Raman study of the dissolution of gases under high pressure on conformational equilibria of some n-alkanes

The conformational changes of some odd-numbered n-alkanes by increasing pressure agree with a greater volume of the trans conformations. Dissolving nitrogen, helium, argon or carbon dioxide displaces the conformational equilibria. The evolution is sensitive to the nature of the gas. For solutions of nitrogen or argon, all-trans forms appear with increasing pressure just before the solid organized phase. We show the effect of the quantity of dissolved gas on the conformational equilibria from the study of the vibrational spectrum of nitrogen. The interaction between the nitrogen molecules and the conformations of alkanes is also discussed.     

Sample decomposition in an electrically heated cold-trap inlet system for high speed gas chromatography

Thermal decomposition of some hydrocarbon and chlorinated hydrocarbon compounds in metal capillary tubes used in an inlet system for high speed gas chromatography has been investigated. The metal tube is cooled to about ?75°C by a flow of cold nitrogen gas in order to focus a vapor sample cryogenically. A capacitive discharge power supply is then used to heat the metal tube resistively in order to revaporize the sample and introduce it to the separation column as a plug 5-10 ms wide. The effects of tube temperature, tube material, sample vapor residence time, and type of carrier gas on thermal cracking are described. Use of a copper-nickel alloy tube resulted in less cracking than either pure platinum or pure nickel. Cracking is more significant with hydrogen as carrier gas than with helium. Cracking also increases with increasing sample residence time in the hot tube. Quantitative sample injection with minimum decomposition can be obtained for a variety of aliphatic and aromatic hydrocarbons and chlorinated hydrocarbon compounds.     

Spectroscopic characterization of a neon surface-wave sustained (2.45 GHz) discharge at atmospheric pressure

In this paper a spectroscopic study of a microwave (2.45 GHz) neon surface-wave sustained discharge (SWD) at atmospheric pressure in a quartz tube has been carried out in order to determine the plasma characteristic parameters (e.g. electron temperature and density, gas temperature, absorbed power per electron) and also to identify possible deviations from the thermodynamic equilibrium for this kind of microwave discharge. The results have been compared to experiments in the literature for other noble gas (helium and argon) SWDs generated under similar experimental conditions. Intermediate values between those of argon and helium plasmas were obtained for characteristic neon plasma parameters (temperatures and electron density). An important departure from the Saha equilibrium was exhibited by neon SWDs.     

Emission characteristics of mixed gas plasmas in low-pressure glow discharges

Glow discharge optical emission spectrometry (GD-OES) with mixed plasma gases is reviewed. The major topic is the effect of type and content of gases added to an argon plasma on the emission characteristics as well as the excitation processes. Emphasis is placed on argon–helium, argon–oxygen, and argon–nitrogen mixed gas plasmas. Results for non-argon-matrix plasmas, such as neon–helium and nitrogen–helium mixtures, are also presented. Apart from the GD-OES, glow discharge mass spectrometry and furnace atomization plasma emission spectrometry with mixed plasma gases are also discussed.     

Parametric study of the flow and temperature fields in an inductively coupled r.f. plasma torch

A theoretical investigation of the effect of different parameters on the flow and the temperature fields in a radiofrequency inductively coupled plasma is carried out. The parameters studied are: central injection gas flow rate, total gas flow rate, input power, and the type of plasma gas. The results obtained for argon and nitrogen plasmas at atmospheric pressure indicate that the flow and the temperature fields in the coil region, as well as the heat flux to the wall of the plasma confinement tube, are considerably altered by the changes in the torch operating conditions.     

碰撞气体的种类和压力对离子阱质谱性能的影响

基于数字离子阱技术,研究了离子阱质谱分析实验过程使用的碰撞气体种类及压力对离子阱质谱性能,如质量分辨能力、信号强度、串级质谱分析,以及低质量截止效应等的影响.实验过程中,在离子的激发和碰撞诱导解离阶段,分别采用质量数不等的氦气(质量数=4 amu)、氮气(质量数=28 amu)、氩气(质量数=40 amu)等作为碰撞气体,以及不同的气体压力,研究了它们对质谱性能的影响.结果表明,当采用质量数较大的氩气作为碰撞气体时,可以有效改善低质量数截止效应和提高离子碰撞过程中的能量转移效率,同时提高离子捕获和解离效率,但是质量分辨率会明显降低.在获得较高质量分辨率方面,氦气作为碰撞气体时效果最好.在气压相同的情况下,质量数大的碰撞气体有利于提高串级质谱分析效率,即获得更多碎片离子峰和更多有关母体离子结构的信息.     

