Spatial distributions of metastable argon,temperature and electron number density in an inductively coupled argon plasma

Spatially resolved radial distributions of excitation temperature and electron number density in an argon ICP were obtained. The argon excitation temperature and electron number density near the plasma center were found to 7000 K and 5 × 10¹⁵ cm^?3, respectively, at an RF power of 1.5 kW and a carrier argon flow rate 0.65 1 min^?1.Various distributions of the absorbance at the Ar I 811.5 nm line, which has one of the metastable levels as the lower level, were obtained with and without carrier argon flow, where an MIP was used as a light source. Introduction of a large amount of potassium did not influence the distribution of the absorbance. The emission intensities at Ar I 811.5 nm were also measured for comparison.     

Some spatial effects observed in the axially viewed filament argon microwave induced plasma with solution nebulization

Spatial profiles of analyte emission in an axially viewed argon filament microwave induced plasma sustained in the TE₁₀₁ rectangular cavity have been measured along a discharge tube cross-section for neutral atoms as well as ion lines of several elements. The filament diameter was approximately 1 mm. The analyte solution was introduced by means of an ultrasonic nebulizer without desolvation. The radial emission distribution depends on the operating parameters and is different for each of the analytes examined. Spatial distributions of excitation temperature (4000–6000 K) measured with Ar I lines by the Boltzmann plot method as well as electron temperature (6000–8000 K) by line to continuum emission ratio measurements at Ar I 430 nm and electron number density (1–1.5×10¹⁵ cm⁻³) by the Stark broadening method of the H_β line were determined to support the evidence of plasma processes. In the presence of excess sodium the enhancement of emission intensity and its shift to the plasma center appears to be the result of increased analyte penetration to the plasma. Changes in spatial emission profiles for Ca atoms and ions suggest that for this element ambipolar diffusion may be important as an additional interference mechanism. A possibility of minimizing spectral interferences from argon emission lines by choosing an off-axis plasma region for emission intensity measurements is indicated.     

Charge transfer reactions between xenon ions and iron atoms in an argon-xenon inductively coupled plasma

Xenon is added to the axial channel of an argon inductively coupled plasma (ICP) at doses up to 1.5% of the aerosol gas flow. Emission is collected from the gas flowing into the sampling orifice of a mass spectrometer (MS). These Xe doses have little effect on the electron density n_e or on the intensities of Fe (I) emission lines. Certain Fe (II) lines are enhanced when Xe is added, particularly those from Fe⁺ states that can be populated by near-resonant charge transfer between Xe⁻ and neutral Fe. Calculations based on measured values of n_e indicate that Xe⁺ should be present at densities of up to 7 × 10¹⁴ cm⁻¹, which should be sufficient Xe⁺ to drive the proposed charge transfer reactions.     

A spatial study of electron density and analyte emission in a d.c. argon plasma

Spatially resolved electron density measurements have been performed on a three-electrode d.c. plasma using a linear photodiode array based spectrometer. The electron density values measured are between 1× 10¹⁵ and 1 × 10¹⁶ cm^?3 depending on spatial position. The spatial distribution of Ca I (422.7 nm) and Ca II (393.4 nm) emission has also been measured and the Ca II-Ca I emission intensity ratio evaluated. Using the n_e values measured, an analagous LTE ratio has been calculated and this has been compared to the experimental values. Measured ratios are found to be from 28 to 100 times less than LTE ratios. Some possible sources leading to these infrathermal ratios are discussed.     

