Extinction and flame bifurcations of stretched dimethyl ether premixed flames

Extinction limits and flame bifurcation of lean premixed dimethyl ether–air flames are numerically investigated using the counterflow flame with a reduced chemistry. Emphasis is paid to the combined effect of radiation and flame stretch on the extinction and flammability limits. A method based on the reaction front is presented to predict the Markstein length. The predicted positive Markstein length agrees well with the experimental data. The results show that flow stretch significantly reduces the flame speed and narrows the flammability limit of the stretched dimethyl ether–air flame. It is found that the combined effect of radiation and flow stretch results in a new flame bifurcation and multiple flame regimes. At an equivalence ratio slightly higher than the flammability limit of the planar flame, the distant flame regime appears at low stretch rates. With an increase in the equivalence ratio, in addition to the distant flame, a weak flame isola emerges at moderate stretch rates. With a further increase in the equivalence ratio, the distant flame and the weak flame branches merge together, resulting in the splitting of the weak flame branch into two weak flame branches, one at low stretch and the other at high stretch. Flame stability analysis demonstrates that the high stretch weak flame is also stable. Furthermore, a K-shaped flammability limit diagram showing various flame regimes and their extinction limits is obtained.     

Measurements of burning velocities of dimethyl ether and air premixed flames at elevated pressures

Laminar burning velocities of dimethyl ether (DME) and air premixed flames at elevated pressures up to 10 atm were measured by using a newly developed pressure-release type spherical bomb. The measurement system was validated using laminar burning velocities of methane–air flames. A comparison with the previous experimental data shows an excellent agreement and demonstrates the accuracy and reliability of the present experimental system. The measured flame speeds of DME–air flames were compared with the previous experimental data and the predictions using the full and reduced mechanisms. At atmospheric pressure, the measured laminar burning velocities of DME–air flames are in reasonable agreement with the previous data from spherical bomb method, but are much lower than both predictions and the experimental data of the PIV based counterflow flame measurements. The laminar burning velocities of DME–air flames at 2, 6, and 10 atm were also measured. It was found that flame speed decreases considerably with the increase of pressure. Moreover, the measured flame speeds are also lower than the predictions at high pressures. In addition, experiments showed that at high pressures the rich DME–air flames are strongly affected by the hydrodynamic and thermal-diffusive instabilities. Markstein lengths and the overall reaction order at different equivalence ratios were extracted from the flame speed data at elevated pressures. Sensitivity analysis showed that reactions involving methyl and formyl radicals play an important role in DME–air flame propagation and suggested that systematic modification of the reactions rates associated with methyl and formyl formations are necessary to reduce the discrepancies between predictions and measurements.     

Benzene precursors and formation routes in a stoichiometric cyclohexane flame

Benzene formation was found to be dominated by stepwise radical dehydrogenation of cyclohexane in a stoichiometric flat flame of cyclohexane/O₂/32.5% Ar, 30.00 Torr pressure, and 35.0 cm s⁻¹ feed velocity. This route, involving H-abstractions and β-scissions, is in contrast to conventional propargyl routes. Three types of analyses lead to this conclusion: identification of key flame species by mass and ionization energy; measurement and use of mole-fraction profiles in the flat flame; and mechanistic reactive-flow modeling of the flame, interpreted by analyzing the dominant reaction steps giving rise to the prediction. For relevant species, profiles of mole fraction were mapped by molecular-beam mass spectrometry in separate apparatuses with identical burners using electron ionization (UMass Amherst) and synchrotron VUV photoionization (LBNL ALS), respectively. In the latter, recently developed apparatus, ionization energies can be measured with greatly enhanced resolution, yielding improvements in species identification that include precise resolution of hydrocarbon isomers, crucial to the findings of this study.     

Role of hydroxyl production and heat release in the two-zone fuel-rich adiabatic dimethyl ether/air flames at atmospheric pressure

Fuel-rich laminar adiabatic flames of premixed dimethyl ether/air mixtures at a high initial temperature and atmospheric pressure have been studied by numerical simulation and sensitivity analysis. These flames, having two heat release zones, are of great interest as an unusual and little-studied subject. We have investigated the chemical processes occurring in the two zones and analysed the mechanism of heat release in the flame. It has been found that the key reactions that have a significant influence on the flame speed are those involving dimethyl ether and the products of its incomplete oxidation. Calculation of the heat release rate confirms the presence of two heat release zones in the flame. A comparison of the reactions making a major contribution to the heat release with those significantly affecting the flame speed indicates that the main factor determining the flame speed is the formation of hydroxyls, rather than heat release. Analysis of the flame speed sensitivity shows that in the case of a two-zone structure of the flame, its speed is mainly determined by the reactions taking place in the low-temperature zone. That is, the cool zone with a higher temperature gradient is the leading one.     

