New coil geometries for the study of secondary discharge in atmospheric-pressure helium inductively coupled plasma mass spectrometry

Several coil geometries are proposed to reduce the secondary discharge in atmospheric-pressure helium inductively coupled plasma mass spectrometry (He ICPMS) using a crystal-controlled 40-MHz generator. The effectiveness of the proposed geometries in controlling the plasma potential is investigated by the Langmuir probe method and ion kinetic energy (IKE) measurements. The influence of RF power on plasma gas temperature (T_g) is investigated through measurements of ICP-mass spectrometer interface pressure and IKEs for new and conventional coil geometries. Trends in plasma potential and T_g are well correlated, revealing that T_g is elevated (to ∼3500 K) at high power levels mainly as a consequence of the interaction between the ICP and the grounded sampler. The reduction of the secondary discharge results in lower T_g values (∼2600 K), necessitating the use of a membrane desolvator to remove water-related polyatomic interferences observed in the mass spectrum. Solvent removal also improves the sensitivity for bromine as a high-ionization-potential element by one order of magnitude compared to the previous studies.     

Electron production and loss processes in a spectrochemical inductively coupled argon plasma

The behavior of inductively coupled plasmas for spectroscopic purposes has been studied extensively in the past. However, many questions about production and loss of electrons, which have a major effect on this behavior, are unanswered. Power interruption is a powerful diagnostic method to study such processes. This paper presents time resolved Thomson scattering measurements of the electron density n_e and temperature T_e in an inductively coupled argon plasma during and after power interruption. In the center of the plasma the measured temporal development of n_e and T_e can be attributed to ambipolar diffusion, three-particle recombination and ionization. However, at the edge of the plasma an additional electron loss process must be involved. In addition, the high electron temperature during power interruption indicates the presence of an electron heating mechanism. The energy gain by recombination processes is shown to be insufficient to explain this electron heating. These discrepancies may be explained by the formation and destruction of molecular argon ions, which can be present in significant quantities.     

Langmuir probe measurements of electron temperature in an inductively coupled plasma

The feasibility of using double Langmuir probes to measure electron temperature (T_e) in an Ar inductively coupled plasma (ICP) was evaluated. Experimental methods for probing the plasma and for reducing rf interference were devised. Despite these measures, the probe signal was noisy and erratic if the ICP had the normal analytical configuration with a hole through its center, so measurements were restricted to an ICP without an axial channel. Theoretical criteria indicated that Langmuir probe measurements in an atmospheric pressure ICP were in a borderline regime in which the measured T_e values may have been depressed somewhat (relative to the actual T_e values in the ICP) due to cooling of electrons as they approached the probe. The T_e values obtained from the center of the ICP were 7500 K at a forward power of 1.0 kW and 10 000 K at 1.25 kW for a measurement position 8 mm above the load coil. Electron density (n_e) measurements by the Langmuir probe method were comparable to or higher than n_e values calculated from the Saha equation at the measured T_es. The T_e and n_e values were high enough to indicate that, if electron cooling and ion-electron recombination occurred near the probes, these effects were not extreme and/or the use of two probes compensated for them in some fashion. The probe measurements also indicated that T_e increased with the potential difference between the probes. This latter observation provided tentative evidence that the electron kinetic energy distribution was non-Maxwellian with an excess of higher energy electrons relative to lower energy electrons.     

Comparison of mass spectrometric and optical measurements of temperature and electron density in the inductively coupled plasma during mass spectrometric sampling

Electron density (n_e) and ionization temperature (T_ion) are measured using atomic emission spectrometry (AES) from the small funnel of gas just outside the sampling orifice of an inductively coupled plasma-mass spectrometer (ICP-MS). Rotational temperature (T_rot) is measured using an OH emission band. T_ion is also determined for the same elements (Zn and Cd) by using M⁺ ion signal ratios by MS. For matrix-free solutions, typical values are n_e=1.6×10¹⁵ cm⁻³, T_rot=3340 K, T_ion (MS)≈T_ion (AES)≈7000 K. This agreement between the T_ion values supports other observations that, for atomic analyte ions M⁺ of similar m/z values in matrix-free solutions, the relative signals in the mass spectrum reflect the corresponding relative abundances in the ICP region being drawn into the sampler. Using either MS or AES, T_ion for Cd is 300–400 K higher than that for Zn, which indicates that T_ion can vary for different elements in the ICP. Sodium nitrate matrix at levels up to 1000 ppm Na does not cause a measurable change in n_e; 2000 ppm Na causes n_e to increase to 2.1×10¹⁵ cm⁻³. Sodium matrix has a large effect on the MS signal levels but does not greatly change the resulting T_ion values measured optically.     

