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obtained by the indiscriminate use of Raman scattering cross sections from the literature. The technique used in this work also provided for the f i t time insight into the detection limits of LRM spectroscopy, which is an important piece of information in the interpretation of analyses on natural fluid inclusions. Although the reported quantification factors and detection limits apply only to our specific instrument and analytical conditions, the method of calibration can be applied to any LRM. Our calibration approach solves two problems that have to be faced by every fluid inclusion researcher using quantitative LRM spectroscopy. ACKNOWLEDGMENT We appreciate reviews by Mark Andersen and Paul Dhamelincourt on a previous version of the manuscript, although we retain full responsibility for all information presented. This work was supported in part by NSF Grant EAR 8408004. Registry No. SO2, 7446-09-5; COP,124-38-9; 02,7782-44-7; CO, 630-08-0; H,S, 7783-06-4; CH4, 74-82-8; H,, 1333-74-0; Nz, 7727-37-9. LITERATURE CITED (1) Dhamelincourt, P.; Beny, J. M.; Dubessy, J.; Poty, 9. Bull. Mineral. 1979, 702, 600. (2) Dubessy. J.; Geisler, D.; Kosztolanyi, C.; Vernet, M. Geochim. Cosmochim. Acta 1983, 47, 1. (3) Ramboz, C.; Schnapper, D.; Dubessy, J. Geochim. Cosmochim. Acta 1985, 4 9 , 205 (4) Touray, J. C.; Beny-Bassez, C.; Dubessy, J.; Guilhaumou, N. Scanning Necfron Microsc. 1985, 103. ( 5 ) Higgins, K. L.; Stein, C. L. Microbeam Anal. 1988, 31. (6) Roedder, E. Fluid Inclusions ; Mineralogical Society of America: Washington, DC, 1984.
(7) Shepherd, T.; Rankin, A. H.; Alderton, D. H. M. FluM Inclusions Studies; Blackie: London, 1985. (8) Kotra, R. K.; Gibson, E. K. €OS 1982, 6 3 , 450. (9) Sommer, M. A.; Yanover, R. N.; Bourcier, W. L.: Gibson, E. K. Anal. Chem. 1985, 5 7 , 449. (10) Barker, C.; Smith, M. P. Anal. Chem. 1986, 5 8 , 1330. (1 1) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1978. (12) Piaczek, G. I n Handbuch der Radioiogie; Marx, E., Ed.; Akademische Verlagsgesellschaft: Leipzig, Germany, 1934. (13) Fenner, W. R.; Hyatt, H. A.: Keliam, J. M.; Potto, S. P. S.J . Opt. SOC. Am. 1973, 6 3 , 73. (14) Schrotter, H. W.; Klockner, tl. W. I n Raman Spectroscopy of Gases and Liquids; Weber, A., Ed.; Springer-Verlag: New York, 1979. (15) Bernstein, H. J. I n Vibrational Intensifies in Infrared and Raman Spectroscopy; Person, W. B., Zerbi, G., Eds.; Elsevier: New York, 1982. (16) Cheliletz, A.; Dubessy, J.; Kosztolanyi, C.; Masson-Perez, N.; Ramboz, C.; Zimmermann, JAL. Bull. Mlneral. 1984, 107, 1969. (17) Dubessy, J.; Guilhaumou, N.; Mullis, J.; Pagel, M. Bull. Mineral. 1984, 107, 189. (18) Wopenka, 9.; Pasteris, J. D. Appl. Specfrosc. 1988, 4 0 , 144. (19) Malyj, M.; Griffiths, J. E. Appl. Spectrosc. 1988, 40, 52. (20) Pasteris J. D.; Wopenka, 9.; Seitz, J. C., submitted for publication in Geochim . Cosmochim . Acta. (21) Turrell, G. J. Raman Specfrosc. 1984, 15, 103. (22) Bremard, C.; Dhamelincourt, P.; Laureyns. J.; Turrell, G. Appi. Specfrosc. 1985, 3 9 , 1036. (23) Andersen, M. E.; Muggli, R. 2. Anal. Chem. 1981, 53, 1772. (24) Hopkins, J. B.; Farrow, L. A. J. Appi. Phys. 1988, 59, 1103. (25) Wang, C. H.; Wright, R. 9. Chem. Phys. Len. 1973, 2 3 , 241. (26) Malyj, M.; Griffiths, J. E. Appl. Specfrosc. 1983, 3 7 , 315. (27) Hill, R. A.; Mulac, A. J.; Smith, D. R. Appl. Specfrosc. 1976, 3 0 , 183. (28) Montero, S.; Bermejo. D.; Lopez, M. A. Appl. Spectrosc. 1978, 3 0 , 628. (29) Pasteris, J. D.; Seitz, J. C.; Wopenka, B. Microbeam Anal. 1985, 25. (30) Seitz, J. C.; Pasteris, J. D.; Wopenka, 9. Geochim. Cosmochim. Acta, in press.