Molecular-beam mass spectrometry study of oxy-combustion in a novel coal-plate experiment

Oxy-fuel coal combustion could play a significant role in the foreseeable future for its application in carbon capture and storage (CSS) technologies. Therefore, detailed knowledge about the ongoing chemical kinetics in the combustion process is necessary. Here, we present an explorative approach to study volatile species gas phase kinetics in a novel coal-plate experiment probed with molecular-beam mass spectrometry. This coupling allows for time-resolved quantitative measurements of the gas-phase directly above the surface of solid fuels, which aid gaining more insight into the gas phase chemistry during coal combustion by detailed speciation information. Two coal samples, a rhenish lignite and a coal manufactured from hydrothermal carbonization, were chosen for this investigation due to their similar classification but different molecular structures. For both samples, our measurements show a two-stage devolatilization phase separated by a char-oxidation phase which can be attributed to the interaction of oxygen consumption in the gas phase and on the surface and the release of volatiles from deeper layers of the plate. Furthermore, detailed speciation data of light, oxygenated, and tar species allowed to identify fuel structure-specific decomposition patterns of the two different coal materials, thus providing comprehensive data that can be used for future model validation purposes.     

Atomic spectrometry and atomic mass spectrometry in bioanalytical chemistry

Abstract

Atomic spectrometry and atomic mass spectrometric (MS) techniques have been playing crucial roles in the field of biosciences. They detect elements with relatively high sensitivities and are thus applicable to a wide range of analytical targets. In the past decade, determination of bio-relevant metallic elements continues to be of interest, while particularly noteworthy are methods developed for small molecules, peptides, proteins, nucleic acids and even cells that well exploited the bio-analytical strengths of atomic spectrometry and atomic MS, either in a direct or indirect manner. Quantitation, as well as speciation and imaging analyses are all involved. The present review aims to assimilate recent advances in bio-analysis utilizing atomic spectrometry and atomic MS, primarily covering the period of 2013–2018, in an attempt to provide readers insight into the developing trends of this research frontier. Followed by concluding remarks and perspectives, the applications are divided into the following four catalogs: (i) toxicologically important metal-containing species, with an emphasis on quantitative and imaging analysis; (ii) quantitation of biomolecules using naturally occurring heteroatoms; (iii) exogenous metal ion or nanoparticle tagging-based strategies in bioassays; and (iv) label-free detection of biomolecules.     

Identification of Intermediates in Pyridine Pyrolysis with Molecular-beam Mass Spectrometry and Tunable Synchrotron VUV Photoionization

利用可调谐同步辐射真空紫外光电离结合分子束取样技术研究吡啶的热解,温度是1255―1765 K、压力为267 Pa. 通过测量光电离质谱和光电离效率谱,鉴别了近20种产物和中间物,并给出了物种随温度变化的摩尔分数. 主要的产物为H2、HCN、C2H2、C5H3N、C4H2和C3H3N. 根据实验结果分析了吡啶热解的一些主要反应路径.     

挥发性有机物同分异构体光电离-离子迁移谱检测研究

自行研制的紫外光电离-离子迁移谱装置,对碳链异构、官能团异构和官能团位置异构的11种同分异构体挥发性有机物进行了实验研究,结果表明,这些有机同分异构体能够在离子迁移谱中实现分离。得到了11种挥发性有机物离子的约化迁移率,对实验所研究的同分异构体而言,迁移率大小符合以下规律:直链<分支<环状,一级<二级<三级,对位<间位<邻位,醇类<酮类<芳香类。实验中用指数稀释法对11种样品进行了定量分析,利用该方法得到装置检测限达到ppb～ppm量级。其中对丙醛、叔丁醇、乙苯、乙醚等同分异构体的光电离离子迁移谱研究均为首次报道。     

Photoionization and Dissociative Photoionization Study of Cholesterol by IR Laser Desorption/Tunable Synchrotron VUV Photoionization Mass Spectrometr

介绍了一种将红外激光解吸/真空紫外光电离质谱技术应用于分析胆固醇的新方法. 由于近阈值单光子电离作用,可以在低能量下只产生纯净的胆固醇分子离子峰;增加光子能量则可以使碎片离子峰大量出现. 为了验证碎片离子的归属,利用商用高分辨电子轰击电离-飞行时间质谱仪分析并指认了胆固醇主要的碎片峰. 此外,采用量子化学从头算的方法研究了胆固醇母体离子和碎片构型,并讨论了部分主要的光解离机理     

Shock tube measurements of ignition delay times and OH time-histories in dimethyl ether oxidation