Diagnostics in an air inductively coupled plasma

Radial temperature distributions in an air inductively coupled plasma discharge, operated at atmospheric pressure, are calculated from measurements of the absolute intensities of two atomic nitrogen lines (746.9 and 493.5 nm), the first negative band system of the nitrogen molecular ion at 391.4 nm, and the air continuum at 560.0 nm. The radial intensity distribution of the Mg I 285.2 and Mg II 279.6 nm lines are employed with the determined radial temperature distribution to calculate the radial electron number density throughout the normal analytical zone. The temperatures ranged from about 6000 to 10,000 K, and the electron number density varied from 5 × 10¹³ to 2 × 10¹⁶ cm^?3 in the regions above the induction coil where differences of less than 3 fold were observed between experimental and calculated Ca II to Ca I intensity ratios. On the basis of agreement among the measured temperatures and calcium ion-to-atom intensity ratios, the extent of local thermodynamic equilibrium is evaluated.     

Spatial profile measurement of ionization and excitation temperatures in an inductively coupled plasma

The developed instrument for spatial profile measurement [1] has been applied to the measurement of ionization and excitation temperatures in an inductively coupled plasma (ICP). The silicon intensified target (SIT) detector allowed it to measure a large number of emission spectra in a short period. The ease of acquisition enabled building up complete contour maps of ionization and excitation temperatures. The contour maps of various temperatures reveal that local thermal equilibrium does not exist in the whole ICP. The comparison between ionization temperature profiles for Ar and Ca indicates that in the normal analytical zone of the ICP, Ca is ionized as expected from the Ar ionization temperature. Excitation temperatures derived from low-level Fe I lines are lower than those derived from high-level Fe I lines over a large part of the plasma. The result confirms that for Fe I lines the ICP is characterized as an ionizing plasma in the whole ICP and the low atomic levels are overpopulated with respect to the high atomic levels.     

Impact of copper vapor contamination on argon arcs

A fast, accurate, and comprehensive emission spectroscopic set-up has been employed to study the impact of copper vapor on an Ar-Cu mixture plasma. Temperature profiles in the arc have been determined in the absence of Cu vapor and then in its presence, using the absolute line intensity method for an Ar spectral line; these profiles have been compared with temperature profiles derived from relative intensities of Cu I lines. Temperature profiles derived from relative intensity of Cu I lines have been used to calculate the radial density distribution of copper atoms in the arc. The following observations have been made from the resulting atomic number densities: (1) the copper vapor concentrates in the fringes of the arc, with atomic number densities up to 8.6×10¹¹ cm⁻³; and (2) Cu atomic number densities in the core of the arc are small.     

Appreciation of a collisional-radiative model for the description of an inductively coupled argon plasma

The collisional-radiative model has been applied to the argon ICP discharge in order to elucidate the excitation mechanism in the plasma. The population density distributions of 25 argon energy levels were calculated under a steady-state approximation by using the literature values of electron number density, 5 × 10 ¹⁴cm^?3 and electron temperature, 9000 K. In the case of an optically thin plasma, in which the induced absorption can be neglected, the calculated population densities showed an overpopulation for low lying states, and were very close to LTE values for the upper levels. These results suggest the following excitation mechanisms in the argon ICP; corona model for lower levels and ladder-like excitation and ionization by electron impact for upper levels. According to the present calculation, the non-overpopulation of argon metastable can be interpreted by the interconversion between metastable and radiative states. It has been found that the induced absorption of resonance lines in an optically thick plasma and the motion of species in an inhomogeneous plasma have significant effects on the population densities. The non-linear processes by collision between heavy particles were not predominant compared to electron impact processes.     

Plasma diagnostic on a low-flow plasma for inductively coupled plasma optical emission spectrometry

In order to elucidate the fundamental properties of a low-flow inductively coupled plasma (ICP) operated under total Ar consumption of 0.6 L min^− 1, excitation temperatures, rotational temperatures, ionization temperatures, electron temperatures, and electron number densities were studied with optical emission based methods. The plasma was operated in the SHIP torch (Static High Sensitivity ICP), which was designed for optical emission spectrometric detection.     

