Izumi Ishii

A tutorial discussion on measurements of rotational temperature in inductively coupled plasmas

Abstract The intensities in the rotational fine structure of electronic bands such as B2Σ+u → X2Σ+g of N+2, A2Σ+ → X2Πi, of OH and B3Σu− → X3Σ−g of O2 are often used for measurements of rotational temperature prevailing in plasma sources such as the inductively coupled plasmas (ICP) and microwave induced plasmas (MIP). Certain misconceptions (such as the incorrect use of Hunds rule, the use of transition probability in conjuction with the equation for line strength, and the incorrect assignment of rotational lines) in the calculations of rotational temperatures have led to erroneous results. The main point of this paper is to emphasize that the interpretation of the rotational intensity distribution depends on both the angular momentum coupling and on the resolution of the instrument used. In this report, the introductory theory of rotational temperature measurement is reviewed and examples of correct and incorrect calculations are cited for ICP and MIP discharges sustained in argon, helium, argon-nitrogen, argon-oxygen and argon-air.

Radial excitation temperatures in argon-nitrogen inductively coupled plasmas

Abel-inverted excitation temperatures are measured for Ar-N2 inductively coupled plasmas (ICPs) formed in a conventional torch using a 27.12-MHz crystal-controlled generator. The dependence of temperature on gas composition (0–100% N2 in the outer flow), forward power (1.2–3.0 kW), observation height (6 and 15 mm) and injector gas flow-rate (1 and 2 l min–1) are studied using Fe as the thermometric species. When 5–10% N2 is used in the outer gas flow instead of pure Ar, the axial temperature is approximately 1000 K greater than that of the commonly used Ar ICP. Substitution of Ar in the outer flow by pure N2 reduces the axial temperature from 6200 to 4900 K at 1.2-kW forward power. At 2.5 kW, the Ar-N2 plasma, with pure N2 in the outer flow, exhibits a temperature of approximately 6200 K, similar to that of a 1.2-kW Ar ICP.

Rotational temperatures of argon-nitrogen ICP discharges measured by high-resolution Fourier transform spectrometry

Abstract The intensity distribution in the rotational structure of the electronic band B 2 Σ + g → X 2 Σ + g of N + 2 (0,0) was measured by high-resolution Fourier transform spectrometry. This spectral information enabled estimates of the rotational temperatures in atmospheric-pressure argon-nitrogen inductively coupled plasmas (Ar-N 2 ICPs). These rotational temperatures were compared to the excitation and the Doppler temperatures of Ar ICP and Ar-N 2 ICPs having from 0 to 100% nitrogen in the outer gas flow. The rotational temperature of the mixed-gas plasmas ranged from 5500 to 10000 K, depending on the percentage of nitrogen in the outer gas flow, compared to 7800 K for the Ar ICP. The lowest and the highest rotational temperatures were obtained when the outer gas flow contained 100% and 17% nitrogen, respectively.

Line widths and temperatures of Ar-N2 ICP discharges measured by high-resolution Fourier transform spectrometry

Abstract The high-resolution Fourier transform spectrometer (FTS) of the Los Alamos National Laboratory was used for diagnostic studies of Ar-N 2 ICP discharges. High-resolution FTS data were obtained to: (a) conduct analysis of line widths and line shapes for Fe lines to ascertain contributions from the Gaussian and Lorentzian components; (b) to calculate the Doppler or translational temperatures of emitting species by using the half width of the Gaussian component; and (c) to determine excitation temperatures based on the relative intensities of many spectral lines. The effect of gas composition and plasma operating conditions on line widths, Doppler and excitation temperatures were examined.

Helium ICP-AES: effects of induction frequency and forward power on plasma formation and analytical and fundamental properties

Abstract Helium inductively coupled plasmas (He ICPs), formed at 6.8, 27.1 and 40.7 MHz with a single generator, were investigated at power levels of up to 2.5 kW for atomic emission spectrometry (AES). A 27.1-MHz generator and an impedance matching network were modified for operation at the cited frequencies. Detection limits for Br I (827.2 nm), Cl I (837.5 nm), II (804.3 nm), and S I (921.2), excitation and rotational temperatures, and electron number densities were measured. In addition, spectral features of He ICPs at atmospheric and reduced pressure were compared for the wavelength range between 200 and 600 nm for the 27.1 MHz plasmas. For the reduced-pressure He ICP, the series limit of He (3S-3P) was used to estimate electron number densities in the energy addition region. At higher operation frequencies, detection limits were improved by approximately a factor of 3 for the 1.5-kW atmospheric-pressure plasmas (2, 3 and 9 μg ml for Br, Cl and I, respectively), while no significant change in excitation and rotational temperatures and electron number densities were noted. Operation of a 27.1-MHz He ICP at 2.5 as compared to 1.5 kW, increased electron number density and rotational temperature by approximately a factor of 2 and 400 K, respectively. However, no significant change in detection limits or excitation temperature were noted at the higher power. These results were compared to data obtained for a 1.1-kW, 27.1-MHz Ar ICP.

