过渡金属原子对掺杂g-CN单层作为高效氮电催化剂性能

由于氨是药物、肥料和树脂等领域的基础,氨合成一直广受关注.工业中主要通过Haber-Bosch反应制备氨,反应需要在高温高压下进行.因此,探索其它氨合成技术对减轻能源消耗和缓解温室效应具有重大意义.在溶液条件下,采用水作为氢质子源,电化学还原氮合成氨方法受到了极大关注.然而,大多数电催化剂难以活化氮气分子且电催化氮气还原过程中存在副反应竞争,因此,研发高效的电催化材料仍然是一个重要研究领域.研究人员探索了多种电催化材料,其中,双原子对催化剂成为电催化领域的研究热点.与单原子催化剂相比,双原子对催化剂不仅具有低配位的金属原子,而且可以通过调节额外分散的金属原子来改善多数电催化反应性能.作为一种新型碳氮材料,二维g-CN具有高表面积、多孔结构以及出色的光学活性和热力学稳定性,可以与金属原子对良好地适配,是一种有潜力的基底材料.然而,目前有关金属双原子对负载在g-CN单层上作为电催化剂催化N₂分子还原性能尚不清楚.本文采用密度泛函理论计算研究了N₂分子在过渡金属原子对(TM=Sc~Zn)掺杂g-CN单层上的吸附和活化,根据吉布斯自由能详细地研究了电催化合成氨的电化学机理.计算发现,在Fe₂@CN和Co₂@CN催化剂上,其决速步骤的自由能变化分别为0.47和0.78 eV.对于Fe₂@CN,N₂电还原反应机制遵循末端路径,而在Co₂@CN上,其还原过程为末端或混合路径.由于Co₂@CN对析氢反应的抑制效果较好,因此该电催化材料体系极具竞争力.相比于Co₂@CN,Fe₂@CN具有较好的氮气活化性能,但选择性较差.另外,N₂分子与Fe₂@CN和Co₂@CN之间存在电荷的接受-给予过程,这在活化惰性N₂分子中氮原子间的三键上起到了关键作用.第一性原理分子动力学模拟结果表明,Fe₂@CN和Co₂@CN表现出较高的结构稳定性.因此,本文深入探讨了过渡金属原子对掺杂g-CN单层催化剂上的氮气还原效率及机制,为合理设计该系列的高效、低成本电催化剂提供理论依据.     

用于水中有机磷农药检测的离子迁移率谱仪预富集进样方法

建立了检测水中有机磷农药的离子迁移率谱仪预富集进样方法。预富集器由表面覆盖有吸附薄膜的微热板、聚四氟乙烯电路板和管座组成,具有操作简单,无需有机溶剂,自加热,热容小,功耗低等优点。以马拉硫磷检测为例,分析了富集器解吸升温速率和离子迁移率谱仪半透膜温度对检测结果的影响。采用高温短时脉冲加热和低温维持加热相结合的解吸方式,既可形成较高的进样浓度脉冲,又可减少进入漂移管的杂质,有利于提高离子迁移率谱仪检测灵敏度。实验表明:采用所述预富集及两阶段加热解吸进样方法,对水中马拉硫磷的检出限为3.9μg/L,达到了国家标准对水中有机磷检测的要求。     

Microfabricated electroosmotic pump for capillary-based sequential injection analysis

A microfabricated electroosmotic pump with an integrated serpentine isolation channel was developed on a glass chip and applied to a capillary-based sequential injection analysis (SIA) system for an enzyme inhibition assay. The pump chip contains an anode reservoir, an ion-exchange membrane electric field decoupler (EFD) that also serves as a cathode reservoir, parallel pump channels and an isolation channel. A two-step etching process was adopted to etch the pump channels to a depth of 20 μm and the isolation channel to a depth of 90 μm. The pump flow rate was very stable: the relative standard deviation (RSD) of the pump rate was 1.9% for propulsion and 2.3% for aspiration. The pump chip was successfully applied to a capillary-based sequential injection analysis system with a confocal fluorescence detector. For repetitive analysis of a 13 μM fluorescein solution, an RSD of 0.6% was attained, which demonstrated the flow stability of the EOF pump. The system was then applied to an enzyme inhibition assay, the diethylenetriaminepentaacetic acid (DTPA) inhibition of the β-galactosidase-catalyzed hydrolysis of fluorescein di(β-d-galactopyranoside). Reproducible results (RSD<3.0%) were obtained.     