Ignition delay times and OH concentration time-histories were measured in DME/O₂/Ar mixtures behind reflected shock waves. Initial reflected shock conditions covered temperatures (T₅) from 1175 to 1900 K, pressures (P₅) from 1.6 to 6.6 bar, and equivalence ratios (?) from 0.5 to 3.0. Ignition delay times were measured by collecting OH^∗ emission near 307 nm, while OH time-histories were measured using laser absorption of the R₁(5) line of the A-X(0,0) transition at 306.7 nm. The ignition delay times extended the available experimental database of DME to a greater range of equivalence ratios and pressures. Measured ignition delay times were compared to simulations based on DME oxidation mechanisms by Fischer et al. [7] and Zhao et al. [9]. Both mechanisms predict the magnitude of ignition delay times well. OH time-histories were also compared to simulations based on both mechanisms. Despite predicting ignition delay times well, neither mechanism agrees with the measured OH time-histories. OH Sensitivity analysis was applied and the reactions DME ↔ CH₃O + CH₃ and H + O₂ ↔ OH + O were found to be most important. Previous measurements of DME ↔ CH₃O + CH₃ are not available above 1220 K, so the rate was directly measured in this work using the OH diagnostic. The rate expression k[1/s] =  1.61 × 10⁷⁹T^−18.4 exp(−58600/T), valid at pressures near 1.5 bar, was inferred based on previous pyrolysis measurements and the current study. This rate accurately describes a broad range of experimental work at temperatures from 680 to 1750 K, but is most accurate near the temperature range of the study, 1350-1750 K. When this rate is used in both the Fischer et al. and Zhao et al. mechanisms, agreement between measured OH and the model predictions is significantly improved at all temperatures.     

Photoionization mass spectrometry and modeling study of premixed flames of three unsaturated C5H8O2 esters

The detailed chemical structures of low-pressure premixed laminar flames fueled by three simple unsaturated C₅H₈O₂ esters, the methyl crotonate (MC), methyl methacrylate (MMA), and ethyl propenoate (EPE), are investigated using tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. Significant differences in the compositions of key reaction intermediates between these flames under similar flame conditions are observed. The results enable further refinement and validation of a detailed chemical kinetic reaction mechanism, which is largely based on a previous model for saturated esters. Detailed kinetic modeling describes how these differences are related to molecular structures, leading to unique fuel destruction pathways for each of these isomers. Meanwhile, the effect of carbon carbon double bonds on the combustion chemistry of small fatty acid esters is addressed.     

Penning trap mass spectrometry for nuclear structure studies

High-precision mass measurements as performed at the Penning trap mass spectrometer ISOLTRAP at ISOLDE/CERN are an important contribution to the investigation of nuclear structure. Precise nuclear masses with less than 0.1 ppm relative mass uncertainty allow stringent tests of mass models and formulae that are used to predict mass values of nuclides far from the valley of stability. Furthermore, an investigation of nuclear structure effects like shell or sub-shell closures, deformations, and halos is possible. In addition to a sophisticated experimental setup for precise mass measurements, a radioactive ion-beam facility that delivers a large variety of short-lived nuclides with sufficient yield is required. An overview of the results from the mass spectrometer ISOLTRAP is given and its limits and possibilities are described.      

An experimental and kinetic modeling study of a premixed nitromethane flame at low pressure

A premixed nitromethane/oxygen/argon flame at low pressure (4.67 kPa) has been investigated using tunable vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. About 30 flame species including hydrocarbons, oxygenated and nitrogenous intermediates have been identified by measurements of photoionization efficiency spectra. Mole fraction profiles of the flame species have been determined by scanning burner position at some selected photon energies. The results indicate that N₂ and NO are the major nitrogenous products in the nitromethane flame. Compared with previous studies on nitromethane combustion, a number of unreported intermediates, including C₃H₄, C₄H₆, C₄H₈, C₂H₂O, C₂H₄O, CH₃CN, H₂CNHO, C₃H₃N and C₃H₇N, are observed in this work. Based on our experimental results and previous modeling studies, a detailed oxidation mechanism including 69 species and 314 reactions has been developed to simulate the flame structure. Despite some small discrepancies, the predictions by the modeling study are in reasonable agreement with the experimental results.     

Probing fuel-specific reaction intermediates from laminar premixed flames fueled by two C5 ketones and model interpretations

