Preliminary spectroscopic experiments with helium microwave induced plasma produced in air by use of a new structure: the axial injection torch

In this experimental study we have used spectroscopic methods to characterize helium plasma obtained by means of a novel waveguide-fed microwave plasma torch at atmospheric pressure, the axial injection torch. This device produces a “plasma flame” by coupling high frequency (HF) power at 2.45 GHz to the discharge. Various flame parameters (namely the electron density number and the electron and gas temperatures) have been determined by using spectroscopic diagnostic techniques that provided an estimate in terms of the helium flow rate, absorbed HF power and axial position in the experiments. These preliminary results suggest some departure from local thermodynamic equilibrium (LTE) and seem to indicate the utility of the discharge as an excitation source for emission spectroscopy. Comparison with other microwave torches already described in the literature is made in terms of the electron density and the electron and gas temperature.     

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.     

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.     

Corrective correlation method for the analysis of gases by plasma/laser-excited fluorescence spectrometry

Traces of gaseous impurities in permanent gases can be quantitatively determined by recording their fluorescence produced in a dual process of excitation in a plasma discharge combined with excitation in a resonant field of laser radiation. However, variations in the composition of the gas samples can cause systematic errors of determination. Such errors can be corrected using a correlation relationship established between the fluorescence intensity and concentrations of components of the sample. As an example, the possibility of accurate determination of low neon concentrations in a sample of helium containing an undetermined amount of argon is presented. The correlation relationship between the level of the saturated fluorescence of neon and the saturation parameter (spectral density of laser power required to reach the 50% level of neon fluorescence intensity relative to the level of the completely saturated fluorescence) over a broad argon content range in helium was established and used. The objective was to neutralize possible obscuring impacts of unknown uncontrolled impurities that may or may not be present in a gas sample on the analytical results.     

Fundamental studies of mixed-gas inductively coupled plasmas

The effects of adding foreign gases to the central-gas flow or the intermediate-gas flow of an argon inductively coupled plasma are presented. In particular, the influence of up to 16.7% added helium, nitrogen or hydrogen on radially-resolved electron number density, electron temperature, gas-kinetic temperature and calcium ion emission profiles is examined. It is shown that these gases affect not only the fundamental parameters and bulk properties of the plasma, but also how energy is coupled and transported through the discharge and how that energy interacts with the sample. For example, added helium causes an increase in the gas-kinetic temperature, most likely due to the higher thermal conductivity of helium compared to argon but, in general, does not appear to affect significantly either the electron temperature or electron concentration. The shift in the calcium ion emission profile towards lower regions in the discharge with added helium may be attributable to higher droplet desolvation and particle vaporization rates. In contrast, the addition of nitrogen or hydrogen to an Inductively Coupled Argon Plasma (Ar ICP) results in dramatic changes in all three fundamental plasma parameters: electron number density, electron temperature, and gas-kinetic temperature. The net effect of these molecular gases (N₂ or H₂) on calcium ion emission and on the fundamental plasma parameters is shown to be dependent on the amount of gas added to the plasma and whether the gas is introduced as part of the central- or intermediate-gas flow. In general, nitrogen added to the central-gas flow causes a significant reduction in the number of electrons throughout most of the discharge (over an order of magnitude in certain regions), mainly in the central and upper zones of the ICP. A drop of 3000–5000 K in the central channel electron temperature and a smaller drop in the gas-kinetic temperature are also observed when N₂ is added to the central-gas flow. In contrast, the introduction of nitrogen in the intermediate flow causes about a 1 × 10¹⁵ electrons cm⁻³ increase in the electron concentration in the low, toroidal regions of the plasma and an increase in the gas-kinetic temperature of around 1000 K throughout most of the discharge. As seen with the addition of nitrogen to the central-gas flow, the electron temperature is found to increase in the toroidal zones of the plasma when N₂ is added to the intermediate flow. These combined effects cause a 20-fold depression in the calcium ion emission intensity only a 1.7-fold depression when N₂ is added to the central- or intermediate-gas flows, respectively. On the other hand, hydrogen causes a depression in the electron concentration in the upper areas of the plasma when this gas is added to the central flow but increases the number of electrons in the same region when added to the intermediate flow. Hydrogen also causes a dramatic effect on the electron and gas-kinetic temperatures, significantly increasing both of these parameters throughout the discharge. An increase in the calcium ion emission intensity, accompanied by a downward shift, elongation and broadening of the calcium ion emission profile is also observed with H₂ addition.     