The relation between internal and external parameters of a spectrochemical inductively coupled plasma

The excitation kinetics in a spectrochemical plasma are governed by the electron density n_e, electron temperature T_e, and heavy particle (gas) temperature T_h. Therefore, knowledge of these ‘internal’ plasma parameters is important for an understanding of the relation between the sample concentration in the plasma and light emission. Because of the small size of the plasma, the internal plasma parameters are related rather directly to the ‘external’ operational parameters of the plasma, such as the plasma dimensions, power density, and pressure. This relation is established by the various particle and energy balances, and can be used to estimate the internal plasma parameters and predict trends for a change in the operational parameters. In the present work, this approach was applied to spectrochemical inductively coupled plasmas under various gas-flow, gas-composition, and plasma-power conditions, and validated by Thomson scattering experiments. The measured values and trends of the internal plasma parameters are in close agreement with those expected on the basis of the operational parameters of the plasma.     

Real-time analysis of CuO by inductively coupled plasma emission without external calibration

The study of a method, devoted to real-time detection of metallic pollutants present in stack gas, is investigated. This method is based on spectroanalysis using an inductively coupled plasma (ICP) emission system without external calibration. The fluidized bed technology is employed to inject metallic species into the ICP emission. The mass fluxes of copper oxide (CuO) are then determined by using the intensity ratios of the metallic element spectral lines with those of the plasma gas element (argon or dry air). These ratios and the plasma characteristics (atomic excitation temperature, degree of thermal disequilibrium θ=T_e/T_h) are inserted into a calculation code of plasma composition to determine the mass flux. The results are in good agreement using either argon plasma or dry air plasma. A study of the fluidized bed properties is made to compare our values with those resulting from the elutriation calculation of the copper oxide.     

The use of different plasma gases (Ar, N 2 and air) for CMP-OES: figures of merit and temperature measurements

A comparative study of a 600 W capacitively coupled microwave plasma (CMP) operated with different plasma gases (Ar, N₂ and air) with respect to the achieved detection limits for Fe, Cr, Zn, Ca and Mg have been carried out. Radially and axially resolved rotational temperatures (T_rot), excitation temperatures (T_exc) and electron number densities (n_e) of these plasmas have been determined using OH (T_rot), Fe (T_exc) and Mg (n_e) as thermometric species. The influence of different gas flow rates on T_rot, T_exc and n_e, and of Li as an easily ionized element on T_exc has been investigated.     

Fundamental studies on a planar-cathode direct current glow discharge. Part I: characterization via laser scattering techniques

A laser-scattering-based instrument was used to study an argon d.c. planar-diode glow discharge. The gas-kinetic temperature (T_g) was determined via Rayleigh scattering and the electron number density (n_e), electron temperature (T_e), and shape of the electron energy-distribution function were determined by Thomson scattering. Axial profiles of these parameters were obtained as the discharge current, voltage, and pressure were varied. Trends in the profiles of T_g and in the other parameters show the interdependence of these plasma species and properties. The results will be compared with current theoretical computer models in order to improve our understanding of the fundamental processes in glow discharges sustained under conditions appropriate for spectrochemical analysis.     

Thomson scattering on argon surfatron plasmas at intermediate pressures: Axial profiles of the electron temperature and electron density

The axial profiles of the electron density n_e and electron temperature T_e of argon surfatron plasmas in the pressure range of 6–20 mbar and microwave power between 32 and 82 W have been determined using Thomson Scattering of laser irradiation at 532 nm. For the electron density and temperature we found values in the ranges 5 × 10¹⁸ < n_e < 8 × 10¹⁹ m^− 3 and 1.1 < T_e < 2.0 eV. Due to several improvements of the setup we could reduce the errors of n_e and T_e down to 8% and 3%, respectively. It is found that n_e decreases in the direction of the wave propagation with a slope that is nearly constant. The slope depends on the pressure but not on the power. Just as predicted by theories we see that increasing the power leads to longer plasma columns. However, the plasmas are shorter than what is predicted by theories based on the assumption that for the plasma-wave interaction electron–atom collisions are of minor importance (the so-called collisionless regime). The plasma vanishes long before the critical value of the electron density is reached. In contrast to what is predicted by the positive column model it is found that T_e does not stay constant along the column, but monotonically increases with the distance from the microwave launcher. Increases of more than 50% over 30 cm were found.     