A microwave induced plasma system for the maintenance of moderate power plasmas of helium,argon, nitrogen and air

A microwave induced plasma system capable of maintaining stable plasmas of each of the gases helium, argon, nitrogen and air is presented. The system is capable of operation at powers of up to 500 W. The TM₀₁₀ cavity design is similar to that previously described in the literature with some modifications. A demountable torch facilitates centering of diffuse plasmas of helium, nitrogen and air by providing 6 flows directed tangentially within the quartz tube. This torch was not useful for argon plasmas. Toroidal argon plasmas were maintained with a threaded quartz tube arrangement. The heat generated by these plasmas was dissipated by an outer sheath of coolant air. Details of the design and preliminary characterization of each plasma system is presented.     

Thermodynamic and transport properties of argon/carbon and helium/carbon mixtures in fullerene synthesis

The equilibrium composition and thermodynamic and transport properties of argon; carbon and helium/carbon mixtures are calculated in the temperature range 300–20,000 K. The curves for the composition of mixtures of 50%, carbon in argon or helium are shown fir a pressure of 1.33 × 10⁴ Pa. The calculations for the heat capacity at constant pressure (C_p) and transport coefficients are validated with other studies, for the cases or pure argon and pure helium at a pressure of 10⁵ Pa. The properties of mixtures with various proportions of carbon in argon and helium are calculated. Results are presented at pressures of 105 and 1.33 × 10⁴ Pa, typical of reactors for the synthesis of fullerenes and nanotubes. It is observed that the properties of carbon and mixtures of carbon with a buffer gas (argon or helium) are very different from those of the buffer gas, thus the need to consider this effect in simulations. In general, the mixtures follow trends intermediate to those of the pure gases from which they are composed except for the thermal conductivity which shows a deviation from this tendency in the region between 11,500 and 19,000 K for argon/carbon mixtures and between 8,000 and 12,000 K for helium/carbon mixtures. Also, the electrical conductivity of mixtures of low carbon concentration is very close to that ofpure carbon. A datafile containing the transport properties of mixtures for pressures between 10⁴ and 10⁵ Pa is available free of charge from the authors.     

Matrix effect in the cryotrapping of gas samples in the ppbv range

A gas mixture containing ppbv concentrations of volatile chlorinated and aromatic hydrocarbons was prepared in helium, nitrogen, or argon in a plastic (Nalophan) bag, in which it was found the mixture could be safely stored for several days. Samples of the mixture were subsequently analyzed by gas chromatography after direct cryotrapping in an empty metal U-tube. The cooling medium used was liquid nitrogen, liquid argon, or a dry ice-acetone slurry (197 K). The efficiency of cryotrapping in liquid nitrogen was over 90% when the aromatic hydrocarbon mixture was prepared in the helium matrix, but between 50 and 70% when it was prepared in nitrogen or argon, the recovery from the argon matrix being somewhat higher. The poor recovery in nitrogen at 77 K and argon at 87 K was explained by the reduced rate of diffusion to the wall owing to mist or aerosol formation. At 197 K condensation was negligible if the partial pressures were lower than the saturated vapor pressures.     

Gas‐Hydrate Packing and Stability at High Pressures

The decomposition temperatures of double gas hydrates of tetrahydrofuran with noble gases from krypton to helium at pressures up to 15 kbar were found by differential thermal analysis. The stability of hydrates was shown to rise as their packing coefficient increases. Krypton and argon hydrates retain the original cubic structure II in the whole pressure range. In neon and helium systems, polyhedral double hydrates have upper stability limits at 7.4 and 6.0 kbar, respectively.     

Static pressure and heat flux measurements of a thermal argon plasma flow in a water-cooled tube

The momentum and energy transfer phenomena with large temperature difference were investigated experimentally and theoretically, using an argon atmospheric thermal plasma. The plasma was generated by an arc discharge, 4–6 kW, and flowed into a water-cooled copper tube for static pressure measurements and into a copper block with the same size hole (8 mm i.d.) for measuring heat fluxes using a transient method. The argon flow rate was 2.77–8.31×10^–4 kg/s. The static pressure of the plasma flow shows a different variation from that of an ordinary flow and does not decrease monotonically. The axial distributions of the numerical calculations are in fair agreement with those of the experiments, and it is concluded that the contributions of recombination and of physical properties play important roles in the behavior of the confined thermal plasma flow.     