Boltzmann statistical consideration on the excitation mechanism of iron atomic lines emitted from glow discharge plasmas

A Boltzmann plot for many iron atomic lines having excitation energies of 3.3–6.9 eV was investigated in glow discharge plasmas when argon or neon was employed as the plasma gas. The plot did not show a linear relationship over a wide range of the excitation energy, but showed that the emission lines having higher excitation energies largely deviated from a normal Boltzmann distribution whereas those having low excitation energies (3.3–4.3 eV) well followed it. This result would be derived from an overpopulation among the corresponding energy levels. A probable reason for this is that excitations for the high-lying excited levels would be caused predominantly through a Penning-type collision with the metastable atom of argon or neon, followed by recombination with an electron and then stepwise de-excitations which can populate the excited energy levels just below the ionization limit of iron atom. The non-thermal excitation occurred more actively in the argon plasma rather than the neon plasma, because of a difference in the number density between the argon and the neon metastables. The Boltzmann plots yields important information on the reason why lots of Fe I lines assigned to high-lying excited levels can be emitted from glow discharge plasmas.     

Some fundamental measurements of the atmospheric-pressure,microwave-induced discharge

Measurements of several fundamental parameters in a microwave-induced atmospheric-pressure flowing plasma are presented. Optical and electrical measurements were performed on argon and argon/nitrogen plasmas in the region 1–7 cm outside the cavity, as the applied microwave power, and plasma composition were varied.The stability of the plasma, atomic emission from the argon support gas, and emission from the analyte species, are proportional to the electron density. The observed electron density was varied when the power was changed,-when an electrophilic species was added, and as the observation zone was moved relative to the microwave field. In all cases, the change in the emission signal followed the change in electron density.The electron temperature, as measured by the double-probe method, is related to the kinetic energy of the fastest-moving electrons in the plasma. It is unchanged by variations in power, plasma gas composition, flow rate, and is independent of the location of the probes relative to the cavity. The spectroscopic and electrical data are consistent with excitation by ion—electron radiative recombination.     

Experimental check of the high-frequency behavior of the electrons in molecular and atomic gas plasmas

The high-frequency behavior of the electron component in collision-dominated nitrogen plasmas of dc glow discharges, acted upon by an additional microwave field, has been studied on an adequate kinetic basis for field frequencies exceeding the characteristic frequency for energy dissipation in electron collisions with nitrogen molecules. In particular, the phase delay of the electron current density with respect to the driving microwave field has been calculated. To check the validity of the results obtained by the electron kinetic approach, the phase delay has been experimentally determined adapting an appropriate microwave resonator method to the dc plasma. The comparison of the theoretically and experimentally determined phase delay of the ac electron current in the nitrogen plasma leads to a good agreement in the entire range of high-field frequencies and confirms the conclusions on the high-frequency behavior of the electrons deduced from the electron kinetic approach. Using previous results for a neon plasma, the remarkable impact of the atomic data of the collision processes in different gases on the high-frequency behavior of the electron component in these gas plasmas is additionally evaluated.     

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.     

Some properties of a microwave boosted glow discharge source using neon as the operating gas

The use of neon as the operating gas for the analysis of aluminium samples with the microwave boosted glow discharge source has been studied. A new type of anode tube allowed the gas to enter the source near the sample surface so that more material was transported into the discharge. Erosion rates have been measured under conditions optimised for high line-to-background ratios and found to be lower than with argon (9 and 21 n/s, respectively). Despite the lower erosion rate the detection limits measured for a number of elements in aluminium are in the range 0.02-1 microg/g and comparable to those obtained with argon as the operating gas.     

Low power cross-flow atmospheric pressure Ar + He plasma jet : Spectroscopic diagnostic and excitation capabilities