RECEIVED for review February 9,1987. Accepted May 11,1987.
Preliminary Characterization of a Graphite Vapor Plasma in a Theta-Pinch Magnetic Field E. T. Johnson' and R. D. Sacks*
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109
The plasma produced by the capacitive discharge vaporization of a graphite fiber bundle is compressed by an external magnetk Held. The plasma is formed by the discharge of an underdamped RCL tank circuit. The plasma current also is used to generate a magnetlc field in the kilogauss range In a large coil surrounding the plasma. The plasma has cylindrical symmetry, and the coaxial magnetic field resuits In a diamagnetic fluid drift of charged particles. The resulting azimuthal drift current couples wlth the magnetlc field to produce a radial Lorentz force, which compresses the plasma. While the presence of the magnetlc field has relatively little effect on the electrical properties of the plasma, radiative properties are significantly altered. Design features of the plasma generath devke along wtth preHminary electrical and spectral characterlzatlon are presented.
High-temperature plasma devices are finding widespread application in analytical chemistry as sample cells for the Present address: AT&T Bell Laboratories, Murray Hill, NJ 07066.
generation of atomic vapor, as radiation sources for atomic spectroscopy, and as ion sources for mass spectrometry. The control of plasma properties including sampling characteristics, particle number densities, electron excitation temperature, and ionization temperature is of great importance in the effective utilization of these devices for analytical applications. The simple control of plasma power often is relatively ineffective in altering these properties. Internal processes in the plasma may regulate these properties by altering the plasma volume so as to keep power density relatively constant ( I , 2 ) . Several recent studies have shown that external magnetic fields can be very effective in altering plasma properties for microwave-induced plasmas ( 3 ) ,high-current capacitive discharge plasmas (4-8), and glow-discharge plasmas with currents in the millampere range (9-11). The presence of the magnetic field causes ions and electrons to precess in circular paths in the plane normal to the field direction. If the plasma contains a macroscopic electric field with a component normal to the external magnetic field, charged particles undergo a drift motion in the direction normal to the plane containing the electric and magnetic field vectors (12, 13). This E X B drift motion has been used to alter the properties of highcurrent capacitive discharge plasmas ( 4 , 5 )and improve their analytical performance (6, 7). Recently, this type of plasma
0003-2700/87/0359-2170$01.50/0 1987 American Chemical Society
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Table I. Coil Properties and Discharge Conditions coil construction material design
coil 1 12-gauge solid copper
coil 2 10-gaugesolid copper
two sections, 25 turns in each section two sections, 55 turns in each section wound in five layers wound in five layers each section: 22.7 o.d., 18.7 id., 2.2 wide, each section: 22.3 o.d., 18.3 i.d., 4.9 wide, 1.8 between sections for viewing 1.8 between sections for viewing
dimensions, cm
plasma peak field strength in first discharge half-cycle, kG 1.1 discharge conditions charging voltage, kV inductance, pH capacitance, pF energy, J peak current, kA ringing frequency, kHz support gas pressure
