Anal. Chem. 1987, 59, 2176-2180
2176
current conduction in the plasma transfers to these species with the rapid decay of radiation from ionic atmospheric species. While the results reported here are preliminary, they clearly show that a relatively modest magnetic field in a theta-pinch configuration can alter significantly the radiative properties of an atmospheric pressure plasma. It appears that the magnetic field alters the plasma by reducing its rate of expansion. The successful use of a single power supply to generate both the plasma and the magnetic field is unique to the system reported here. However, an important drawback to this approach is that near each zero crossing of the plasma current waveform, the magnetic field is too small to affect the plasma, and charged as well as neutral species are more rapidly lost from the system. Walters (25) and Coleman and Walters (26) have described a technique for the generation of unidirectional capacitive discharges. This would eliminate the zero crossings of the current and could improve plasma-magnetic field coupling. Registry No. Graphite, 7182-42-5.
LITERATURE CITED (1) Boumans, P. W. J. M. Theory of SpectrochemicalExcitations:Plenum: New York, 1966. (2) Walters, J. P. Appi. Spectrosc. 1989, 23,317-331. (3) Goode, S . R.; Pipes, D. T. Spectrochim. Acta, Part B 1981, 368, 925-929. (4) Albers, D.:Johnson, E.:Tisack, M.; Sacks, R. Appl. Spectrosc. 1986, 40, 60-70.
(5) Albers, D.;Sacks, R. Specfrochim. Acta, Part B 1986, 4 7 8 , 968.
(6) Albers, D.: Tisack, M.; Sacks, R. Appl. Spectrosc. 1987, 4 7 , 131-141. Albers, D.;Sacks, R. Anal. Chem. 1987, 59, 593-597. Tisack, M.; Sacks, R. Spectrochim. Acta, Parf 8 . in press. Kamla, G. J.; Scheeline, A. Anal. Chem. 1888, 58, 923-932. Kamla, G. J.; Scheeline, A. Anal. Chem. 1986, 5 4 , 932-939. Trivedi, K.; Tanguay, S.; Matties, M.; Sacks, R. Appl. Spechosc., in press. (12) Chen, F. F. Introduction to Plasma Physics; Plenum: New York, 1974. (13) Boyd, T. J. M.; Sanderson, J. J. Plasma Dynamics; Barnes and Noble: (7) (8) (9) (10) (11)
New Yark. 1969.
(14) Ekdahl, C. A.: Commisso, R. J.; McKenna, K. F. J . Appl. Phys. 1981, 52. 3245-3248. (15) Post, R. F. Rev. Mod. Phys. 1956, 28, 338-362. (16) Shohet, J. L. The Plasma State; Academic: New York, 1971. (17) Johnson, E. T.; Sacks, R. D. Anal. Chem., followlng paper in this issue. (18) Suh, S. Y.; Collins, R. J.; Sacks, R. D. Appl. Spechosc. 1981, 35, 42-52. (19) Salmon, S. C.; Holcombe, J. A. Anal. Chem. 1978, 50, 1714-1716. (20) Goldstein, S. A.; Walters, J. P. Spectrochim. Acta, Part6 1976, 318, 201-220, 295-316. (21) Suh, S. Y.; Sacks, R. D. Spectrochim. Acta, Part B 1981, 368, 1081-1 096. (22) Glasstone, S.; Lovberg, R. Controlled Thermonuclear Reactions; Van Nostrand: Princeton, NJ, 1960. (23) Montaser, A.; Goode, S. R.; Crouch, S. R. Anal. Chem. 1974, 46, 599. (24) Montaser, A.; Crouch, S. R. Anal. Chem. 1974, 46. 1817. (25) WaRers, J. P. Anal. Chem. 1972, 4 0 , 1672-1682. (26) Coleman, D. M.; Walters, J. P. Spectrochim. Acta, Parf B 1978, 378, 547.
RECEIVED for review February 25,1987. Accepted May 4,1987. This work was supported by the National Science Foundation through Grant No. CHE 8411290.
Analytical Features of a Graphite Vapor Plasma in a Theta-Pinch Magnetic Field E. T. Johnson' a n d R. D. Sacks* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109
A magnetic field of a few kilogauss parallel to the axis of a high-current Capacitive discharge plasma is used to campress the plasma and improve its performance as an atomization cell and excltatlon source for atomic emlsslon analysis. A solution or powder suspenslon microsample Is applied to a graphke fiber bundle where It Is dispersed by capillary action. A discharge with peak current of 1 kA vaporlzes the fiber bundle and sample and generates a Irlgh-temperature plasma in the vapor. The plasma current also generates the magnetk field in a cdl surrounding the plasma. The flber bundle is doped with a high concentration of Na to Increase charge denstty In the plasma and thus Increase the plasma-magnetlc field interaction. The magnetically altered, Na-doped plasmas exhibit slgnificantty lower contlrmum background Intensity and more linear analytical curves than unaltered, undoped plasmas.