Some considerations on the microwave electrodeless discharge: Plasma diagnostics in argon,helium or nitrogen atmospheres

Plasma diagnostics of several microwave plasmas are determined by making electrical (with double floating probes) and optical measurements in pure Ar, He or N₂ plasmas, and also in Ar plasmas containing various metals, i.e. Cs, Tl or Zn; plasma parameters, such as, electric field (E), electron (j_e) and ion (j_i) current densities, electron density (n_e), electron temperature (T_e) electron conductivity (σ_e), ion density (n_i), electron mean free path (λ_e) electron (μ_e) and ion (μ_i) mobilities and electron [(v_e)_drift] and ion [(v_i)_drift] volocities are either directly measured or calculated. The reversal temperature (T_r) of excited (0.96 eV lower level) thallium atoms is measured, and the steady-state conditions of the plasma are analyzed by the energy balance equation. The experimental measurements indicate that the electric field strength E decreases as the space charge decreases (ionization extent) increases. Although the plasma appears to be under steady-state conditions, it is not under local thermodynamic equilibrium conditions, i.e. T_e >T_r. In addition, the measurements indicate that there is a deficiency of electrons in the plasma (n_e < n_i), probably due to electron affinity processes; and the plasma has a small positive space charge.     

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 possible steady state kinetic model for the atomization and excitation processes during inductively coupled plasma atomic emission spectrometry: Application to interference effects of lithium on calcium

A possible steady state kinetic model is presented for the atomization and excitation processes during inductively coupled plasma atomic emission spectrometry. The model takes into account the relative rates of (a) thermal dissociation of analyte salt, (b) recombination of counter atom and analyte atoms, (c) charge transfer between analyte and interferent species, (d) charge transfer between analyte and argon species, and (e) ion/electron collisional de-ionization. Number density ratio data, n_u′/n_u, where n_u denotes the excited state and the prime denotes the presence of an interferent element, are presented showing that the predictions of the model are consistent with the signal enhancement observed at low analyte concentrations when Ca is determined by ICP in the presence of excess Li.     

High resolution studies of the origins of polyatomic ions in inductively coupled plasma-mass spectrometry,Part I. Identification methods and effects of neutral gas density assumptions,extraction voltage,and cone material

Common polyatomic ions (ArO⁺, NO⁺, H₂O⁺, H₃O⁺, Ar₂⁺, ArN⁺, OH⁺, ArH⁺, O₂⁺) in inductively coupled plasma-mass spectrometry (ICP-MS) are identified using high mass resolution and studied using kinetic gas temperatures (T_gas) determined from a dissociation reaction approach. Methods for making accurate mass measurements, confirming ion identifications, and correcting for mass bias are discussed. The effects of sampler and skimmer cone composition and extraction voltage on polyatomic ion formation are also explored. Neutral species densities at several locations in the extraction interface are estimated and the corresponding effects of the T_gas value are calculated. The results provide information about the origins of background ions and indicate possible locations for their formation or removal.     

Spatially resolved emission using a geometry-dependent system function and its application to excitation temperature profile measurement

As typical emission spectroscopy involves chord integration along the line of sight, a local measurement with high spatial resolution is attempted using simple lens optics in this work. In the experiment, chord integrated optical plasma emission profile was measured by moving a scanning lens located outside the plasma. The measured emission intensities were spatially reconstructed by employing a geometry-dependent system function, and the local (i.e., only from the lens focal point) emission intensities were obtained with all out-focused emissions subtracted. The 34 different Ar I emission lines spatially reconstructed in this way were used to determine excitation temperature (T_exc) of the argon plasma by the Boltzmann plot method. Being different from the plasma driven at 13.56 MHz where a rather uniform profile was obtained, the spatial profile of T_exc from the plasma driven at 90 MHz showed a hollow profile, which is similar to that of the electron temperature (T_e) measured by a Langmuir probe. This hollow profile is attributed from the electromagnetic phenomena such as skin effect and standing wave effect. The similar spatial tendency of T_exc and T_e implies that T_exc can be a representative of T_e. This is particularly useful for the cases in which conventional Langmuir probe measurements are limited, such as in large size plasmas.     

Spatially resolved measurements of ion density behind the skimmer of an inductively coupled plasma mass spectrometer

Ions are extracted from the inductively coupled plasma through a conventional sampler and skimmer and then deposited on an array of graphite targets at the exit of a set of electrostatic ion lenses. The Sc⁺ signal is enhanced by choosing appropriate potentials on the ion lenses. The Sc⁺ signal is suppressed by the presence of concomitant Cs ions at high concentrations. Comparisons of grounded ion lenses and two different ion lens potential settings are made. The signal is enhanced more extensively by the ion lenses when there are no concentrated concomitant ions. This study indicates that matrix effects in inductively coupled plasma mass spectrometry could possibly be alleviated by choosing ion lens potentials such that the ions enter the ion optics with a relatively broad beam cross section, the beam then being focused to a smaller size. A photon stop inside the ion lens stack reduces ion transmission and changes the shape of the beam profile from conical to bimodal.     