Line broadening mechanisms in the low pressure laser-induced plasma

The spectral profiles of Ca and Rb lines have been studied in a laser induced plasma as a function of pressure (1–10 torr) and delay time with respect to the plasma initiation (1–10 μs). Measurements were made in a plasma induced by the 1064-nm output of a Nd:YAG laser on a calcium carbonate matrix, doped with Rb. Spectral profiles were measured in absorption using a narrow-band cw Ti:Sapphire laser. It was shown that in the case of a trace element (Rb in a CaCO₃ matrix), the broadening mechanism was Doppler-dominant, whereas for a major matrix component (Ca), resonance broadening was the main contributor to the line shape. The plasma was shown to be non-equilibrium provided by the difference between the kinetic (3000 K) and the excitation (8000 K) temperatures. The electron number density at delay times of 5–10 μs and pressures of 1–10 torr was estimated not to exceed 10¹⁵ cm⁻³. The number densities of Ca atoms in the ground and the excited (23 652 cm⁻¹) states were evaluated by measuring line width and peak absorption at 732.6 nm. They were found to be in the range of (1.5–2.2)×10¹⁷ cm⁻³ for the ground state and (1.5–33)×10¹¹ cm⁻³ for the excited state.     

A steady-state approach to evaluation of proposed excitation mechanisms in the analytical ICP

A method is described for the evaluation of proposed analyte ionization and excitation mechanisms in the analytical ICP. In the method, a steady-state kinetic expression is derived for each proposed mechanism; the resulting expression predicts a linear relationship between analyte emission and the concentrations or concentration products of species important in the excitation and deactivation processes. The application of this approach to Ca ionization and excitation is described for the region 15 mm above the load coil and 0–5 mm from the center of the plasma. Importantly, the concentration of ground-state Ca seems not to be important in the production of either excited Ca atoms or ions. Rather, Ca atom excitation appears to occur by means of three-body or radiative Ca⁺-electron recombination. In contrast, Ca⁺ seems to be excited directly by electron impact. Among the mechanisms evaluated here, depopulation of excited-state calcium species by collisional deactivation appears to be less significant than radiative decay to the ground state.     

Stark broadening of Mg I and Mg II spectral lines and Debye shielding effect in laser induced plasma

We report Stark broadening parameters for three Mg I lines and one Mg II line in the electron number density range (0.67–1.09) · 10¹⁷ cm^− 3 and electron temperature interval (6200–6500) K. The electron density is determined from the half width of hydrogen impurity line, the H_α, while the electron temperature is measured from relative intensities of Mg I or Al II lines using Boltzmann plot technique. The plasma source was induced by Nd:YAG laser radiation at 1.06 μm having pulse width 15 ns and pulse energy 50 mJ. Laser induced plasma is generated in front of a solid state surface. High speed photography is used to determine time of plasma decay with good homogeneity and then applied line self-absorption test and Abel inversion procedure. The details of data acquisition and data processing are described and illustrated with typical examples. The experimental results are compared with two sets of semiclassical calculations and the results of this comparison for Mg I lines are not unambiguous while for Mg II 448.1 nm line, the results of Dimitrijević and Sahal-Bréchot calculations agree well with our and other experimental results in the temperature range (5000–12,000) K and these theoretical results are recommended for plasma diagnostic purposes. The study of line shapes within Mg I 383.53 nm multiplet shows that the use of Debye shielding correction improves the agreement between theoretical and experimental Stark broadening parameters.     

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.     

Langmuir probe study of the charged particle characteristics in an analytical radio frequency-glow discharge. Roles of discharge conditions and sample conductivity

