Fundamental studies of mixed-gas inductively coupled plasmas

Spectrochrmm

Fundamental

Am.

studies of mixed-gas inductively

Vol

49B. Nos 12-14.

pp 1259-1282. 1994 Elsevier Science Ltd Printed m Great B&am

coupled plasmas*t

NORMAN N. SESI, AMELIA MACKENZIE, KATHRYN E. SHANKS, PENGYUAN YANG and GARY M. HIEFTJE$ Department of Chemistry, Indiana University, Bloomington, IN 47405, U.S.A. (Received

2 June 1994; accepted

6 August

1994)

Abstract-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 (N, or HZ) 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 N, is added to the central-gas flow. In contrast, the introduction of nitrogen in the intermediate flow causes about a 1 x lOi electrons cm-3 increase in the electron concentration in the low, toroidal regions of the plasma and an increase in the gaskinetic 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 N, 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 N, is added to the central- or intermediategas 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 H, addition.

1. INTRODUCTION OVER the years, several research groups have experimented with the use of mixed-gas discharges in attempts to improve the analytical utility of the inductively coupled I n a mixed-gas ICP, one or more of the plasma-gas flows plasma (ICP) [l-36]. (central, intermediate or outer) is partially or totally replaced with another gas. Argon plasmas have been supplemented with gases such as helium [l, 13, 16, 24-26, 32-341, nitrogen [l, 3, 4, 6, 7, 9, 11-17, 20-23, 27, 29, 30, 32-351, hydrogen [lo, 11, 16, 18, 19, 32-341, air [l, 13, 16, 221, oxygen [l, 5, 8, 11, 13-16, 20-22, 321, xenon [28, 321 * This paper was published in the Special Honor Issue of Spectrochimica Actu Part B, dedicated to J. D. Winefordner. t This work has been presented in part at the Twentieth Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Detroit, Michigan, 17-22 October 1993, and at the Pittsburgh Conference on Analytical Chemistry, Chicago, Illinois, 27 February -4 March, 1994. $ Author to whom correspondence should be addressed. 1259

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and methane [2, 16, 29, 31, 321, to name just a few. Most of these studies were aimed at overcoming some of the analytical and economic limitations of conventional argon ICPs. In certain parts of the world, argon is more expensive than some of the abovementioned gases; hence, the partial or total replacement of argon with other support gases could lead to lower operating costs. From an emission spectrometry viewpoint, traditional argon ICPs do not efficiently excite elements with high excitation energies such as the halogens. In addition, some analytes have emission lines which spectrally overlap those of argon. Similarly, in mass spectrometry, isobaric interferences from argon-containing polyatomic species can interfere with the determination of several elements (e.g. 40Ar40Ar+ with *‘Se+ and 40Ar160+ with 56Fe+). The mass interference can be a serious limitation when one must determine elements with only one isotope (e.g. 40Ar35C1+ with 75As+). A short review of the literature dealing with the analytical utility of mixed-gas ICPs reveals interesting results. MONTASERand MORTAZAVI[7] measured detection limits for several elements using optical emission spectrometry and found the limits of detection in an argon-nitrogen ICP to be inferior to those obtained with a pure argon plasma under similar operating conditions. MONTASERet al. [9] repeated the above measurements under optimized conditions for both Ar-N2 and conventional Ar ICPs and found similar results. However, under appropriate conditions, the detection of neutral atom lines was shown to be improved by the addition of nitrogen to the outergas flow. In a series of articles, CHOOT and HORLICK[12-151 described the characteristics of Ar-N,, Ar-O,, Ar-air and Ar-He mixed-gas ICPs. In general, their data revealed improved measurement precision, increased analyte signal-to-noise ratios, and minimization of vaporization-related and easily ionizable element matrix-effects. LAM and HORLICK [22], BEAUCHEMINand CRAIG [27, 301, and WANG et al. [29] employed Ar-N, mixed-gas ICPs as ion sources for mass spectrometry. All three studies showed that the addition of nitrogen to an argon plasma reduced certain polyatomic interferences and enhanced analyte ion signals. In two studies, better isotope-ratio measurement precision and accuracy [27], and lower mass discrimination were also observed [30]. SCHRAMELand LI-QIANG [lo] studied an argon-hydrogen ICP and found that detection limits were improved by the addition of hydrogen for most of the elements they studied, but especially for heavy elements. MURILLO and MERMET[19] also characterized an argon-hydrogen ICP and noticed an enhancement in energy transfer between the ring plasma and the central channel. The improvement in energy transfer was postulated to be due to the higher thermal conductivity of hydrogen than argon. However, no improvement in the detection limits was observed for the Ar-H, plasma. Characterizations of argon-helium mixed-gas plasmas by SHEPPARDet al. [25, 261 and WANG et al. [29] indicate improved elemental detection limits for both emissionand mass-based analysis methods. However, mass-dependent bias effects in mass spectrometry were shown to be present in both argon and argon-helium ICPs. Research work conducted by SMITHet al. [28], and HILL et al. [31] using argon-xenon and argon-methane mixed-gas plasmas, respectively, also indicate reductions in polyatomic-ion interferences in ICP-Mass Spectrometry (ICP-MS). In both studies, detection limits were either comparable to or better than in a conventional Ar-ICP. The exact mechanisms behind the observed effects are not clear. However, it is generally agreed that the effects are related to changes in the bulk properties of the plasma. The bulk properties include thermal conductivity, electrical conductivity, heat capacity, viscosity, density and radiant energy. Obviously, different gases have different bulk properties, so the use of one or more gases in combination may permit the development of user-tailored ICPs. Unfortunately, the development of customized inductively coupled plasma sources is easier said than done. A number of questions arise. Which gases should be used and in what fractional concentrations? Into which gas-inlet port should the supplemental gas or gases be introduced: central, intermediate, or outer flows or in some combination?