A cross-flow atmospheric plasma jet with distilled water or analyte solution nebulization has been investigated. The plasma gas flows perpendicularly to the RF powered electrode (11.21 MHz) and a grounded electrode was added for plasma stabilization. The working parameters of the plasma generator can be controlled in order to maximize either the plasma power (75 W) or the voltage on the RF powered electrode (plasma power, 40 W). The plasma gas, pure argon (0.4 l min⁻¹) or a mixture of argon (0.3–0.4 l min⁻¹) and helium (0–0.2 l min⁻¹), was also used for liquid nebulization. Optical emission of the plasma, collected in the normal viewing mode, was used for plasma diagnostics and for evaluating its excitation capabilities. The influence of helium content in the mixed-gas plasma on the plasma characteristics and on the emission axial profiles of the plasma gas constituents and of the analytes originate from the wet aerosol was studied. The addition of helium to the argon plasma, generally determines decreases in the emission of the plasma gas constituents (with the exception of molecular nitrogen), in the rotational temperature and in the electron number density and increases in the excitation temperatures and in the emission of easily excitable analytes. Based on the determined electron number densities, it was concluded that in the plasma zone which presents interest from analytical point of view the plasma is not very far from the partial thermodynamic equilibrium. In function of the helium content in the plasma gas and of the axial distance from the powered electrode the excitation temperatures are in the range of 2420–3340 K for argon, 2500–5450 K for oxygen and 900–2610 K for ionic calcium and the electron number densities are in the range of 1.2 10¹²–1.25 10¹³ cm⁻³. Some elements with excitation energy lower than 6 eV were excited in the plasma. The plasma excitation capability depends on the working conditions of the plasma generator (maximum power or maximum voltage on the RF powered electrode) and on the helium content in the mixed-gas plasma. The estimated detection limits for the studied elements (Na, Li, K, Ca, Cu, Ag, Cd, Hg and Zn) are in the range of 7 ng ml⁻¹ to 28 μg ml⁻¹.     

Fundamental properties characterizing low-pressure microwave-induced plasmas as excitation sources for spectroanalytical chemistry

Experimental measurements of the spectroscopic temperature and the electron temperature in low-pressure rare gas plasmas sustained by a microwave generator operating at 2450 MHz have revealed divergent values. These measurements have been interpreted on the basis of a radiative recombination model originally proposed by Schlüter. The importance of Penning ionization by metastable rare gas atoms in the excitation of foreign atoms has been discussed in terms of this model.On the basis of the radiative recombination model for these plasmas, the parameters of analytical importance are the concentration and energy of electrons in a high energy electron group, the concentration and energy of electrons in a low energy electron group, and the concentration of metastable rare gas atoms. Measurements of the spectroscopic temperature of an argon plasma have revealed that the energy of electrons in the low energy electron group is not greatly affected by applied microwave power and pressure over the range from 1–25 torr. The energy of electrons in the high energy electron group is not greatly affected by pressure and applied microwave power over the range studied, but has been shown to depend on the ionization potential of the plasma gas. The total electron concentration is not greatly affected by gas pressure for low applied powers, but varies with applied power, particularly at low pressures. The concentration of metastable argon atoms has been shown to depend on both the applied power and pressure. Studies of the excitation of mercury by these plasmas have led to results which are consistent with the radiative recombination model.     

Glow discharge excitation and matrix effects in the Zn–Al–Cu system in argon and neon

Matrix effects and other deviations from the standard model of glow discharge optical emission spectroscopy (GD-OES) have been investigated in the Zn–Al–Cu system in a Grimm-type discharge in argon and neon. In ionic spectra of the elements that can be ionized by asymmetric charge transfer with ions of the discharge gas, most observed deviations from the standard model can be explained by variations of the number density of ions of the discharge gas, caused by asymmetric charge transfer reactions with the matrix element. Similar mechanism, but involving metastables of the discharge gas, was observed for the Cu II spectrum in neon. Some matrix effects in atomic spectra of aluminium and possibly also copper suggest that three-body recombination of ions of the discharge gas, assisted by an analyte atom, is responsible for excitation of certain atomic levels of the analyzed elements. Excited atomic states of the analyzed elements have higher fractional populations in neon than argon, by factors that are similar for all three elements and the median of which is slightly less than 3. It is shown which lines are free of matrix effects and suitable for highly accurate analysis of Zn–Al–Cu alloys by GD-OES and how to optimize the calibration model. Neon can be a reasonable alternative to argon as the discharge gas for some applications.     

A study of the density of sputtered atoms in the plasma of the modified Grimm-type glow discharge source

The sputtering of atoms from the cathode of a modified Grimm-type glow discharge source was studied using hollow cathode lamps as primary sources. Absorption of copper atoms at a distance of 1.5 mm from the cathode was measured, using different discharge conditions, with helium, neon, argon, krypton and nitrogen as carrier gases. For conditions with voltages at and above 800 V, the greatest absorption (copper atom concentration) was obtained using argon as carrier gas. Absorption by copper and chromium, measured at varying distances from the cathode and at different discharge conditions, shows a maximum between 1 and 2 mm from the cathode. This phenomenon can only be explained by cluster sputtering or cluster formation in the plasma. By using the Doppler temperatures of the emission and absorption sources to calculate line profile halfwidths, measured absorbances can be converted to atom number densities.A diffusion model has been formulated to describe the diffusion of sputtered atoms through the plasma which is in a steady state. From the agreement obtained with experimental results, it is concluded that in principle this diffusion model can be used to predict the spatial distribution of sputtered atoms in the plasma.     