drift motion also has been used to alter the properties of arc and glow discharge plasmas (11). If the plasma contains a significant charge density gradient in the direction normal to the external magnetic field, a different type of drift motion occurs in the direction normal to the plane containing the density gradient vector and the magnetic field vector. This is not a particle drift motion but rather a fluid drift motion, which generates a magnetic field in the plasma. This induced magnetic field opposes the externally applied magnetic field, and thus this drift motion is called a diamagnetic drift. If the plasma has cylindrical symmetry, and if the external magnetic field is coaxial with the plasma axis, the diamagnetic drift motion is in the form of closed loops around the plasma axis, and an azimuthal current J is generated in the plasma. This current couples with the axial magnetic field to generate a radial Lorentz force ( J X B ) , which compresses the plasma (14-16). Because of the azimuthal or theta direction of the drift current, this often is referred to as a theta-pinch configuration. Goode and Pipes (3) used a theta-pinch to compress a microwave plasma in a quartz confinement tube. The axial magnetic field was generated by discharging a capacitor through a single turn coil, which was coaxial with the confinement tube. Recently, Kamla and Scheeline (9, 10) developed a theta-pinch plasma device designed specifically for sampling refractory solid materials. Their system uses a low-pressure preionizing plasma discharge in a confinement tube. The plasma is pinched by discharging a capacitor through a multiturn helical coil surrounding the plasma. The combination of irreversible shock wave heating and adiabatic compression results in high plasma temperature. This report describes a novel theta-pinch plasma device, which is designed as a high-temperature atomization cell and excitation source for solution residue and solid powder suspension microsamples. This plasma device operates a t atmospheric pressure and is relatively simple and convenient to use. Design features as well as preliminary electrical and spectroscopic chracterization of the device are presented here. Analytical characteristics are presented in a companion report (17). EXPERIMENTAL S E C T I O N Experiment Design. The plasma is generated by the highvoltage capacitive discharge heating of a low-mass graphite fiber bundle. The bundle consists of several hundred individual filaments. The bundle may or may not vaporize completely from the discharge depending on the experimental conditions. After a period of ohmic heating of the graphite by the discharge current, dielectric breakdown of the surrounding gas results in the formation of a highly luminous plasma. A solution or powder mi-
plasma
5.9
coil 1 4.0
800 30 240 0.74
coil 2 8.0
1700 30 960
1.0 1.02 0.70 air, N2,Ar(60%)/02(40%) atmospheric
crosample applied to the fiber bundle is atomized thermally from the hot graphite as well as by surface ablation from the hot plasma gases. An oscillating magnetic field with a peak value of a few kilogauss parallel to the fiber bundle axis is used to alter the plasma properties. The magnetically confied plasma system described here differs in several ways from the typical theta-pinch system. First, the system is operated at atmospheric pressure, and some of the plasma heating processes that are present in a low-pressure magnetic pinch may be inoperative at atmospheric pressure. Second, no mechanical confinement tube is present, and the plasma can expand without limit. The magnetic field may not reduce the plasma diameter but rather reduce its rate of expansion. Third, the current used to generate the magnetic field in a coil surrounding the plasma is also used to generate the plasma. Thus, the plasma current is larger than those in typical theta-pinch systems. Since the high-current plasma exists with or without the magnetic field, a meaningful comparison of plasma properties with and without the field is possible. Finally, a multiturn high inductance magnet coil is used, and no attempt is made at obtaining the large dB/dt values usually associated with theta-pinch experiments. Discharge Circuit and Magnet Coils. Figure 1 shows a simplified diagram of the discharge circuit. The graphite fiber bundle F and magnet coil L, are in series with a 30-pF capacitor C, which is initially charged to several kilovolts from a high-voltage