Ideal velocity focusing in a reflectron time-of-flight mass spectrometer

A single-stage ion mirror in a time-of-flight (TOF) mass spectrometer (MS) can perform first order velocity focusing of ions initially located at a start focal plane while second order velocity focusing can be achieved using a double-stage reflectron. The situation is quite different when an ion source extraction field is taken into account. In this case which is common in any practical matrix-assisted laser desorption/ionization (MALDI) TOF-MS a single-stage reflectron, for example, cannot perform velocity focusing at all. In this paper an exact, analytic solution for an electric field inside a one-dimensional reflectron has been found to achieve universal temporal focusing of ions having an initial velocity distribution. The general solution is valid for arbitrary electric field distributions in the upstream (from the ion source to the reflectron) and downstream (from the reflectron to an ion detector) regions and in a decelerating part of the reflectron of a reflectron TOF mass spectrometer. The results obtained are especially useful for designing MALDI reflectron TOF mass spectrometers in which the initial velocity distribution of MALDI ions is the major limiting factor for achieving high mass resolution. Using analytical expressions obtained for an arbitrary case, convenient working formulas are derived for the case of a reflectron TOF-MS with a dual-stage extraction ion source. The special case of a MALDI reflectron TOF-MS with an ion source having a low acceleration voltage (or large extraction region) is considered. The formulas derived correct the effect of the acceleration regions in a MALDI ion source and after the reflectron before detecting ions.     

An economical fluorescence detector for lab-on-a-chip devices with a light emitting photodiode and a low-cost avalanche photodiode

An economical fluorescence detector was developed with an LED as the exciting source and a low-cost avalanche photodiode (APD) module as a photon sensor. The detector was arranged in an epifluorescence configuration using a microscope objective (20× or 40×) and a dichroic mirror. The low-cost APD was biased by a direct current (DC) high voltage power supply at 121 V, which is much lower than that normally used for a PMT. Both DC and square wave (SW) supplies were used to power the LED and different data treatment protocols, such as simple average for DC mode, software based lock-in amplification and time specific average for SW mode, were tested to maximize the signal-to-noise ratio. Using an LED at a DC mode with simple data averaging, a limit of detection of 0.2 nmol L(-1) for sodium fluorescein was attained, which is among the lowest ever achieved with an LED as an excitation source. The detector was successfully used in both capillary and chip electrophoresis. The most significant advantages of the detector are the compact size and low cost of its parts. The aim of the work is to prove that widely available, low-cost components for civilian use can be successfully used for miniaturized analytical devices.     

Simple algorithms for nonlinear calibration by the classical and standard additions methods

In univariate calibration by both the classical method and the standard additions method, calibration data are fitted to a response function y=f(x), from which the amount of an unknown x(0) is estimated by solving an equation of form y(0)=f(x(0)). Most such calibrations are limited to linear response functions f, for which the uncertainty in x(0) can be estimated from well-known expressions. The present work describes and illustrates one-step algorithms, in which x(0) is treated as an adjustable parameter in a nonlinear fit of the calibration data, with its standard error thus obtained numerically as a direct outcome of the fit. The computations are easily implemented with a number of data analysis programs and are illustrated here for a representative one, KaleidaGraph. The methods handle heteroscedastic data as easily as homoscedastic and nonlinear functions as easily as linear, permitting the analyst to experiment with different response functions in the quest for an optimum calibration. The estimates of sigma (x0) are obtained from the variance-covariance matrix V for the fit, so for weighted fitting with commercial programs, it is important to know which V--a priori or a posteriori--is being used.     