Spectroscopic characterization of atmospheric pressure argon plasmas sustained with the Torche à Injection Axiale sur Guide d'Ondes

An argon microwave (2.45 GHz) plasma produced by a microwave plasma torch known as “Torche à Injection Axiale sur Guide d'Ondes” (TIAGO) in air ambience at atmospheric pressure was experimentally characterized. The conditions for appropriate TIAGO torch operation in argon and flame morphology were researched under several experimental conditions of gas flow (0.15–5.00 L · min^–1) rates and microwave input powers (100–1000 W). Gas temperature and electron density values were studied by means of optical emission spectroscopy. Besides, the processes inside the discharge and their interaction with the surrounding atmosphere were described according to the recorded spectra.     

Double clathrate hydrates of tetrabutylammonium fluoride + helium, neon, hydrogen and argon at high pressures

Decomposition curves of double ionic clathrate hydrates of tetrabutylammonium fluoride with helium, neon, hydrogen and argon were studied at pressures up to 800 MPa. Formation of double hydrates with helium, neon and hydrogen does not lead to any significant increase of the temperatures of decomposition of these hydrates; at high temperatures the hydrates may decompose even at lower temperatures than the hydrate of pure tetraalkylammonium salt does. Decomposition temperatures of double hydrates with argon in all cases were 4–8 °C higher in comparison with the decomposition temperature of ionic clathrate hydrates of tetrabutylammonium fluoride. We suppose that this behavior is caused by simultaneous effect of three factors on hydrate decomposition temperature: (1) partial filling of the small cavities in the framework of the hydrate with water molecules, (2) weakness of the van der Waals interactions between the gas molecules and the host water molecules, and (3) dissolution of helium, hydrogen and neon in the solution of tetrabutylammonium salt causing a decrease of melting temperatures of the hydrates formed from these solutions.     

Microwave sputtering of conductors and insulators for optical emission and mass spectrometry

A microwave-powered slab-line cavity was used to excite a discharge in low pressure argon or neon and to demonstrate the sputtering of conducting and non-conducting samples by a microwave excited discharge. Both optical emission spectroscopy and mass spectrometry were used as detection systems. The dependence of the signals on gas pressure and net microwave power was investigated.     

Microwave sputtering of conductors and insulators for optical emission and mass spectrometry

A microwave-powered slab-line cavity was used to excite a discharge in low pressure argon or neon and to demonstrate the sputtering of conducting and non-conducting samples by a microwave excited discharge. Both optical emission spectroscopy and mass spectrometry were used as detection systems. The dependence of the signals on gas pressure and net microwave power was investigated.     

Comparison of temperature measurements in ICP and MIP with Ar and He as plasma gas

Various temperature measurements have been carried out in microwave induced plasmas (MIP) generated with a surfatron and inductivcly coupled plasmas (ICP) both with argon and helium as plasma gas. Iron has been used for the determination of excitation temperature, and OH and N⁺₂ for rotational temperatures. In the case of the Ar ICP, equilibrium is attained between the various temperatures (4500 K), as previously described. On the other hand, in the He ICP and the MIPs, iron provides the highest temperature (4500 K) but discrepancies are obtained with results from N⁺₂ and OH. These two species show lower values, especially OH (2000 K).     

On the electron temperatures and densities in plasmas produced by the “torche à injection axiale”

The electron temperature and the electron density of plasmas created by the “Torche à Injection Axiale” (TIA) are determined using Thomson scattering. In the plasma with helium as the main gas, temperatures of around 25 000 K and densities of between 0.64 and 5.1 × 10²⁰m⁻³ are found. In an argon plasma the electron temperature is lower and the electron density is higher: 17 000 K and around 10²¹ m⁻³ respectively. From these results it can be established that the ionisation rates of both plasmas are much larger than the recombination rates, which means that the plasmas are far from Saha equilibrium. However, deviations from a Maxwell electron energy distribution function, as reported for the “Microwave Plasma Torch” (MPT), are not found in the TIA. The excitation and ionisation power of the TIA appears to be stronger than that of the MPT.