power supply, HV. The capacitor charging voltage is monitored with voltage divider R,-& and a DVM. The discharge is initiated on command by closing switch SI, which is either a welding ignitron or a gravity-operatedspark gap. These switches and their associated trigger circuits are described in ref 18. Switch S2, which is also a gravity-operated spark gap, is used in conjunction with R,to discharge any residual capacitor voltage after each experiment. The plasma current is monitored at point I by a Pearson Electronics Model 1025 wide-band current transformer. The plasma voltage drop is monitored at point V by a Tektronix Model P6015 capacitively compensated high-voltage probe. Two theta-pinch coils were constructed for these studies. Coil properties and discharge conditionsare described in Table I. Both coils were constructed in two sections with a gap of about 1.8 cm between the sections for viewing the plasma. Plywood spacers between the sections were used to prevent axial movement during the discharge. The two sections were connected in series and wound in the same direction. The coils were formed into ridged structures that could be removed easily from the plasma chamber. However, the coils were always in the discharge circuit. This allowed the direct comparison of the plasma properties with and without the theta pinch magnetic field under conditions of nearly identical circuit inductance. Additional details of the coil design are found in ref 4. Plasma Chamber and Fiber Bundle Cell. Experiments were conducted in an 18-cm4.d. cylindrical nylon chamber (see Figure 2). The chamber is oriented with its axis in the horizontal plane and normal to the optical axis. The fiber bundle axis is oriented
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Figure 3. Plasma cell: F, graphite fiber bundle; B, polycarbonate base; E, Graphite electrodes: C, copper clamp; S, polycarbonate sparkover
V
Figure 1. Simplified drawing of the capacitive discharge circuit used to generate the plasma and the magnetic field. The magnetic field is generated by a large coil surrounding the fiber bundle and coaxial with it. HV, high-voltage power supply; C, 30 HF discharge capacitor; F, graphite fiber bundle; L, theta-pinch coil; SI, main discharge switch; S,, gravity-operated spark gap-switch for removing residual charge; R,, current limiting resistor; Rl-R2, voltage divider; DVM, digital voltmeter; I, discharge current monitor probe: V, plasma voltage probe.
-r N
P
1
t
I
Figure 2. Plasma chamber and cantilever mount: N, nylon chamber; P, thetapinch coil; S, coil spacers to prevent deformation; W, quartz window; B, graphite fiber bundle; F, electrical feedthroughs; G, gas line connections; E, end plate for accessing chamber interior; C, cantilever mount; 0, optical bar.
along the chamber axis. Radiation is viewed through a quartz window in the cylindrical chamber wall. One end of the chamber is connected to the optical bar by a cantilever mount. The theta-pinch coil freely slides over the other end of the chamber. This end is sealed with a removable cap for accessing the interior. The fixed chamber end plate is fitted with high-voltage electrical feedthroughs and gas-line connections. The graphite fiber bundle is supported in the cell shown in Figure 3. The portions of the cell that are exposed to the plasma are made from polycarbonate plastic. The fiber bundle is anchored at each end between a pair of graphite blocks, which provide electrical contact. These blocks are held in copper clamps, which are supported on the electrical feedthroughs in the chamber end plate. The cell contains a pair of polycarbonate shields, which prevent sparkover to other parts of the cell and improve the positional stability of the plasma. Each shield contains a 3.0mm-diameter hole through which the fiber bundle passes. These holes enlarge slightly with use through the ablative action of the plasma. The shields are washed after every experiment and are replaced after about 100 experiments. The length of the fiber bundle between the faces of the cell is 4.5 cm. Optical and Electrical Monitoring. All spectra were obtained with 1.0-m or 0.75-m Czerny-Turner spectrometers