A novel high-current, capacitive discharge plasma device was designed and constructed for the analysis of metallic elements in solution or powder suspension samples. The device uses a low-mass bundle of graphite fibers to atomize 'Present address: AT&T Bell Laboratories, Murray Hill, N J 07066.
the sample and initiate the plasma. A magnetic field parallel to the plasma axis is used to compress the plasma and alter its properties. The magnetic field alters the trajectories of ions and electrons in the plasma and constrains these charged particles to move in circular orbits around the field lines (I, 2). In a cylindrically symmetric plasma with a radial concentration gradient, the magnetic field induces a rotational (theta) current, which couples with the magnetic field to yield a radial compression force in the plasma (3). Since highcurrent capacitive discharge plasmas typically have charged particle densities in the 1016-1018-cm-3range (4,5), they are very susceptible to this type of magnetic field interaction. The plasma device is quite simple and convenient to use since it is designed for atmospheric pressure operation and a single power supply generates both the plasma and the magnetic field. The design of the plasma device and a preliminary spectral and electrical characterization are found in a companion report (6). In the present paper, some qualitative and quantitative analytical features of the plasma are presented. These include sample introduction, background spectral features, the spacial distribution of continuum background and analyte radiation, and the effect of the magnetic field on reproducibility and sensitivity. The use of a relatively high concentration of an easily ionized element to alter the nature of the plasma-magnetic field interaction also is discussed.
0003-2700/87/0359-2176$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987 2177 Table I. Apparatus and Experimental Conditions capacitive discharge circuit and discharge conditions underdamped RCL circuit; 4 X 7.5 pF, 12-kV capacitors in parallel; charging voltage 8.0 kV, inductance 1700 p H (all in magnetic field coil), total discharge energy 960 J, peak current 1.0 kA in first half-cycle, ringing frequency 1.02 kHz; all experiments at atmospheric pressure in 60% Ar-40% O2 coil constructed from 10-gauge solid Cu wire; two 55-turn sections wound in five magnetic field generation layers; each section is 22.3-cm o.d., 18.3-cm id., 4.9 cm wide; 1.8-cm space between sections for viewing plasma; coil oriented coaxial with plasma; peak field strength 5.9 kG at 1.0 kA Union Carbide type WCB graphite tape dissected into lengthwise strands; each graphite fiber bundle strand has 700 10-pm diameter filaments; 4.5-cm length vaporized for each shot; mass vaporized 6.75 mg Jarrell-Ash Model 78-462, 1-m, f/8.6 Czerny-Turner with 1200 line/mm grating, spectrometer blazed for 300 nm in first order; exit and entrance slit widths 100 pm 1P28 photomultiplier tube biased at 700 V; photocurrent measured with current photoelectric detection system follower using 1.0-kQ load; radiation waveforms displayed on Nicolet Model 2090 digital storage oscilloscope with 2-MHz sampling rate and floppy-disk memory, channel A used for discharge current monitoring via Pearson Model 1025 wide-band current transformer and channel B used for radiation monitoring Evans Associates Model 4130 gated integrator board with current follower input analog integrator and 555 timer network for automatic reset and delayed gated integration; output displayed on DVM or digital oscilloscope
1 7T H T H - 1 -7-7- I --Trigger
I
Discharge circuit
Integrator
I I
Spectrometer
I
i
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Flgure 1. Block diagram of the plasma generation and radlation measurement system.
EXPERIMENTAL SECTION The experimental apparatus is described in detail in a companion report (6). Figure 1shows a block diagram of the system. Experimental conditions are sulIllIlllllzed inTableI. The plasma is generated by the capacitive-dischargevaporization of a low-mass bundle of very thin graphite filaments. The capacitive discharge circuit and associated trigger apparatus are described in detail in ref 7. The magnetic field that compresses the plasma is generated by the plasma current in a large coil surrounding the plasma. The plasma current is monitordby a Rogowski coil and a digital storage oscilloscope. The output from the Rogowski coil also triggers a high-speed analog integrator, which integrates the photocurrent from the photomultiplier tube. The analog integrator has been described (8). Spacially resolved photographic spectra were obtained on Kodak SA 1 plates. An astigmatic image transfer system (9) consisting of two 50-mm diameter, 310-mm focal length spherical mirrors was used to obtain spacial resolution in the vertical direction (normal to the plasma axis) on the focal plane of the spectrometer. The system used a lateral magnification of 0.25, and spacial resolution in the plasma was better than 0.5 mm. Optical density scans were obtained on a Joyce-hebl Model Mark IIIB recording microdensitometer. Stock solutions (1000 ppm) were prepared from reagent grade salts. All standard solutions contained 1.0% Triton X-100 (Rohm and Haas), a nonionic surfactant that was added to improve sample wetting and promote capillary dispersion into the fiber bundle. A 5-pL aliquot of standard solution was transferred to the fiber bundle with a micropipet. After sample application, the fiber bundle was air-dried under an incandescent lamp for about 3 min. Solid powder samples were separated by particle size on an ATM Sonic Sifter, and particle size fractions of