Plasma polymerization of tetrafluoroethylene. IV. Comparison of ethylene and tetrafluoroethylene by measurement of electron temperature and density of positive ions

The double probe method was applied to plasma of tetrafluoroethylene (TFE) and ethylene and the electron temperature (T_e) and density of positive ions (n_p) were measured at various discharge wattages. The probe current-probe voltage diagrams for TFE were different from those for ethylene. The shape of its diagram indicates that a considerable number of negative ions exist in TFE plasma. The levels of n_p for TFE were also nearly six times greater than those for ethylene at the same discharge current. The dependence of TFE polymer deposition and the chemical structure of the polymer, based on ESCA data on discharge current, was related to T_e and n_p measured by the probe method. The values of T_e and n_p may not be directly related to the polymer formation in a plasma; the method provides a direct measure of plasma energy density where plasma polymerization takes place, whereas it cannot be accurately estimated by the input energy of a discharge. It was found that plasma energy density based on (n_pT_e) for TFE plasma and that for ethylene differ significantly at the same level of input parameter (W/FM), where W is the discharge wattage, F is the volume flow rate, and M is the molecular weight of the monomer.     

Statistical mechanics of Ar2+ in an inductively coupled plasma

In the mass spectrum of an argon inductively coupled plasma (ICP), there is a peak due to the presence of the argon dimer ion, Ar₂⁺. Using elementary statistical mechanics, an attempt is made to elucidate the mechanism responsible for this ion's presence in the ICP, The assumption of local thermodynamic equilibrium (LTE) in the ICP leads to three possible mechanisms that could be responsible for the presence of the argon dimer ion, however, the results of the calculations show that only one of the mechanisms agrees with experiment. The experimental measurements of the number density ratio of Ar₂⁺ to Ar⁺, against which the theoretical values are compared, were taken using inductively coupled plasma mass spectrometry (ICP-MS),     

Spatial profile measurement of electron number densities and analyte line intensities in an inductively coupled plasma

A versatile instrument for spatial profile measurement has been developed and applied to the measurement of electron number densities and analyte emission intensities in an inductively coupled plasma (ICP). A precise Y-Z stage on which the ICP source was mounted was set on a rail-based optical bench. By translating the ICP source with a precision of ± 0.01 mm, the H_β Stark broadening and analyte line intensities were measured with the use of a silicon intensified target (SIT) and a photomultiplier (PMT). Micro-computer assisted data acquisition allowed it to measure a large number of emission profiles in a short period. The ease of acquisition enabled to build up complete contour maps of electron number densities, Ca neutral atom (Ca I) and Ca ion (Ca II) line intensities, and intensity ratios of the Ca II and Ca I lines. The maximum electron number density was 4 × 10¹⁵ cm^?3 occurring low in the plasma and 5 mm off axis. In a contour map of the electron number densities a hollow region was found low in the plasma, and the distribution pattern looked like a deep “trench”. Along the central channel of the ICP, the peak position of Ca II emission occurred higher than that of Ca I emission, and the spatial distribution of Ca II emission was wider and taller than that of Ca I emission. It has been verified that Ca I is emitted mainly at the region where the electron number density is less than 1 × 10¹⁴ cm^?3.     

Langmuir probe measurements of the ion extraction process in inductively coupled plasma-mass spectrometry. Part 2. Measurements of floating voltage and radiofrequency voltage

A single Langmuir probe is thrust axially through the skimmer into the supersonic jet of a typical interface for inductively coupled plasma-mass spectrometry (ICP-MS). Floating voltage V_f is measured from current-voltage curves at various axial positions and is generally in the range +2 to +7 V in the supersonic jet. A disturbance at the skimmer, when present, induces a sharp increase in V_f in the vicinity of the skimmer tip. Floating voltage generally decreases as the probe is retracted farther behind the skimmer. The measured d.c. voltages are attributed primarily to plasma rectification and to the calorelectric effect between two metal surfaces at different temperatures in a plasma. An r.f. voltage of up to 25 V (peak-to-peak) at the frequency of the plasma generator can also be measured behind the sampler.     

Spatial profiles of electron density in the inductively coupled plasma

Spatially resolved, radial electron number density (n_e) profiles have been measured at rf power settings of 1.00, 1.25, 1.50, 1.75 and 2.00 kW, and vertical heights of 4, 8, 12, 16 and 20 mm above the load coil. These profiles have been condensed and presented as electron density contour plots for each input power. The precision of the method was evaluated by doing ten replicate measurements of electron density. The relative standard deviation varied between 2 and 10 % with the maximum at the plasma centre. Electron density was measured with and without the presence of the easily ionizable element—Cs, and no significant difference was observed.