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In order to assist in the development of a rational procedure to design mixed-gas plasmas, the fundamental parameters of the plasma (which include electron number density, electron temperature and heavy-particle temperature) need to be determined under various operating conditions. The measurement of gas-kinetic temperatures, and of electron concentrations and temperatures, will aid in the understanding of how mixed-gas plasmas function since these parameters can be used to describe several of

the bulk properties and are directly responsible for sample volatilization, excitation and ionization. Previous measurements of electron number densities in Ar-He, Ar-N2 and Ar-HZ mixed-gas ICPs have been limited to Ar-N2 or Ar-H, plasmas with the molecular gas introduced into the outer-gas flow [15, 37, 381. MONTASER and FASSEL [37] used the series-limited, line-merging technique to measure the electron number density at two laterally resolved, on-axis heights in an Ar-N, ICP with nitrogen introduced into the outer-gas flow. The non-radially resolved results indicated that under similar ICP operating conditions the electron concentrations measured in a conventional argon ICP were higher than those in a pure nitrogen outer-gas flow Ar-N, ICP. CHOOT and HORLICK [15] determined electron number densities in an argon-nitrogen (N* in outergas flow) ICP using Stark broadening of the hydrogen-beta line. Their Abel-inverted measurements at five positions above the load coil (ALC) indicated that the electron concentrations in the Ar-N2 plasma were generally higher in the lower regions of the discharge but lower in the upper plasma zone as compared to an all-argon ICP. Similarly, BATAL et al. [38] used the Stark broadening method to determine electron concentrations but in an argon ICP with hydrogen added to the outer-gas flow. Their radially-resolved measurements at 2 mm ALC showed an increase in the electron number density with the addition of hydrogen. Temperature determinations have been based on the use of Boltzmann plots and a suitable thermometric species (rotational and excitation temperatures) [17, 19, 23, 38, 391 or on the determination of non-radially resolved Doppler (gas-kinetic) temperatures [23]. The temperature measurements have been conducted on Ar-N, discharges with nitrogen added to the outer-gas flow [17, 23, 391 or on Ar-H, ICPs with hydrogen mixed with the argon carrier-gas [19] or outer-gas [38] flow. In the present study, computed tomography, Thomson scattering and Rayleigh scattering have been used as methods to measure radially-resolved calcium ion emission profiles, electron temperatures and concentrations, and gas-kinetic temperatures with5 Ar, Ar-He, Ar-N, and Ar-H, plasmas. Results showing the effects of these gases added to the central- or intermediate-gas flow of a conventional argon inductively coupled plasma are presented. 2.

EXPERIMENTAL

2.1. ThomsonlRayleigh scattering spectrometer The Thomson/Rayleigh scattering instrument used in this study is similar to that described in earlier papers from our laboraotry [40, 411. A complete description of the current instrument can be found in Ref. [42]. It has been shown that the electron energies in an ICP can be described by the Maxwell distribution [43]. Therefore, in order to improve the accuracy of the electron temperature and concentration determinations, a non-linear least-squares program based on the Levenberg-Marquardt algorithm [44] is used to fit the known Thomson scattering equation to the measured profile. Electron temperature and number density are then simultaneously extracted from the best fit. 2.2. Optical imaging spectrometer The monochromatic imaging spectrometer (MIS), utilized to obtain two-dimensional images of the ICP, is similar in form to the one that has been described previously [45], except the MIS employed in this study uses a one meter monochromator (Jobin-Yvon model HRlOOO, reciprocal linear dispersion of -0.28 nm/mm at 532 nm) and is a part of the Thomson/Rayleigh scattering instrument. In addition, the plasma image was not rotated, so the vertical orientation

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Table 1. ICP operating conditions rf generator Impedance matcher Torch

Nebulizer gas flow rate (1 min-‘) Total central gas flow rate (1 min-‘) Total intermediate gas flow rate (I min-‘) Outer gas flow rate (1 min-‘) Incident rf power (kW) Reflected rf power (W) Sample introduction system”

Sample uptake rate (ml min-‘) Solvent loading (mg min-‘)b

27.12 MHz Leco Instruments (Model RTG-27-CT) PlasmaTherm (currently RF Plasma Products) ICP 2500 Conventional 18 mm i.d. outer tube, 1.5 mm i.d. injector tube (Leco model from Precision Glassblowing of Colorado) 1.00 1.20 1.00 14 1.25 < 10 concentric nebulizer (Type C, Precision Glassblowing of Colorado) with water-cooled Scott-type spray chamber. The aerosol was partially desolvated