Revised, fast, flexible algorithms for determination of electron number densities in plasma discharges

Abstract This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hard copy text is accompanied by a disk with two electron number density ( n e ) calculation programs written in Turbo Basic (NNE.EXE) and C language (NE.EXE), data mes ( ∗ .DAT), a text file (NE.WP) with the hard copy paper in WordPerfect 5.1 and a text file (README) for the introduction of the data files and programs. The theoretical aspects and applications of NNE.EXE and NE.EXE are described in the main article for three plasmas having a wide range of electron number density (4 × 10 13 to 1 × 10 17 cm −3 ). The descriptions of the procedure, data format, and hardware requirements can be found in the Appendix and the README file. The determination of n e . is accomplished by least-squares fitting of the entire emission profile or the wing portions of the emission profile of the H β line (486.13 nm) to the theoretical Stark broadened profiles, compiled at electron temperature 2500, 5000, 10000 and 20000 K. This approach can reduce errors in the determination of n e owing to the structural dip in the center of the Stark profiles depending on n e value. The curve-fitting routine employs an interval-halving algorithm to produce new interpolated Stark profiles, reduces number of iterations and time required for calculation, and allows the calculation of n e to the limits set by instrument spectral resolution and the availability of theoretical spectral data. Additionally, each program provides graphic displays allowing the reader to observe the interpolated Stark profiles, the fitting process, and magnitude of residuals during the course of calculation, and the effects of plasma temperatures and spectral resolution on the estimation of n e . In addition to H β , theoretical profiles also are included for H α and H γ to illustrate the suitability and drawbacks of these lines for n e estimation. These programs are extended versions of a program reported earlier in Spectrochimica Acta 44B , 175 (1989).

Noise power spectra comparing He and Ar ICP discharges

Abstract Noise power spectra (NFS) of helium inductively coupled plasmas (He ICPs) were measured using tangential- and laminar-flow torches. To identify the noise sources originating from the sample introduction systems and the plasmas, NPS were measured for two frequency ranges: 0–35 and 0–1060 Hz. Spectra were compared with those obtained for the commonly used Ar ICP. Examination of the NPS reveals and reconfirms that: (a) the He ICP generated in a tangential-flow torch experiences a greater level of high-frequency fluctuation than the Ar ICP, (b) no high-frequency peaks are observed when a laminar-flow torch is used to form the He ICP, and (c) the major contribution to noise at low frequency results from the injection of aqueous samples into the plasma. Results are discussed in terms of plasma asymmetries, vortex ring phenomenon, and instabilities introduced by pneumatic and ultrasonic nebulizers.

Fundamental properties of helium inductively coupled plasmas measured by high-resolution Fourier transform spectrometry

Abstract Intensities and widths of atomic spectral lines of He, H, and Fe, excited in an atmospheric-pressure helium inductively coupled plasma (He ICP) were measured with a high-resolution Fourier transform spectrometer. These data along with measured intensities of rotational bands, such as B 2 Σ + u → X 2 Σ + g of N + 2 and A 2 Σ + → X 2 Π i of OH, were used to estimate excitation, rotational, and Doppler temperatures, and electron number densities. Similar to an Ar ICP, the widths of Fe lines ranged from 3 to 4 pm. The line widths of H and He lines in the spectra from a dry He ICP generally were larger than those observed from a wet plasma. The line-width data for H were used to evaluate electron number densities in wet and dry He ICP discharges. The excitation temperature of He ICP was dependent upon the energy levels of the selected thermometric species. The rotational temperatures measured from OH (3000 K) and N + 2 (2200 K) were substantially different from those of an Ar ICP. The presence of water in the aerosol resulted in an increase in the excitation temperature of the He ICP, similar to the trend observed earlier for the Ar ICP. The implications of these results in He ICP spectrochemical analysis are discussed

Radial excitation temperatures in argon-oxygen and argon-air inductively coupled plasmas

Abel-inverted excitation temperatures are measured for Ar-O2 and Ar-air inductively coupled plasmas (ICPs) formed in a conventional torch with a 27.12-MHz crystal-controlled generator. The dependencies of temperature on gas composition (0, 5, 10, 20, 50 and 100% of either O2 or air in the outer flow), forward power (1.2 and 2.5 kW), observation height (6, 10 and 15 mm) and injector gas flow-rate (1 and 2 l min–1) are studied using Fe and Mo as the thermometric species. Results are compared with temperatures measured for Ar and Ar-N2 plasmas. When 5–100% O2 or 5–100% air is used in the outer flow instead of pure Ar, the axial temperatures measured at the 10-mm observation height and 1.2-kW forward power are greater than that of the commonly used Ar ICP, except when pure O2 is used in the outer flow. The highest axial temperatures are achieved when only 10% O2(6800 K) or 10% air (6400 K) are introduced into the outer flow. At 2.5 kW and 6 mm, the Ar-air plasma, with air in the outer flow, exhibits a temperature of approximately 6200 K, similar to that of a 2.5-kW Ar-N2 ICP or a 1.2-kW Ar ICP. In contrast, the axial temperature measured for a 2.5-kW Ar-O2 ICP with pure O2 in the outer flow is approximately 1500 K lower than that of the commonly used 1.2-kW Ar ICP. In general, the trends noted for the Ar-air ICPs are similar to those described earlier for the Ar-N2 ICPs.

Extraction discharge source for argon inductively coupled plasma atomic emission spectrometry: examination of analytical potentials in the detection of a range of elements and fundamental properties

An extraction discharge was formed at the tail flame of an Ar inductively coupled plasma (ICP) for atomic emission spectrometry. Analytical performance was measured for 19 elements at 46 different wavelengths for the supplementary discharge and for the Ar ICP alone. Detection limits of the Ar ICP used with the extraction discharge were generally lower than those for the Ar ICP alone. While detection limits measured at Ca II 370.60, Ca II 373.69, Zn II 202.55, Zn I 213.86, Mg II 279.55 Mg II 280.27, Mn II 293.30, Mn II 293.93 and Mn II 294.92 nm were improved by a factor of 8–44 in the presence of the extraction discharge, the enhancement in detection power was not significant (less than a factor of five) for the majority of the elements tested. Compared with the conventional Ar ICP, higher excitation and rotational temperatures were measured for the extraction discharge. Electron number densities measured in the presence of the extraction discharge (at 26 mm) were approximately the same as that of an Ar ICP alone (at 18 mm). These results were compared with the data obtained previously for an extraction discharge and with those found in ICP mass spectrometry.