The flame chemistry was explored for two C₅ ketones with distinct structural features, cyclopentanone (CPO) and diethyl ketone (DEK). Quantitative information for numerous species, including some reactive intermediates, was probed from fuel-rich (?= 1.5) laminar premixed flames fueled by the ketones with a photoionization molecular-beam mass spectrometer (PI-MBMS). Furthermore, a new kinetic model was proposed aimed at interpreting the high-temperature combustion chemistry for both ketones, which could satisfactorily predict the current flame speciation measurements. Experimental observations in combination with modeling analyses were used to reveal the similarities and differences between the compositions of the species pools of the two flames, with emphasis on the effects of the carbonyl functionality on pollutants formations. Besides some primary species which preserve fuel-specific features produced from initial steps of fuel consumptions, basic C₁C₄ intermediates also differ much between the two flames. More abundant intermediates were observed in the CPO flame because the cyclic fuel structure enables ring-opening processes followed by formations of C₃ and C₄ hydrocarbons which cannot be easily produced from the two isolated ethyl moieties in DEK under flame conditions. The consumptions of C₃C₄ hydrocarbons in the CPO flame further lead to larger C₅C₆ species which were under the detection limit in the DEK flame. In both flames, the tightly bonded carbonyl groups in the fuels tend to be preserved, leading to carbon monoxide through α-scissions of fuel-related acyl radicals. The carbonyl moieties in most detected C₁C₃ aldehydes and ketones form through oxidations of hydrocarbon species rather than directly originating from the fuels.     

Quantification routines for adsorption studies in static secondary ion mass spectrometry and the effect of ionisation probability

It is shown that the usual method of quantification of surface composition in static secondary ion mass spectrometry (SSIMS), which is purely comparative in nature, is unsuitable for adsorption studies by SSIMS. This is because of the effect of the ionisation efficiency and ion stability of a particular ion produced from a molecule adsorbed on the surface of a substrate. The established routine results in a non-linear relationship between calculated relative surface coverage and the ion selected to characterise the adsorbate. The application of a new normalisation routine to time-of-flight secondary ion mass spectrometry (ToF-SIMS) data has been used to account for this discrepancy, and also takes into account the effect of a possible contribution from the clean substrate to the ion selected to characterise the adsorbate molecule. This routine is suggested only for use with organic secondary ions, where the ionisation potential of such ions is of a comparable magnitude, and should prove particularly useful in the application of surface analysis techniques to adsorption studies.     

加速器质谱法测量41Ca及其应用研究

Ca在自然界中含量丰富，其长寿命放射性同位素^41Ca不仅是理想的生物医学示踪剂，同时在地质年代的测定、环境科学和核天体物理的实验研究等方面都有着非常重要的作用．加速器质谱法目前是测量^41Ca的理想方法．文章介绍了用加速器质谱测量^41Ca的方法，介绍了目前的国际研究现状和潜在的应用研究领域以及中国原子能科学研究院在加速器质谱测量^41Ca方面的研究情况。     

应用激光质谱法选择探测一氧化碳

此文解决了激光质谱法中从含同质量数的N2分子的混合气体中选择CO分子这一问题,由于用紫外266nm激光不能从含N2分子的混合气体中选择CO分子,因此采用了可见激光来探测,发现用可见激光430～435nm的激光能成功地将CO分子选择探测出来。此外还着重分析了在可见波段和266nm激光下产生CO+离子的机理：在可见光波段443.55nm所对应的共振峰是CO分子吸收3光子的激光能量与A1Π态的振动量子数为2的振动态共振产生的;434.3nm所对应的共振峰是CO分子吸收3光子的激光能量与A1Π态的振动量子数为3的振动态共振产生的;CO分子吸收两光子的266nm激光能量与A1Π态第7振动态的某一高转动态相共振,处于该振动态的CO分子再继续吸收一个光子的266nm激光的能量至其电离态电离产生CO+离子。     

在线光电离质谱方法研究HUSY选择性中毒对聚丙烯催化热解行为的影响

利用热解-光电离飞行时间质谱方法,在线研究了不同酸性强度的HUSY沸石催化剂对聚丙烯（PP）催化热解的影响.将HUSY以不同程度的氨气毒化处理,得到不同酸强度和酸位点数量的催化剂,在线获得PP与酸性强度不同HUSY在不同热解温度下的光电离质谱图,通过程序升温热解实验,获得PP与酸性强度不同的HUSY经热解后产物随温度升高的变化趋势.结果表明PP热解产物的生成温度、选择性和转化率均与HUSY的酸性有较强关联.     

Synthesis of metallic silicide fullerenes and the characteristics thereof by mass spectrometry

Direct current arc discharge is used for the study on the synthesis of metallo-fullerenes (MFs) to discover whether there exist metallic silicide fullerenes and silicon fullerenes. The resultant components are isolated by the multistage high-performance liquid chromatography (HPLC) and analyzed with the Time-of-Flight (TOF) mass spectrometry. Results show that there exist fullerenes such as SiC69, YSi2C64, YSi2C78, Y3Si2C78 as well as Y2Si2C90 which are structurally similar to (Y2C2)@C82.     

一门十分活跃的核分析技术--加速器质谱(AMS)最新进展

文章介绍了一门十分活跃的核分析技术——加速器质谱的基本原理、设备与技术发展,以及在地球科学、生物医学、环境科学、核天体物理应用方面的最新进展.