shields. Table 11. Properties of the Graphite Fiber Bundles material length, cm number of fibers fiber diameter, pm bundle mass/cm, mg resistance, s2
Union Carbide Type WCB graphite tape 4.5 between plastic shields 700 (typical) 10
1.5 21 (typical)
equipped with 1200 line/mm gratings. Reciprocal linear dispersions are 0.81 and 1.1nm/mm, respectively. Both instruments were used in the first order. Photographic spectra were obtained on Kodak SA 1 emulsion, which was processed according to manufacturer recommendations. Photographic spectra with spacial resolution normal to the plasma axis were obtained by using an astigmatic imaging system (19) consisting of a pair of 50-mm-diameter, 310-mm-focal-length 0 spherical mirrors. The mirrors were used in an over-and-under configuration to correct for the astigmatism of the side-by-side mirror configuration in the spectrometers (20). Since the radial dimension of the plasma is significantly greater than the 20 mm length of the spectrometer entrance slit, a magnification of 0.25 was used with the image transfer system. Spacial resolution in the plasma was better than 0.5 mm. Some spectral line distortion from coma was observed near the ends of the photographic plate. All microdensitometer measurements were made in the central one-third of the plate where no line distortion was apparent. A few time-gated, spacially integrated photographic spectra were obtained by using the apparatus described in ref 8 and 21. These spectra were recorded on Kodak Royal X Pan emulsion developed to an ASA speed of 2400. Optical density traces from photographic spectra were obtained on a Joyce-Loebl Mark IIIB recording microdensitometer. Photoelectric spectra were obtained with a 1P28 photomultiplier tube. Bias voltage was adjusted as needed. Spacially resolved, photoelectric radiation measurements were made by using the astigmatic imaging system to form an image of the plasma on a mask placed in front of the spectrometer entrance slit. The optical system used a lateral magnification of 0.33 and yielded spacial resolution along a vertical axis in the plane of the mask of about 0.1 mm. The mask was 1.0 mm long in the vertical direction. This limited the height of each observation zone in the plasma to about 3.0 mm. With this optical system, some integration occurs in the horizontal direction (parallel to the plasma axis). This was useful in improving shot-to-shot reproducibility without the loss of information regarding magnetic-field-induced changes in the plasma. Photoelectric radiation waveforms were recorded on a Nicolet Model 2090-111 digital storage oscilloscope equipped with a floppy-disk memory. All photoelectric radiation waveforms were background corrected by subtracting the waveform from a bare fiber bundle. Plasma current and voltage waveforms also were recorded on the digital oscilloscope. Magnetic field strength was measured with a probe coil and a passive RC integrator as described by Glasstone and Lovberg (22). Materials and Reagents. The graphite fiber bundles were obtained from lengthwise strands of 6-cm-long sections of woven graphite tape (Union Carbide type WCB). Each section of tape yielded 15 fiber bundles. Properties of the fiber bundles are given in Table 11. Montaser et al. (23,24) used graphite braids with
ANAL.YTICAL CHEMISTRY, VOL. 59. NO. 17. SEPTEMBER 1. 1987
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n
4 f
L,
I
0
0.2
0.4
0.6
0.8
Time,mr
Figure 5. ElecMcal waveforms fw EkV. 9604 discharges showing plasma current (a) and voltage drop (b) with theta-pinch magnetic fleld ( d d line) and withwt tha field (broken line).
(b)
Flpun 4. Ewlron microscope photographs of a typical fiber bundle lOOOX (b). Properiles of the fiber bundle are found
at BOX (a) and In Table 11.