Characterization of complex hydrocarbon mixtures using on-line coupling of size-exclusion chromatography and normal-phase liquid chromatography to high-resolution gas chromatography

An on-line coupling of size-exclusion Chromatography (SEC), normal-phase liquid Chromatography (NPLC), and gas Chromatography (GC) for the characterization of complex hydrocarbon mixtures is described. The hyphenated system separates according to size, polarity, and boiling point. The use of size exclusion as the first separation step allows for the direct injection of complex (“dirty”) samples withont prior clean-up. SEC-NPLC coupling was realized using an on-line solvent evaporator based on fully concurrent solvent evaporation (FCSE) using a modified loop-type interface, vapor exit and co-solvent trapping. Complete reconcentration of the analytes was realized by the introduction of a cryogenic cold trap. For the subsequent hydrocarbon group-type separation an ammo-silica column with n-heptane as eluent was used. The NPLC-GC coupling was based on an on-column interface using partially concurrent solvent evaporation (PCSE) and an early vapor exit. Initial results obtained on the analysis of a residue from the atmospheric crude-oil distillation (a so-called long residue) are presented as an example of the enormous separation power of the SEC-NPLC-GC system. The application of the system for quantitative analysis has not yet been studied.     

Ti2: accurate determination of the dissociation energy from matrix resonance Raman spectra and chemical interaction with noble gases

UV-visible and resonance Raman spectra of Ti(2) isolated in Ar, Kr, and Xe matrices at temperatures of 10 K were measured by using the 514 nm line of an Ar ion laser. The data show that the Ti(2) molecule interacts strongly with Xe, leading to a significant weakening of the Ti[bond]Ti bond strength. The f(Ti[bond]Ti) force constant decreases in the series Ar>Kr>Xe, from 232.8 Nm(-1) in Ar and 225.5 Nm(-1) in Kr to 199.7 Nm(-1) in Xe. Additional experiments in an Ar matrix containing 2 % of Xe indicate the formation of a molecule of the formula Ti(2)Xe. Our spectra for Ti(2) in an Ar matrix give evidence for several previously not observed members of the Stokes progression. The sum of experimental data allows for an improved estimation of the dissociation energy on the basis of a LeRoy-Bernstein-Lam analysis. A dissociation energy of 1.18 eV was derived from this analysis. The UV-visible data give evidence of the vibrational levels of an excited state of Ti(2).     

Direct classification of olive oils by using two types of ion mobility spectrometers

In this work, we explored the use of an Ion Mobility Spectrometry (IMS) device with an ultraviolet (UV) source, and of a Gas Chromatographic (GC) column coupled to an IM Spectrometer with a tritium source, for the discrimination of three grades of olive oil, namely: extra virgin olive oil (EVOO), olive oil (OO) and pomace olive oil (POO). The three types of oil were analyzed with both equipment combinations as coupled to a headspace system and the obtained ion mobility data were consecutively processed with various chemometric tools. The classification rate for an independent validation set was 86.1% (confidence interval at 95% [83.4%, 88.5%]) with an UV-IMS and 100% (confidence interval at 95% [87%, 100%]) using a GC-IMS system. The classification rate was improved by using a more suitable ionization source and a pre-separation step prior to the IM analysis.     

Novel criteria for fast searching for optimal method in gradient ion chromatography: an integrated approach

In this article, an integrated approach for prediction and optimization in ion chromatography (IC) was presented. The approach provides a fast and reliable insight in the elution behavior of an IC system. The predictions are based on a mathematical model that predicts ion retentions (for both isocratic and gradient modes) by using an empirical isocratic model. Other chromatographic values significant for the optimal elution conditions (resolution, peak asymmetry) are calculated quickly and easily from the predicted retention values of characteristic points of a chromatographic peak. Every day, IC users might find this approach a suitable tool for finding optimal IC elution conditions in a given system.     