1.0 18.9

a no sample or solvent were introduced during the scattering measurements b obtained using the silica-gel trapping method [66] of the ICP in these experiments lies perpendicular to the slit width. The entrance and exit slit height was set to 3 mm and the entrance and exit slit width to 1 mm. These slit settings were chosen as a compromise between spectral and spatial resolution, and light throughput. The horizontal (radial) and vertical (height above load coil) spatial resolution of the imaging instrument were measured using a United States Air Force test target [46, 471 and found to be 1.41 and 1.12 line pairs mm-‘, respectively. 2.3. ICP operating conditions The ICP operating conditions are listed in Table 1. All gas flow rates were mass flow controlled (Tylan Corp. models FC-260 and FC-280, and MKS Instruments type 1159A), with the exception of the outer-gas flow where a rotameter was used (Matheson model 604). The amount of added gases is expressed as a percentage of the total central or intermediate channel flow rate. The intermediate channel flow rate was held constant at 1.00 1 min-‘, so 16.7% added gas implies that the intermediate channel flow consisted of 0.167 1 min-’ supplementary gas and 0.833 1 min-’ argon. In the case of the central-channel gases, the total flow rate was maintained at 1.20 1 min-‘, with the argon nebulizing gas held fixed at 1.00 1 min-‘. Since the nebulizing-gas flow was maintained constant, the highest added-gas percentage we could go up to in this study was 16.7% (0.2 x 10011.2 = 16.7). The nebulizing gas was held constant in order to eliminate effects associated with changes in the nebulization efficiency and other sample-introduction-related parameters (e.g. solvent loading). For the emission-based studies, the supplementary gases were added after the sample introduction system along with additional argon make-up gas when required. The scattering experiments (for the determination of electron temperature, T,, gas-kinetic temperature, Tg, and electron number density, n,) were performed on argon mixed-gas plasmas using no sample introduction system. Thus, different conditions in the plasma exist when analyte emission features are being measured and when the fundamental parameters (T,, Tgr n,) are mapped. The presence of incompletely desolvated droplets and incompletely vaporized particles in the central channel of an ICP have been shown to cause changes in the local, time-resolved emission intensities of both atoms and ions [48-501. In addition, changes in the number of atoms, ions and electrons near droplets and particles have also been noted [50]. While quantitative comparisons between results obtained in the presence and absence of an aerosol would be suspect, the qualitative explanations given here should be valid. The presence of foreign gases in an Ar ICP cause global changes in the properties of the plasma, in contrast to the localized effects of droplets and particles. It is possible that the presence of both added gases and droplets and particles can cause significant changes in the properties of the ICP. Future studies are intended to address these issues. 2.4. Gases All gases used in these studies were obtained from Air Products (Allentown, PA, U.S.A.). Argon gas was obtained from boil-off of liquid argon with a purity of 99.997%. Helium,

Fundamental studies of mixed-gas ICP nitrogen and hydrogen were used directly from pressurized 99.999%) 99.9985% and 99.995%) respectively.

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gas cylinders with stated purities of

2.5. Reagents The calcium solution used for the imaging studies was prepared from a reagent-grade nitrate salt (J. T. Baker Chemical Co.). The salt was dissolved in a 0.1 M nitric acid matrix which was prepared from concentrated nitric acid (E. M. Science).

3. MEASUREMENTTECHNIQUES:BRIEFTHEORY

3.1. Incoherent

Thomson scattering Thomson scattering is the scattering

of electromagnetic radiation by unbound electrons in a plasma. Since the electrons are moving with respect to an incident laser beam as well as with respect to the detector, the scattered radiation is doubly Doppler-shifted. By measuring the scattering spectrum at various Dopplershifted wavelengths, one can determine the electron energy distribution. If the electrons follow a Maxwellian velocity distribution, the width of the spectrum can be related to an electron temperature. In addition, by using a calibrated instrument, obtained by normalizing the Thomson-scattering signal to the intensity of the room-temperature Rayleigh scattering signal, the total scattered intensity (integrated area underneath the Thomson-scattering spectrum) can be related to the electron number density. In comparison with other diagnostic methods for the determination of electron concentration and electron temperature, Thomson scattering has the advantages of not physically perturbing the plasma, of not assuming plasma local thermodynamic equilibrium, and of being inherently radially resolved, with the observation volume being defined by the intersection of the laser beam with the optical axis of the detection system. Further discussion of Thomson scattering may be found in the cited Refs [X-56]. Incoherent