The application of a tuned Langmuir probe to the measurement of the charged particle characteristics of electron number density, ion number density, electron energy distribution function, average electron energy and electron temperature, in an analytical radio frequency (r.f.)-glow discharge is described. Studies focus on the roles of discharge operating conditions and plasma sampling position for conductive (copper) and nonconductive (Macor) samples. Based on the data obtained here, apparent differences in plasma characteristics between conductive and nonconductive samples can be reasonably explained. For example, the sputtering of conductive samples results in plasmas with obviously higher electron and ion number densities than the sputtering of nonconductive samples (e.g. n_i = 1.8 × 10¹⁰ cm⁻³ and n_e = 1.5 × 10⁹ cm⁻³ for copper, and n_i = 8 × 10⁹ cm⁻³ and n_e = 5 × 10⁸ cm⁻³ for Macor under the conditions of argon pressure = 4 Torr, r.f. power = 30 W and sampling distance = 4.5 mm). Conversely, nonconductive samples yield electrons with higher energies (average electron energies of 15 and 7.5 eV and temperatures of 6.5 and 3.5 eV respectively for the Macor and copper samples). Lower d.c. bias potentials for the case of sputtering nonconductive samples yield reduced sputtering rates and charged particle densities, though the electrons in the latter case have higher energies and thus improved excitation capabilities. The differences between r.f.- and d.c.-glow discharge optical emission spectra are also discussed relative to reported electron energy characteristics. Studies such as these will lay the ground-work for extensive evaluation of inter-matrix type standardization for r.f.-glow discharge atomic emission spectrometry.     

Excitation temperature,degree of ionization of added iron species,and electron density in an exploding thin film plasma

Time and spacially resolved spectra of a cylindrically symmetric exploding thin film plasma were obtained with a rotating mirror camera and astigmatic imaging. These spectra were decouvolved to obtain relative spectral emissivity profiles for nine Fe(II) and two Fe(I) lines. The effective (electronic) excitation temperature at various positions in the plasma and at various times during the first current halfcycle was computed from the Fe(II) emissivity values using the Boltzmann graphical method. The Fe(II)/Fe(I) emissivity ratios together with the temperature were used to determine the degree of ionization of Fe. Finally, the electron density was estimated from the Saha equilibrium. Electronic excitation temperatures range from 10,000–15,000 K near the electrode surface at peak discharge current to 7000–10,000 K at 6–10 mm above the electrode surface at the first current zero. Corresponding electron densities range from 10¹⁷-10¹⁸ cm^?3 at peak current to 10¹⁵-10¹⁶cm^?3 near zero current. Error propagation and criteria for thermodynamic equilibrium are discussed.     

Evaluation of hydrogen line emission and argon plasma electron concentrations resulting from the gaseous sample injection involved in hydride generation-ICP-atomic emission spectrometric analysis

The simultaneous injection of volatile hydride species and hydrogen gas, originating in reagent decomposition, was monitored during the operation of a continuous hydride generation manifold employed for the determination of trace arsenic by HG-ICP-AES. Line and background intensities as well as the FWHM of the hydrogen Hγ and Hδ lines were measured, and electron number densities (n_e) estimated from Stark broadening of the line profiles. Results were compared with those obtained by conventional pneumatic injection of aqueous solutions. Overlapping with atomic nitrogen lines at 410 nm and 411 nm tends to distort the Hδ line profile for the hydrogen-seeded plasma, rendering unreliable results. The N I lines seem to be quenched by the presence of water aerosol. More consistent results were obtained with the Hγ line. When no solutions are pumped through the hydride generation manifold (“dry” plasma), the measured n_e value was (1.57 ± 0.22) × 10¹⁵ cm^–3. Conversely, when the reducing reagent flow was replaced by pure water (corresponding to the injection of water vapor in equilibrium that is swept by the argon carrier gas passing through the phase separator), the electron concentration is 25% higher. In that case the n_e value agrees between the experimental error with that obtained for a plasma in which a water aerosol is introduced at a flow rate of 1 mL/min. An enhancement of 52% relative is observed in n_e when the system is operated under optimized conditions for arsine generation, employing sodium tetrahydroborate in acidic medium as reducing agent (i.e. hydrogen seeded plasma). It was also observed that the continuum emission near 410 nm for the hydrogen containing plasma correlates with the measured electron number density, suggesting that the background enhancement under hydride generation conditions may respond to the ion-electron recombination mechanism.     