High sensitive matrix-free mass spectrometry analysis of peptides using silicon nanowires-based digital microfluidic device

We present for the first time an electrowetting on dielectric (EWOD) microfluidic system coupled to a surface-assisted laser desorption-ionization (SALDI) silicon nanowire-based interface for mass spectrometry (MS) analysis of small biomolecules. Here, the transfer of analytes has been achieved on specific locations on the SALDI interface followed by their subsequent mass spectrometry analysis without the use of an organic matrix. To achieve this purpose, a device comprising a digital microfluidic system and a patterned superhydrophobic/superhydrophilic silicon nanowire interface was developed. The digital microfluidic system serves for the displacement of the droplets containing analytes, via an electrowetting actuation, inside the superhydrophilic patterns. The nanostructured silicon interface acts as an inorganic target for matrix-free laser desorption-ionization mass spectrometry analysis of the dried analytes. The proposed device can be easily used to realize several basic operations of a Lab-on-Chip such as analyte displacement and rinsing prior to MS analysis. We have demonstrated that the analysis of low molecular weight compounds (700 m/z) can be achieved with a very high sensitivity (down to 10 fmol μL(-1)).     

多种类表面活性剂临界胶束浓度定量构效关系

表面活性剂的临界胶束浓度(CMC)是个非常重要的物质特性参数, CMC在研究表面活性剂的工业应用和生物利用方面发挥着关键作用. 本工作提出了一个新的拓扑指数—扩展距离矩阵, 建立了一个稳定的构效关系模型, 并对175种表面活性剂的临界胶束浓度进行了计算预测. 结果表明, 基于新的拓扑指数建立的构效关系模型计算临界胶束浓度能给出稳定可靠的预测结果, 其预测结果相关性系数R²(training set)=0.9295, 相对标准偏差ARD(training set)=8.20%, R²(testing set)=0.9257, ARD(testing set)=6.76%. 与文献中模型预测结果的对比表明, 本工作在稳定性和可靠性上均有显著改善.     

Capillary liquid chromatography-microchip atmospheric pressure chemical ionization-mass spectrometry

A miniaturized nebulizer chip for capillary liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry (capillary LC-microchip APCI-MS) is presented. The APCI chip consists of two wafers, a silicon wafer and a Pyrex glass wafer. The silicon wafer has a DRIE etched through-wafer nebulizer gas inlet, an edge capillary insertion channel, a stopper, a vaporizer channel and a nozzle. The platinum heater electrode and pads for electrical connection were patterned on to the Pyrex glass wafer. The two wafers were joined by anodic bonding, creating a microchip version of an APCI-source. The sample inlet capillary from an LC column is directly connected to the vaporizer channel of the APCI chip. The etched nozzle in the microchip forms a narrow sample plume, which is ionized by an external corona needle, and the formed ions are analyzed by a mass spectrometer. The nebulizer chip enables for the first time the use of low flow rate separation techniques with APCI-MS. The performance of capillary LC-microchip APCI-MS was tested with selected neurosteroids. The capillary LC-microchip APCI-MS provides quantitative repeatability and good linearity. The limits of detection (LOD) with a signal-to-noise ratio (S/N) of 3 in MS/MS mode for the selected neurosteroids were 20-1000 fmol (10-500 nmol l(-1)). LODs (S/N = 3) with commercial macro APCI with the same compounds using the same MS were about 10 times higher. Fast heat transfer allows the use of the optimized temperature for each compound during an LC run. The microchip APCI-source provides a convenient and easy method to combine capillary LC to any API-MS equipped with an APCI source. The advantages and potentials of the microchip APCI also make it a very attractive interface in microfluidic APCI-MS.     