3.2. Rayleigh scattering Rayleigh scattering is the scattering of light by particles (atoms, molecules, etc.) that are much smaller than the incident wavelength of electromagnetic radiation. The scattering particles are generally of large mass compared to an electron and therefore have relatively low thermal velocities, so the effect of Doppler broadening can be neglected. Since Rayleigh scattering is linearly related to the concentration of scatterers and to the particle scattering cross-section, the Ideal Gas Law (n/V = PIRT) can be used to determine the heavy-particle temperature. A two-point calibration is all that is necessary to determine the gas-kinetic temperature. The measurement of roomtemperature scattering from argon gives one point and the other point is determined from room-temperature scattering of helium, which generates a signal that is equivalent to argon scattering at about 17,860 K (ratio of argon to helium photon-scattering cross-sections at 532 nm, multiplied by 293 K). Note that the scattering intensity is inversely proportional to the heavy-particle temperature; gases at high temperatures produce lower scattering intensities than those at low temperatures. The interested reader is directed to Refs [57, 581 for more information about Rayleigh scattering. 3.3. Monochromatic imaging spectrometer and computed tomography A detailed explanation of monochromatic imaging spectrometers and of computed tomography are given in references [45, 591. Briefly; an optical imaging spectrometer generally consists of four components: collimating lens, monochromator, focusing lens and detector (Fig. 1). The collimating lens (placed one focal length away from the object of interest) is used to generate a spatial Fourier transform of the object. The light bundle is then passed into the monochromator where wavelength selection occurs. The wavelength-selected collimated light is then inverse spatially Fourier transformed by passing the light through a second lens (focusing lens) placed one focal length away from a charge-coupled device (CCD)

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Object

1 Rotation Stage

I

Focusing Lens

CCD

1

I

Fig. 1. Block diagram of the monochromatic

imaging spectrometer.

Hence, what is observed at the CCD plane is a two-dimensional picture of the object at a particular wavelength of interest. By measuring a series of two-dimensional projections at various angles around the object, a three-dimensional representation of the object can be generated. The threedimensional reconstruction of an object (e.g. ICP) from a series of two-dimensional projections at several angles is known as computed tomography (CT). The mathematical procedure used in our laboratory to perform the tomographic reconstruction of the ICP is based on an analytical form of the Radon transform and is known as filtered backprojection [45, 60, 611. The advantage of CT in generating radially-resolved emission information from the ICP, in comparison to the more commonly used method of the Abel inversion, is that computed tomography makes no assumptions of object symmetry. This advantage is important since most ICPs are not symmetrical, due to asymmetries in the torch and the load coil. Hence, computed tomography is the method of choice for measuring radially-resolved emission profiles when one attempts to unravel the three-dimensional complexity of the inductively coupled plasma.

detector.

4. RESULTS AND DISCUSSION 4.1. Measurement uncertainty and precision Figures 2 and 3 show the individual measurement uncertainties and day-to-day precision, respectively, for the Thomson and Rayleigh scattering-based measurements. (These and subsequent figures have been scaled to make the most important features as prominent as possible). An individual measurement is defined here as the determination of an electron number density, electron temperature or gas-kinetic temperature value. The individual measurement uncertainties are based on the collection of twenty laser shots per point in the plasma per wavelength. The individual measurement uncertainties for electron number density and electron temperature were obtained from the non-linear least squares fit using the Levenberg-Marquardt algorithm. The gas-kinetic temperature uncertainties were calculated from the laser shot-to-shot fluctuations of the Rayleigh scattering signal. Our current Thomson/Rayleigh scattering spectrometer is a single-channel instrument, meaning that the scattering spectrum is measured by scanning the monochromator are over the wavelengths of interest. In addition, the wavelength measurements performed for each desired location in the plasma. In these experiments, 21 vertical heights ranging from 5 to 25 mm ALC were studied; at each height 31 radiallyresolved points spanning 15 mm were obtained, for a total of 651 measurement locations. As Fig. 2 shows, the individual measurement uncertainties are generally about 10% for electron concentration and electron temperature and about 15% for the heavy-particle temperature. Note that larger percent relative standard deviations (RSD) for electron density and electron temperature are found in the center and far outer regions of the ICP. These uncertainties are higher because the Thomson scattering signals are lower or absent (in the far outer zones there is no plasma) in those areas of the discharge compared to the toroidal regions. Since the Thomson scattering signal is directly proportional to the number of electrons in the scattering volume, lower electron number densities produce smaller scattering signals and thus

Fundamental

Fig. 2. Thomson

and Rayleigh scattering individual measurement argon plasma. See text for discussion.

Fig. 3. Thomson

and Rayleigh

scattering

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day-to-day average).

measurement

uncertainties

for a pure

reproducibility

Fig. 4. Effect of various percentages of helium added to the central-gas plasma on the electron number density.

(Cday

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Fig. 5. Effect of various percentages of helium added to the intermediate-gas plasma on the electron number density.

Fig. 6. Effect of various percentages of helium added to the central-gas plasma on the electron temperature.

Fig. 7. Effect of various percentages of helium added to the intermediate-gas plasma on the electron temperature.

flow of an argon

flow of an argon

flow of an argon

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Fig. 8. Effect of various percentages of helium added to the central-gas plasma on the gas-kinetic temperature.

Fig. 9. Effect of various percentages of helium added to the intermediate-gas plasma on the gas-kinetic temperature.

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flow of an argon

flow of an argon

Fig. 10. Effect of various percentages of helium added to the central-gas flow of an argon plasma on radially-resolved calcium ion (396.8 nm) emission profiles.