A matrix effect and accuracy evaluation for the determination of elements in milk powder LIBS and laser ablation/ICP-OES spectrometry

Laser ablation coupled to inductively coupled plasma optical emission spectrometry (LA-ICP-OES) and laser-induced breakdown spectroscopy (LIBS) were investigated for the determination of Ca, Mg, Zn and Na in milk samples. The accuracy of both methods was evaluated by comparison of the concentration found using LA-ICP-OES and LIBS with classical wet digestion associated with ICP-OES determination. The results were not fully acceptable, with biases from less than 1% to more than 60%. Matrix effects were also investigated. The sample matrix can influence the temperature, electron number density (n _e) and other excitation characteristics in the ICP. These ICP characteristics were studied and evaluated during ablation of eight milk samples. Differences in n _e (from 8.9 to 13.8 × 10¹⁴ cm⁻³) and rotational temperature (ranging from 3,400 to 4,400 K) occurred with no correlation with trueness. LIBS results obtained after classical external calibration procedure gave degraded accuracy, indicating a strong matrix effect. The LIBS measurements clearly showed that the major problem in LA-ICP was related to the ablation process and that LIBS spectroscopy is an excellent diagnostic tool for LA-ICP techniques.     

Comparison of a very high frequency 148 MHz inductively coupled plasma to a 27 MHz ICP

Physical parameters and analytical performance are determined for an analytical ICP operated at 148 MHz, a frequency nearly three times higher than any previously reported. The iron(I) excitation temperatures are approximately 1.5 times lower and the electron densities are five times lower than at 27 MHz. The consequences of these changes are lower analyte and background continuum emission intensities, such that the signal to background ratios are decreased at the higher frequency. Freedom from interferences and working curve linearity are unaffected while ease of sample introduction is improved. A shift towards atomic emission indicates a deviation farther from LTE at 148 MHz. These effects are attributed to the decrease in skin depth with increasing frequency.     

The effect of propane on atomic spectrometric signals in the inductively coupled argon plasma

The introduction of propane into an ICP operated in the pencil plasma mode is shown to alter analyte emission signals. These effects do not result from propane affecting the plasma excitation temperature or electron number density, but are caused by the chemical reactions which occur between the analyte and elemental C in the normal analytical zone and tail plume of the plasma. For signal enhancements, the reaction is believed to involve a reduction of the prevalent metal oxide species by elemental C, from C₃H₈, to form the metal atom and CO in the plasma. Signal depressions, which occur with the addition of C₃H₈ to the plasma may be the result of the formation of metal carbides or deactivation of the excited state analyte species by collisions with elemental C or carbon radicals in the plasma tail plume.     

Excitation of analytes and enhancement of emission intensities in a d.c. plasma jet: a critical review leading to proposed mechanistic models

Experimental data and theoretical criteria are used to critically review existing models for analyte emission enhancement in the 3-electrode d.c. plasma (DCP). The analytical zone is characterized as a non-optically thin recombining plasma in partial thermodynamic equilibrium (PTE). Spectrochemical excitation the authors ascribe largely to: (1) argon resonance line radiative transport; (2) inversion of optically pumped argon states; (3) inversion of analyte populations by Franck-Condon collisions with argon; (4) energy cascading in analytes via a multitude of channels. Adding an easily ionized element (EIE): (1) induces additional resonance line radiative transfer; (2) raises electron densities in cooler, analyte-rich plasma margins; (3) locally increases argon optical absorption cross sections via Stark broadening; (4) redistributes ohmic heating. Coupling between the proposed mechanisms is non-linear. Relationships between radiative transfer and collisional redistribution and (1) background suppression by EIE and (2) analyte emission enhancement by helium are also examined. Similarities between DCP and inductively coupled plasma (ICP) excitation mechanisms are noted and practical implications are addressed.