Excitation of singly ionized argon species in helium-matrix glow discharge plasma—role of the energy transfer from helium metastables

When a small amount of argon is added to the helium plasma in a Grimm-type glow discharge radiation source, the interaction between helium and argon species is investigated from analyzing the intensities of emission lines of of argon ion (ArII). The excitation energy as well as the term multiplicity concerning the optical transitions to which the ArII emission lines are identified are significant factors for determining their emission intensities in the helium-matrix plasma. In the case where the excitation energy of ArII lines is higher than the internal energy of the helium metastable states, the emission intensity in the helium-matrix plasma is observed to be much weaker than that obtained only with argon gas. On the other hand, the intensity is enhanced when the excitation energy is slightly lower. In the excited levels of argon ion having quartet multiplicity, closer interactions with the triplet rather than the singlet metastable level of helium atom are recognized, with the singlet helium metastable in the argon excited levels having doublet multiplicity.     

Emission characteristics of Grimm-style glow discharge plasmas with helium matrix plasma gas containing small amounts of nitrogen

Glow discharge plasmas with helium–(0–16%) nitrogen mixed gas were investigated as an excitation source in optical emission spectrometry. The addition increases the sputtering rate as well as the discharge current, because nitrogen molecular ions, which act as primary ions for the cathode sputtering, are produced through Penning-type ionization collisions between helium metastables and nitrogen molecules. The intensity of a silver atomic line, Ag I 338.29 nm, is monotonically elevated along with the nitrogen partial pressure added. However, the intensities of silver ionic lines, such as Ag II 243.78 nm and Ag II 224.36 nm, gave different dependence from the intensity of the atomic line: Their intensities had maximum values at a nitrogen pressure of 30 Pa when the helium pressure and the discharge voltage were kept at 2000 Pa and 1300 V. This effect is principally because the excitations of these ionic lines are caused by collisions of the second kind with helium excited species such as helium metastables and helium ion, which are quenched through collisions with nitrogen molecules added to the helium plasma. The sputtering rate could be controlled by adding small amounts of nitrogen to the helium plasma, whereas the cathode sputtering hardly occurs in the pure helium plasma.     

Effect of buffer gas on the fluorescence yield of trapped gas-phase ions

We investigated the dependence of three different gases, helium, argon, and nitrogen, on the fluorescence signal intensity of rhodamine 6G cations in the gas phase. The method is based on laser-induced fluorescence of ions trapped in a Fourier transform ion cyclotron mass spectrometer. We found that the use of helium results in the highest fluorescence signal, while no fluorescence was detected when using argon under the same conditions.     

Emission characteristics of Grimm-style glow discharge plasmas with helium matrix plasma gas containing small amounts of nitrogen

Glow discharge plasmas with helium–(0–16%) nitrogen mixed gas were investigated as an excitation source in optical emission spectrometry. The addition increases the sputtering rate as well as the discharge current, because nitrogen molecular ions, which act as primary ions for the cathode sputtering, are produced through Penning-type ionization collisions between helium metastables and nitrogen molecules. The intensity of a silver atomic line, Ag I 338.29 nm, is monotonically elevated along with the nitrogen partial pressure added. However, the intensities of silver ionic lines, such as Ag II 243.78 nm and Ag II 224.36 nm, gave different dependence from the intensity of the atomic line: Their intensities had maximum values at a nitrogen pressure of 30 Pa when the helium pressure and the discharge voltage were kept at 2000 Pa and 1300 V. This effect is principally because the excitations of these ionic lines are caused by collisions of the second kind with helium excited species such as helium metastables and helium ion, which are quenched through collisions with nitrogen molecules added to the helium plasma. The sputtering rate could be controlled by adding small amounts of nitrogen to the helium plasma, whereas the cathode sputtering hardly occurs in the pure helium plasma.     

Gas chromatographic determination of helium in neutron-irradiated beryllium oxide

A method has been developed for the determination of helium produced during neutron irradiation of beryllium oxide ((9)Be + n --> (8)Be + 2n; (8)Be --> 2(4)He). The sample is dissolved by refluxing in a sulphuric acid-phosphoric acid mixture in an argon atmosphere. After the sample has completely dissolved, the released helium is transferred to a gas sampling bulb by means of a Toepler pump. An activated charcoal trap cooled by liquid nitrogen is used to separate helium from argon. The helium is determined by gas chromatography using a 20-ft Linde 5A molecular sieve column.     