similar properties as resistively heated furnaces for atomic absorption and atomic fluorescence applications. Figure 4 shows photographs obtained on a acanning electron mieroseope of a fiber bundle. The small particles observed on the individual filaments appear to be carbon and are not removed hy ultrasonic cleaning. All samples applied to the fiber bundles were aqueous solutions of reagent grade salts. Experiments were conducted in dry air, N,, He, and a 40/60 O,-Ar gas mixture. Moat studies were done at atmospheric pressure with a gas flow rate of about 2 L/min. RESULTS AND DISCUSSION Electrical Characteristics. The RCL discharge circuit is underdamped, and an exponentially damped sinusoidal discharge current is obtained. For the 8-k". 1700-pH discharge (see Table I), the plasma persists for about 6.5 ms with the 5.9-kG peak magnetic field and for about 8.0 ms without the field. Note that the 1700-pH inductance is in the circuit in both cases. Following an initial high-voltage transient a t the start of the discharge, the voltage drop across the plasma is fairly constant at about 500 V. The voltage drop is slightly larger with the field present. The voltage polarity reverses abruptly a t each zero crossing of the discharge current waveform, and thus the voltage waveform appears as a distorted square wave. The exponential damping constant R / 2 L was measured from the current waveform both with and without the magnetic field. The values of 376 and 309 s-', respectively, were obtained for the 8 k V discharges. This gives plasma resistance values of 1.28 and 1.05 R, respectively. The higher resistance with the field probably is the result of comoression of the plasma by the field-induced Lorentz force.
Figure 5 shows the plasma current and voltage waveforms during the f i t half-cycle of the 8 k V discharge. The presence of the magnetic field has relatively little effect on the current waveform. The peak current occurs about 350 w after the start of the discharge and has a value of about 1.0 kA. The current waveform shows a kink or discontinuity about 125 w after the start of the discharge. The voltage waveform is somewhat different with (solid line) and without (broken line) the magnetic field. The voltage drop increases rapidly and reaches a peak value of about 5.0 kV at about the time of the kink in the current waveform. The voltage then drops quite abruptly to a value of about 500 V with the field and about 400 V without the field. The time of the voltage peak and the current kink is quite reproducible from shot to shot, but i t does decrease with increasing capacitor voltage and with decreasing pressure. Qualitatively similar results are obtained for the &kV, 800-pH discharges. However, the first half-cycle peak current is reduced to about 0.74 kA. General Radiative Features. Radiation from the plasma is extremely intense, and continuum background radiation is detected on the SA 1 emulsion a t all wavelengths in the range 250-420 nm from a single 4-kV discharge. For experiments conducted in air, the CN hand system is very intense, and the band head at 388.3 nm is the most prominent feature in the wavelength region investigated. A few experiments were conducted in N2 and in He, but the shot-to-shot reproducibility was very poor. The use of a 60/40 A r l o Zgas mixture resulted in relatively good reproducibility and significantly reduced CN intensity. However, the CN band system remained an important spectral feature. Significant amounts of adsorbed N, or nitrogen-containing compounds may be present in the graphite material or traces of N, may be present in the gas. Acid washing, ultrasonic cleaning in isopropyl alcohol, and strong heating of the fiher bundle in a Bunsen h e had no effect on the CN emission. Heating the graphite fiher bundle to incandescence with a direct current power supply also had no effect. All subsequent data were obtained in the Ar-0, mixture. When experiments are conducted in an oxygen-containing atmosphere, the graphite fiber bundle always is destroyed by the discharge. In an oxygen-free atmosphere, the fiber bundle may or may not survive the discharge depending on the pressure and initial capacitor voltage. At pressures below 100 torr. 4-kV discharges often result in no apparent deterioration
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 17. SEPTEMBER 1. 1987
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$ 0 D i s t a n c e F r o m A x i s , mm
Flguo 6. Microdensitometer traces of continuum background radiatkn from thneintegrated, spacially resolved spectra wlth the magnetic Reld (solid lines) and without the field (broken lines). Scans were made normal to the wavelength axis in a linafree region near 395 nm. (a) 4 k V , 2404 discharges; (b) 8-kV, 9604 discharges.