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Fig. 11 Effect of various percentages of helium added to the intermediate-gas flow of an argon plasma on radially resolved calcium ion (396.8 nm) emission profiles.

Fig. 13. Effect of various percentages of nitrogen added to the central-gas flow of an argon plasma on the electron number density.

Fig. 14. Effect of various percentages of nitrogen added to the central-gas flow of an argon plasma on the electron number density (shown on an expanded scale).

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Fig. 15. Effect of various percentages of nitrogen added to the intermediate-gas argon plasma on the electron number density.

flow of an

Fig. 16. Effect of various percentages of nitrogen added to the central-gas flow of an argon plasma on the electron temperature.

Fig. 17. Effect of various percentages of nitrogen added to the intermediate-gas argon plasma on the electron temperature.

flow of an

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Fig. 18. Effect of various percentages of nitrogen added to the central-gas flow of an argon plasma on the gas-kinetic temperature.

Fig. 19. Effect of various percentages of nitrogen added to the intermediate-gas argon plasma on the gas-kinetic temperature.

flow of an

Fig. 20. Effect of various percentages of nitrogen added to the central-gas flow of an argon plasma on radially resolved calcium ion (396.8 nm) emission profiles.

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Fig. 21. Effect of various percentages of nitrogen added to the intermediate-gas flow of an argon plasma on radially resolved calcium ion (396.8 nm) emission profiles.

Fig. 22. Effect of various percentages of hydrogen added to the central-gas flow of an argon plasma on the electron number density.

Fig. 23. Effect of various percentages of hydrogen added to the intermediate-gas argon plasma on the electron number density.

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Fig. 24. Effect

of various

percentages of hydrogen added to the central-gas plasma on the electron temperature.

Fig. 25. Effect

of various

percentages of hydrogen added to the intermediate-gas argon plasma on the electron temperature.

Fig. 26. Effect

of various

percentages of hydrogen added to the central-gas plasma on the gas-kinetic temperature.

flow of an argon

flow of an

flow of an argon

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Fig. 27. Effect of various percentages of hydrogen added to the intermediate-gas argon plasma on the gas-kinetic temperature.

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flow of an

Fig. 28. Effect of various percentages of hydrogen added to the central-gas flow of an araon plasma on radially resolved calcium ion (396.8 nm) emission-profiles.

Fig. 29. Effect of various percentages of hydrogen added to the intermediate-gas flow of an argon plasma on radially resolved calcium ion (3%.8 nm) emission profiles.

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higher RSDs. Yet, as can be seen from Fig. 3, the precision of the measurements (obtained over a four day period) is around 5% and attests to the stability and reproducibility of our instrument. Since our measurements are shot-noise limited [42], we can improve the uncertainties in the determination of electron concentration, electron temperature and gas-kinetic temperature by performing a larger number of experiments; in other words, by averaging more than 20 laser pulses. Unfortunately, increasing the number of scattering-based experiments will also increase the overall measurement time. It currently takes about 3.5 h to measure the scattering signals from 651 individual plasma volumes; integrating a greater number of laser shots would increase this time proportionately and raise the likelihood of ICP drift. 4.2. Effects of supplementary 4.2.1.

Electron

number

helium added to the central or intermediate channels Figures 4 and 5 show the effects on electron density.

concentration of adding various percentages of helium to either the central or intermediate gas flows of an argon ICP, respectively. The addition of helium to the central channel (Fig. 4) appears to reduce slightly the electron concentration in the low toroidal region of the plasma but to increase the number of electrons present in the central channel. For example, a decrease of about 2.5 X 1014 electrons cme3 is found at 5 mm ALC and a radial position of 4 mm from the central axis when 16.7% helium is added to the central-gas flow. In contrast, an increase of approximately 2.0 X 1014 electrons cme3 is found on-axis (0 mm radial position) at 12 mm ALC. A slight overall depression in the electron number density is found when helium is introduced into the intermediate-gas flow (Fig. 5). In either case, helium addition does not appear to affect significantly the shape or volume of the ICP. It should be noted that the variations in the electron number density for the 0% added helium case in Figs 3 and 4 and subsequent figures are due to the normal day-to-day variations in the ICP. As stated in Section 4.1, it takes approximately 3.5 h to map out 651 individual plasma locations (a cross-section down the center of the plasma) using our current Thomson/Rayleigh scattering spectrometer. In order to account for possible changes in the ICP due to normal day-to-day fluctuations, the “pure” argon ICP was measured several times over the course of the entire suite of experiments (Fig. 3 was constructed based on these experiments). The result of the “pure” argon ICP measurement, which was obtained closest in time to the measurements with the addition of a foreign gas, was chosen so that a fairer comparison could be made between the different cases. 4.2.2. Electron temperature. The effect of helium on the plasma electron temperature is depicted in Figs 6 and 7 for the addition of helium to the central or intermediate channel, respectively. Inspection of these images reveals that the addition of up to 16.7% helium to either the central- or intermediate-gas flows does not significantly affect the temperature of the electrons. The insensitivity of the electron temperature to the addition of helium could imply that the observed increases in the analyte ion signals in ICP-mass spectrometry [25] and ICP atomic emission spectrometry [24] are most likely not due to enhanced analyte ionization, particularly if electron impact is the dominant ionization process. If anything, the additional electrons observed in the central channel upon the addition of helium to the central-gas flow (Fig. 4) would serve to suppress analyte ionization and reduce signal levels. 4.2.3. Gas-kinetic temperature. The effects of helium on the heavy particle temperature when added to either the central or intermediate gas channels are shown in Figs 8 and 9, respectively. Unlike helium’s small effect on the electron number density and temperature, there is a much more noticeable influence on the gas-kinetic temperature. Increases of around 1500 K in the gas temperature throughout the plasma can be found with the addition of 16.7% helium to either the central (Fig. 8) or intermediate (Fig. 9) gas flows. This increase in the heavy-particle temperature is most likely due to the higher thermal conductivity of helium than argon; helium has a thermal conductivity that is greater than that of argon at all temperatures commonly found in the ICP [62].