Integrated photocatalytic micropillar nanoreactor electrospray ionization chip for mimicking phase I metabolic reactions

We developed a nanoreactor chip based system to mimic phase I metabolic reactions of small organic compounds. The microchip, made of silicon, has an anatase-phase titanium dioxide (TiO(2)) nanolayer coating for photocatalysis and an integrated electrospray ionization (ESI) tip for direct mass spectrometric (MS) analysis. This novel method for mimicking phase I metabolic reactions uses an on-chip TiO(2)-nanolayer and an external UV-lamp to induce photocatalyzed chemical reactions of drug compounds in aqueous solutions. The reactions of selected test compounds (verapamil, metoprolol, propranolol, lidocaine, 2-acetamidofluorene, and S-methylthiopurine) produced mostly the same main products as phase I metabolic reactions induced by human liver microsomes, rat hepatocytes, or cytochrome P enzymes, showing hydroxylation, dehydrogenation, and dealkylations as the main photocatalytic reactions. With this method it is possible to detect reactive and toxic products (mimicking reactive metabolites) due to the absence of biological matrices and an immediate analysis. The method used is sensitive: only 20-40 pmol (1-10 ng) of a substrate was needed for the experiment, thus it provides an inexpensive method for screening possible metabolites of new drug candidates. Due to small dimensions of the microchip, diffusion lengths are suitable for the high reaction rates, thus providing a rapid analysis as the reaction products can be detected and identified directly after the photoinduced reactions have occurred. The method shows a similar performance to that of electrochemistry, a commonly used technique for mimicking phase I metabolism.     

A flexible loop-type flow modulator for comprehensive two-dimensional gas chromatography

The present investigation is focused on a simple flow modulator (FM), for comprehensive two-dimensional gas chromatography (GC×GC). The interface is stable at high temperatures, and consists of a metallic disc (located inside the GC oven) with seven ports, which are connected to an auxiliary pressure source via two branches, to the first and second dimension, to a waste branch (linked to a needle valve) and to an exchangeable modulation loop (2 ports). The ports are connected via micro-channels, etched on one of the inner surfaces of the disc. Modulation is achieved using a two-way electrovalve, connected on one side to the additional pressure source, and to the two metal branches, on the other. An FM enantio-GC×polar-GC method (using a flame ionization detector) was optimized (a 40-μL loop was employed), for the analysis of essential oils. As an example, an application on spearmint oil is shown; the method herein proposed was subjected to validation. Finally, an FM GC×GC diesel experiment was carried out, using an apolar-polar column combination, to demonstrate the effectiveness of the modulator in the analysis of a totally different sample-type.     

Zirconia nanoparticles enhanced grafted collagen tri-helix scaffold for unmediated biosensing of hydrogen peroxide

A novel, biocompatible, thermally steady, and nontoxic zirconia enhanced grafted collagen tri-helix scaffold was prepared on a graphite electrode. This scaffold provided a microenvironment for loading biomolecules and helped to retain their natural structure. UV-vis spectroscopy and scanning electron microscopy were used to characterize the scaffold and the structure of immobilized biomolecules. Using horseradish peroxidase (HRP) as an example, this scaffold accelerated its electron transfer and led to its direct electrochemical behavior with a good thermal stability up to 80 degrees C. The surface electron-transfer rate constant of the immobilized HRP was (5.55 +/- 0.43) s(-)(1) in 0.1 M pH 7.0 PBS at 18 degrees C. The immobilized HRP showed an electrocatalytic activity to the reduction of hydrogen peroxide (H(2)O(2)) without aid of an electron mediator. The linear response range of the biosensor for H(2)O(2) was from 1.0 to 73.0 microM with a correlation coefficient of 0.999 (n = 14), a limit of detection down to 0.25 microM and an apparent Michaelis-Menten constant of (0.28 +/- 0.02) mM. The biosensor exhibited high sensitivity, acceptable stability, and reproducibility. The ZrO(2) grafted collagen provided an excellent matrix for protein immobilization and biosensor preparation.     