Molecular kinetic aspects of vaporization of volatile coordination compounds with organic ligands

We present results of the experimental study and numerical simulation of radiation-convective heat and mass transfer during the sublimation of spherical particles of metal β-diketonates in a high-temperature inert gas flow (argon or helium). The sublimation process is visualized, and experimental data on the temperature variation dynamics and particle size are obtained. It is shown that at stable transfer of the compound from the particle surface the sublimation proceeds with the formation of large pores in its structure. The effect of inert gas properties on the kinetics of the vaporization process of precursor particles with various initial diameters is analyzed in the temperature range from 200 °C to 330 °C. Due to a higher thermal conductivity and heat capacity of helium as compared with argon, the choice of helium as carrier gas causes an increase in the sublimation intensity.     

Modeling of gas adsorption equilibrium over a wide range of pressure: a thermodynamic approach based on equation of state

A thermodynamic approach based on the Bender equation of state is suggested for the analysis of supercritical gas adsorption on activated carbons at high pressure. The approach accounts for the equality of the chemical potential in the adsorbed phase and that in the corresponding bulk phase and the distribution of elements of the adsorption volume (EAV) over the potential energy for gas-solid interaction. This scheme is extended to subcritical fluid adsorption and takes into account the phase transition in EAV. The method is adapted to gravimetric measurements of mass excess adsorption and has been applied to the adsorption of argon, nitrogen, methane, ethane, carbon dioxide, and helium on activated carbon Norit R1 in the temperature range from 25 to 70 degrees C. The distribution function of adsorption volume elements over potentials exhibits overlapping peaks and is consistently reproduced for different gases. It was found that the distribution function changes weakly with temperature, which was confirmed by its comparison with the distribution function obtained by the same method using nitrogen adsorption isotherm at 77 K. It was shown that parameters such as pore volume and skeleton density can be determined directly from adsorption measurements, while the conventional approach of helium expansion at room temperature can lead to erroneous results due to the adsorption of helium in small pores of activated carbon. The approach is a convenient tool for analysis and correlation of excess adsorption isotherms over a wide range of pressure and temperature. This approach can be readily extended to the analysis of multicomponent adsorption systems.     

Development of soft plasma ionization (SPI) source for analysis of organic compounds

We present a newly designed soft plasma ionization (SPI) source developed for mass spectrometric study of organic compounds in this study. The SPI cell having a relatively small size consists of a hollow anode and a hollow mesh cathode. The voltage–current characteristic depending on the pressure was investigated, indicating that it has similar characteristics to conventional hollow cathode glow discharges. To investigate the emission characteristics of the SPI source, some molecular band emission spectra (N₂, N₂⁺ and OH⁺) were measured by using argon and helium discharge gases. The SPI source was installed to a commercially used quadrupole mass analyzer for analyzing organic compounds. To demonstrate the SPI source, the mass spectra of some organic compounds (methylene chloride, toluene, benzene, cyclohexane and chloroform) were measured. The organic compounds were ionized with good stability in the plasma, and the fragmentation depended on the applied current. When helium and argon gases were used as the discharge gas, the helium plasma was more suitable for SPI-MS rather than argon because the argon plasma not only suffers from spectral interference but also has lower sensitivity.     

Non-thermal features of atmospheric-pressure argon and helium microwave-induced plasmas observed by laser-light Thomson scattering and Rayleigh scattering

Laser-light Thomson scattering and Rayleigh scattering have been measured from a microwave-induced plasma sustained at atmospheric pressure, using both argon and helium as a support gas. The measurements were performed at several spatial positions in each plasma, and at forward microwave power levels of 350 W for argon, and at 350 W and 100 W for helium. It was found from these measurements that both argon and helium plasmas deviate substantially from local thermodynamic equilibrium (LTE), Measured electron temperatures range from 13 000–21 500 K, whereas gas temperatures are generally lower by a factor of 2 to 10, depending on the support gas and the spatial position in the discharge. At the same forward microwave power, the electron temperature of the helium plasma is about 3500–7000 K higher than that of the argon plasma. Yet, the argon plasma has a higher electron number density than the helium plasma. Electron number densities in both argon and helium plasmas are roughly two to three orders of magnitude lower than what LTE would predict, based on the measured electron temperatures and the Saha Equation. Even more interestingly, signals in the far-wing portion of the Thomson-scattering spectrum were found to be significantly higher than are predicted by a fitted Maxwellian curve, indicating that there exists an over-population of high-energy electrons. It is concluded that, compared to the inductively coupled plasma, the microwave-induced plasma is highly non-thermal and remains in an ionizing mode in the analytical zone.