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Fig. 12. Pictorial representation of possible changes in the droplet desolvation and particle vaporization rates caused by the addition of helium to an argon plasma.

Since the argon atoms are more energetic (higher gas-kinetic temperature) when helium is introduced into the ICP, it would require less additional energy to ionize them. Furthermore, the elevated gas-kinetic temperatures suggest an increased efficiency of energy transport in the discharge, perhaps from the torus to the central channel. The observed increases in the electron number density when the central-gas flow is supplemented with helium (Fig. 4) may be caused by additional argon ionization there. It has been shown [25] that Ar: and ArHe’ show increased signal levels when helium is added to an argon ICP. This postulate also might explain why the electron concentration drops when helium is introduced into the intermediate channel; increased energy-transport efficiency might then raise the fractional heat loss to the atmosphere surrounding the discharge. 4.2.4. Calcium ion emission. Figures 10 and 11 display the effect of helium added to the central- or intermediate-gas flows, respectively, on calcium ion emission (396.8 nm) as a function of height above the load coil (ALC) and radial position. Three trends are evident under both experimental conditions. First, as the amount of helium added to the central channel increases, the overall emission profile becomes more elongated. Second, the peak in maximum emission intensity shifts to lower regions of the plasma. Third, the overall emission intensities increase compared to those obtained from a conventional argon ICP (0% case). As was shown in Figs 4-9, helium has the greatest effect on the gas-kinetic temperature of all three fundamental plasma parameters. In turn, a higher gas-kinetic temperature would be expected to increase the rates of droplet desolvation and particle vaporization [63]. This rate increase would then cause emission to occur sooner in the plasma, leading to an elongation and downward shift in the overall emission profile (see Fig. 12). Furthermore, higher desolvation and vaporization rates would increase the number of analyte species per unit observation volume in the discharge. If the collisional excitation efficiency remained constant, a greater number of analyte species would then be excited and would produce a greater emission intensity. Of course, the observed increase in ion emission might also be due to their collision with “hotter” electrons, shown in Figs 6 and 7 to exist in the lower regions of the ICP. If droplets desolvate and particles

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vaporize sooner in the plasma due to the higher gas-kinetic temperature, a larger fraction of analyte atoms and ions would be produced lower in the plasma as compared to the no-added-gas case. These particular atoms and ions would then interact with the hotter electrons found in the lower regions of the plasma as compared to the higher zones. To determine whether the observed changes in the emission intensity are attributable to an increase in the number of analyte species and/or to an increase in the collisional excitation efficiency will require the measurement of atom and ion number densities using laser-induced fluorescence. These fluorescence measurements will be conducted in our laboratory in the near future.