Determination of trace iron in beer using flow injection systems with in-valve column and bead injection

Three flow injection (FI) systems were investigated for the determination of trace iron in beer: an FI-in-valve column-flame atomic absorption spectrophotometry (FI-FAAS) system, a spectrophotometric FI system with a column placed at the detection point, and an FI-spectrophotometric system with bead injection (FI-BI). Cationic exchange resin Dowex 50W X8 and iminodiacetate chelating resin, Chelex-100, were employed for the FI-spectrophotometric and FI-FAAS systems, respectively. The FI-in-valve column, packed with the resin, enhances the FAAS performance. The spectrophotometric FI system with a column (packed with Chelex-100) placed at the detection point (in a cell holder of a spectrophotometer) is based on the formation of iron (II)–1,10-phenanthroline complex sorbed onto the resin. No eluent has been found to be suitable. The FI-BI for renewable microcolumn has been proven to be an alternative. The FI-FAAS and FI-BI procedures provide online sample preseparation and preconcentration for the determination of iron in beer. Both are simple, rapid, and economical. The procedures also involve sample preparation (decarbonation and suppression of tannin interference by adding ascorbic acid) and standard addition. The results obtained by FI-FAAS and FI-BI agree with those of AOAC spectrophotometric method.     

Simple and Sensitive Determination of Sibutramine in Slimming Tea Beverages Using a Carbon Screen‐printed Electrode with Adsorptive Stripping Voltammetry

The stimulant sibutramine is an anorexic agent found as an adulterant in natural products and multivitamins supplements used for weight‐loss. In this work, a carbon graphite screen‐printed electrode (SPE‐Gr) with adsorptive stripping pulse differential voltammetry (AdSDPV) is presented for the sensitive and simple detection of sibutramine in slimming tea beverages. The proposed electrochemical method shows a linear working range from 2.0 to 120 μM with a low LOD (0.3 μM) for sibutramine determination in slimming tea samples. The analytical performance of the SPE‐Gr with AdSDPV for sibutramine detection suggests its possible application as an easy, fast and low‐cost method to analyse adulterated tea samples with this stimulant at low levels (<0.1 %).     

Determination of folate in infant formula and adult/pediatric nutritional formula by optical biosensor assay: First Action 2011.05

After a review of data from a single-laboratory validation (SLV) study published in the International Dairy Journal 21, 783-789 (2011), a method for folate in infant formula and adult/pediatric nutritional formula was submitted for consideration of adoption by AOAC as an automated assay that is rapid and simple. The method uses an optical biosensor assay to quantitate total folate content in milk and milk-based pediatric and adult nutritional products. The assay uses folate binding protein and a functionalized sensor surface. The SLV showed an instrumental LOD of 0.1 ng/mL (equivalent to 2.5 microg/100 g for a typical infant formula). The method detection limit was 6.5 microg/100 g with a repeatability of 3.48% and an intermediate reproducibility of 4.63% RSD.     

Morphology controls of the melt blending in a novel highly crosslinked bismaleimide system

A novel bismaleimide of 2,2-bis[4-(4-maleimidophenoxy)phenyl]propane (BMIP) with a broad working-temperature-range for the melt blending was successfully synthesized. BMIP possesses a considerably broad working-temperature-range from 75 °C to 250 °C, prior to undergoing cure reactions to form a highly crosslinked network. The morphology types of cured BMIP/clay hybrids can be controlled by varying the shearing temperatures and the contents of the clay. The conditions necessary for achieving an exfoliated or an intercalated BMIP/clay hybrid were thoroughly investigated via X-ray diffractometry, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). All the uncured samples prepared at different shearing temperatures and with an adequate amount of MMT-C (above 3 phr) exhibited an intercalated form of morphology. However, the crosslinking reactions for specified samples prepared at relatively elevated shearing temperatures (above 120 °C) and with a relatively low content of clay (below 15 phr) resulted in morphology changes from the intercalated form to the exfoliated form of morphology. There exists an isotropically mechanical property for the cured matrix of the exfoliated hybrids whereas there exists an anisotropically mechanical property for the cured matrix of the intercalated hybrids.