and nitrophenylaminopropanediol. The extraction procedures require 5 g of muscle, and the quantification limit is 2 micrograms/kg for chloramphenicol. Residues were detected in calf muscle 48 h after intravenous administration of chloramphenicol (25 mg/kg body weight).     

Surface and interface study of ZnO nanoparticles modified by octadecanol phosphate

In this paper, we describe an effective method in which ZnO nanoparticles were prepared through the rapid precipitation transformation reaction in aqueous solution of ZnSO₄ and NaOH with octadecanol phosphate (ODP) as a modifying agent. From the study on the surface and the interface, ZnO nanoparticles modified by ODP exhibited small size, pore structure, good dispersion, and hydrophobicity. The wide variety of surface wettability can be achieved by changing the preparation parameters. The research offers a simple and effective approach to prepare ZnO filler in mild condition and enhances interfacial compatibility between ZnO powders and matrixes by treating the surface with certain capping molecules. Copyright © 2010 John Wiley & Sons, Ltd.     

Electrophoresis of a spherical particle normal to an air-water interface

Electrophoresis of a spherical particle normal to an air-water interface is considered theoretically in this study. The presence of the air-water interface is found to reduce the particle mobility in general, especially when the double layer is very thick. This boundary effect diminishes as the double layer gets very thin. The higher the surface potential, the more significant the reduction of mobility due to the polarization effect from the double layer deformation when the particle is in motion. Local extrema are observed in the mobility profiles with varying double layer thickness as a result. Comparison with a solid planar boundary is made. It is found that the particle mobility near an air-water interface is smaller than that near a solid one when the double layer is thick, and vice versa when the double layer is thin, with a critical threshold value of double layer thickness corresponding roughly to the touch of the interface. The reason behind it is clearly explained as the buildup of electric potential at the air-water interface, which reduces the driving force as a result.