4.3. Effects of supplementary nitrogen added to the central or intermediate channels 4.3.1. Electron

number density. Figures 13, 14 and 15 show the effect of various percentages of nitrogen added to the central (Figs 13 and 14) or intermediate (Fig. 15) channels of an argon ICP. Figure 14 is similar to Fig. 13 but displayed on an expanded electron-density scale to give the reader a better idea of what the electron concentrations are in the upper portions of the plasma. As has been observed by other researchers [4, 9, 12, 221, the addition of nitrogen to the central-gas flow widens the central channel and causes the height of the plasma to shrink, but does not affect the outer diameter of the ICP. Similar trends are observed for the addition of nitrogen to the intermediate channel (Fig. 15) except, in this case, the plasma diameter is also constricted. This behavior supports the earlier assertion, involving helium, that foreign gases added to the intermediate flow affect even the outer zones of the discharge. In general, nitrogen addition depresses the electron concentration in the central channel and in the upper zones of the plasma. The addition of nitrogen to the centralgas flow (cf. Figs 13 and 14) causes the greatest reduction in the electron density and can most likely be attributed to the much higher heat capacity of nitrogen than argon [64]. Nitrogen, unlike argon, can store energy not only in electronic and translational modes, but also in vibrational and rotational modes. Thus, nitrogen can act as an “energy sink”, absorbing some of the available energy in the central channel and reducing the electron concentration. Although this “energy sink” hypothesis can be used to explain the depressive effect of nitrogen, it would not appear to be able to explain why the electron number density in the toroidal region of the discharge increases with the addition of this gas to the intermediate channel (Fig. 15). Before an attempt is made to resolve this apparent inconsistency, let us examine the effect of nitrogen on the electron temperature. 4.3.2. Electron temperature. The depressive effect of nitrogen on the electron concentration in the upper and central zones of the plasma (Figs 13-15) is reflected in the profiles for electron temperature when the same gas is added to the central-gas flow (Fig. 16). In fact, there is more than a 5000 K drop in the electron temperature of the central channel. This temperature drop is consistent with the higher heat capacity or “energy-sinking” capability of nitrogen. However, unlike its effect on the central-channel electron number density, nitrogen raises the electron temperature in the toroidal regions of the discharge and, when it is added to the intermediate-gas flow (Fig. 17), the central-channel electron temperature is also enhanced. This enhancement does not appear to be consistent with the “energy-sinking” capability of nitrogen. To resolve this apparent anomaly, the plasma volume needs to be taken into account. As stated earlier and shown in Figs 13 and 15, the overall plasma volume becomes smaller when N2 is added. Because the amount of power coupled into the discharge is held constant at 1.25 kW, the plasma power density must then increase (same power/smaller volume = greater power density). In contrast to the addition of a supplementary gas to the outer-gas flow [4], heat loss to the gases surrounding the plasma should not occur when the added gas is introduced as part of the central-gas flow and would be expected to be minimal when the foreign gas is introduced into the intermediate-gas flow. With higher power densities one would also expect higher

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electron temperatures and electron number densities near the energy-addition regions of the discharge, consistent with the observed trends. One further question arises: why is some of the additional energy density in the torus (Figs 16 and 17) not coupled back into the central channel? The answer to this question may lie in the kinetics of energy transport. Although some energy is directly coupled from the load coil into the central channel, most of the incident power is coupled into the toroidal regions of the discharge. Energy must then be transported from these “hot” zones to the central channel by collisions and/or diffusional processes. It is possible that the added nitrogen “sinks” energy at a rate that is much greater than can be transported into the central channel. Of course, when nitrogen is added to the intermediate-gas flow, the molecular gas is injected directly into the high-energy region of the ICP. Although the absorption of energy from the plasma by nitrogen also occurs in this region, it is quickly replenished by direct energy coupling from the load coil. If the above hypothesis is correct, the addition of supplementary gases such as nitrogen may be used as a means of affecting not only how energy is transported in the plasma but also as a means of studying the kinetics of energy transport. 4.3.3. Gas-kinetic temperature. Figures 18 and 19 depict the effects of adding various amounts of nitrogen to the central and intermediate channels, respectively, on the gas-kinetic temperature. The effects of nitrogen addition on the heavy-particle temperature are similar to those described above for the electron temperature. When nitrogen is added to the central-gas flow (Fig. 18), depressions in temperature are observed in the central channel and in the upper regions of the plasma. When nitrogen is incorporated into the intermediate-gas flow (Fig. 19), an overall increase in the gas temperature is seen throughout the discharge. Nitrogen, like helium, has a higher thermal conductivity than argon [62] and would be expected to cause an increase in the heavy particle temperature. The same kinetic effect described above (Section 4.3.2) can be invoked here to explain the observed differences between nitrogen added to the central channel or to the intermediate gas flows. 4.3.4. Calcium ion emission. The depressions in electron number density, electron temperature and gas-kinetic temperature in the ICP central channel that occur when nitrogen is added to the central-gas flow would be expected to cause corresponding drops in analyte emission intensities. Lower gas temperatures would be expected to delay droplet desolvation and particle vaporization, thereby causing an upward shift in the analyte emission profiles. The delayed desolvation and vaporization rates might also cause a reduction in the number of analyte species available for excitation and ionization. Furthermore, lower electron temperatures and number densities could decrease the collisional excitation efficiency leading to an overall depression in the measured emission signals. As Fig. 20 shows, the stated effects may be occurring; in fact, a twenty-fold depression is observed in both the 2.5% and 8.3% added-N, cases compared to the all-Ar plasma (0% N2). However, the addition of nitrogen to the intermediate channel (Fig. 21) produces an effect that is similar to that seen with the addition of helium (Fig. 10 or 11); specifically, a downward shift in the emission profile occurs. The main difference is that the emission intensity maximum is about a factor of two lower when nitrogen rather than helium is added, caused perhaps by offsetting effects with N,: although the electron temperature is higher (Fig. 17), the electron number density in the central channel is lower (Fig. 15). The net effect may be a drop in the collisional excitation efficiency that leads to the observed depression in calcium ion emission. The wider calcium ion emission profile observed with added nitrogen (Fig. 20 or 21) is most likely due to the larger central channel that, in turn, causes analyte to be distributed over a greater plasma volume. The resulting analyte dilution may also contribute to an overall lowering in the maximum calcium ion emission intensity. Again, fluorescence measurements are required in order to better address the mechanisms of the effect of nitrogen on an argon ICP.

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4.4. Effects of supplementary

hydrogen added to the central or intermediate channels 4.4.1. Electron number density. The effects of adding hydrogen to either the central-

or intermediate-gas flow on the electron concentration are shown in Figs 22 and 23, respectively. As previously described, a constriction or thermal pinch effect is observed when molecular gases (N2 earlier) are introduced into an argon plasma [3, 4, 7, 9, 11, 16, 22, 331. The observed increases in the electron number density in the low, offaxis regions of the discharge can be explained as being due to an increase in the localized plasma power density; the same incident power but a smaller plasma leads to a higher power density. In contrast to the observed depression in the concentration of electrons in the central, upper portion of the plasma with hydrogen added to the central-gas flow (Fig. 22), a definite increase is obtained when hydrogen is added to the intermediate flow (Fig. 23). 4.4.2. Electron temperature. With the postulated increase in plasma power density caused by the addition of hydrogen, the electron temperature would also be expected to go up. As Figs 24 and 25 show, a general increase of about 1500 K is observed; Fig. 24 depicts the effect of hydrogen added to the central-gas flow and Fig. 25 to the intermediate-gas flow. 4.4.3. Gas-kinetic temperature. Hydrogen, as is well known, has a much higher thermal conductivity than argon [62, 651. Thus, it is not surprising that the gas temperature is observed to increase when higher percentages of hydrogen are added (Figs 26 and 27). An increase of approximately 2000 K in the gas-kinetic temperature is found throughout the plasma when hydrogen is added to the central-gas flow (Fig. 26) and an even greater rise is found when the intermediate-gas flow (Fig. 27) is supplemented with hydrogen. It appears that hydrogen causes a thermalization effect to occur (the gas-kinetic temperature approaches the electron temperature), especially in the upper parts of the discharge (compare Figs 24 and 26, and Figs 25 and 27). 4.4.4. Calcium ion emission. The recorded increases in gas temperature with added hydrogen (Figs 26 and 27) would be expected to raise the droplet desolvation and particle vaporization rates, thereby leading to a downward shift in the calcium ion emission profiles. As is shown in Fig. 29, a downward shift is indeed observed for all percentages of hydrogen studied, as compared to the “pure” Ar (0% HZ) reference case, when hydrogen is added to the intermediate-gas flow. As was stated previously, the observed enhancement in the maximum calcium ion emission signal may be caused by an increase in the collisional excitation efficiency due to the higher electron temperatures and/or a greater number of analyte species that contribute to the observed emission. When the central-gas flow is supplemented with hydrogen (Fig. 28) similar emission behavior is observed as when hydrogen is added to the intermediate channel (Fig. 29), except for the 16.7% case. Since the gas temperatures in a 16.7% central-gas hydrogen-argon plasma are higher than those with less added hydrogen (cf. Fig. 26), one would expect the peak in the calcium ion emission intensity from the 16.7% plasma to occur lower in the discharge than in, for example, the 8.3% added-hydrogen case. This is not observed; in fact, the peak in emission intensity in the 16.7% addedhydrogen plasma is slightly higher than in the 8.3% case. In addition, the emission intensity is lower. To reconcile this apparent anomaly, we must again turn to an explanation based on what is observed visually. Not only is the plasma volume smaller when a molecular gas such as hydrogen is added, but the plasma also begin to pull away from the torch injector tube (see Fig. 30). In other words, the distance the sample aerosol must travel before interacting with the “hot” plasma increases as the amount of added molecular gas goes up. Although the gas temperature rises as well, the amount of its increase is not sufficient to compensate for the increased residence time of the sample in the “cooler” torch region; that is, the region between the bottom of the plasma and the tip of the injector tube.

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Ar Only

Ar + Molecular Gas

Fig. 30. Pictorial representation of the plasma constriction or thermal pinch effect with an added molecular gas to an argon plasma.

5. CONCLUSIONS

It has been shown that adding helium, nitrogen or hydrogen to a conventional argon inductively coupled plasma causes significant changes in the spatial distributions of electron number density, electron temperature, gas temperature and calcium ion emission. The direction and magnitude of these changes are dependent on the type and amount of foreign gas introduced into the plasma and on whether the added gas was introduced as part of the central-gas or intermediate-gas flows. It appears that the addition of molecular gases and helium to an argon plasma not only affects the plasma excitation and ionization conditions but also the rate at which sample is volatilized in the ICP. These effects can have significant implications for the development of customized ICPs. For example, added helium may be used to control droplet desolvation and particle vaporization rates without significantly affecting the electron number densities or electron temperatures of the plasma. This may be important in the development of ICPs that are free from vaporization-related matrix effects. Another interesting possibility is the minimization or elimination of interelement interference effects. It is known that concomitant elements present in a sample affect the electron number density, electron temperature and heavy-particle temperature of the ICP [67]; similar to what added gases do. By a suitable combination of gases, it may be possible to eliminate interelement matrix effects over a region in the discharge larger than the crossover point. However, more fundamental data on the ICP need to be acquired before such a custom plasma can be created. Acknowledgements-Financial assistance from the National Science Foundation (CHE 90-20631) is gratefully acknowledged. N. N. Sesi thanks the American Chemical Society Analytical Division, and Eli Lilly and Company for fellowship support.

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