to the fiber bundle, while 8-kV discharges usually result in the breaking of some of the individual f i i e n t s . The spectra from these low-pressure discharges show relatively little radiation from species in the fiber bundle and thus are less useful for analytical applications. All subsequent studies used atmospheric pressure discharges. The effect of the magnetic field on plasma structure is illustrated by the microdensitometer traces in Figure 6. These traces were obtained from time-integrated, spacially resolved photographic spectra by scanning normal to the wavelength axis in a line-free region near 395 nm. The solid-line traces were obtained with the magnetic field present, and the broken-line traces were obtained without the magnetic field. The top pair of traces was obtained from 4 k V , 240-5 discharges, while the bottom pair was obtained from 8-kV, 960-J discharges. The flat portion of the trace from the 960-J discharge without the field results from the optical density of the emulsion exceeding the dynamic range of the microdensitometer. It is assumed that optical density values are even greater in this region than the value indicated in the figure. For both discharge conditions, the region of intense continuum background radiation is significantly narrower in the magnetically pinched plasma. Without the magnetic field present, the region of intense continuum is considerably wider for the 960-J discharges than for the 240-5 discharges. This difference is smaller with the field present. Figure 7 shows spacially resolved photomultiplier traces of continuum background radiation from the graphite fiber bundle plasma. Again, the region near 395 nm was used. These traces were obtained from 8-kV discharges. All traces are averages from four consecutive experiments. The solid-line traces were obtained with the magnetic field, and the broken-line traces were obtained with no magnetic field. Note that the theta-pinch coil was in the discharge circuit in all cases to maintain a nearly constant circuit inductance. The trace marked d is the discharge current waveform without the field present and serves as a time reference. The current waveform is quite similar with the field present except it terminates about three half-cycles earlier or about 6 me after the start of the discharge. Traces in part c were obtained for an observation zone centered along the plasma axis. Traces in part
WI
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1
2
3
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Flgure 7. Spaciaity resolved radiation Intensity vs. time traces of continuum background wlth (solld lines) and without (broken lines) the magnetic field. Each trace is the average of four shots with &kV discharges. (a) 18 mm from plasma axis; (b) 9 mm from plasma axis; (c) at plasma axis; (d) discharge current waveform without magnetic
field. b were obtained 9 mm from the axis, and traces in part a were obtained 18 mm from the axis. Continuum radiation near the plasma axis both with and without the magnetic field shows intensity oscillations, which correspond to the oscillations in the discharge current amplitude. Continuum intensity is greater without the magnetic field over most of the duration of the discharge. Peak continuum intensity is reached early in the discharge, during the fist or second current half-cycle. The intensity then decreases with decreasing discharge current and is just barely detected by the end of the discharge. The continuum intensity waveform is much different at a distance of 9 mm from the plasma axis. While oscillations of intensity with the discharge current are still observed, they are less dramatic and more erratic. The effect of the magnetic field is more apparent at 9 mm than along the plasma axis, and the time-integrated intensity is about a factor of 5 greater without the field. With the magnetic field, the continuum intensity exhibits a more pronounced oscillatory behavior than that without the field for the first 4 ms of the discharge and then falls to a very low value and remains relatively constant for the remainder of the discharge. Without the field, the continuum radiation shows a broad secondary intensity maximum around 4 ms after the start of the discharge and then decays slowly for the remainder of the discharge. During the interval from 4 to 7 ms after the start of the discharge, the continuum intensity is about a factor of 10 greater without the magnetic field. At 18 mm from the plasma axis, the continuum intensity is again much greater without the magnetic field. With the field, the peak intensity occurs during the second discharge half-cycle. While some oscillations are observed for the first 4 ms, the predominant trend is a gradual decrease in intensity for the entire discharge. Without the field, the continuum intensity gradually increases and does not reach a peak value until about 7 ms after the start of the discharge. The intensity than decays slowly, and considerable background radiation is observed well after the cessation of the discharge current. Figure 8 shows similar spacially resolved photoelectric traces for the Cr neutral-atom line a t 425.4 nm obtained by using the same discharge conditions as those given in Figure 7 . Each
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Figure 8. Spacially resolved radiation intensity vs. time traces of the Cr 425.4-nm neutral-atom line with (solid lines) and without (broken
lines)the magnetic field. Each trace is the average of four shots with 8 k V discharges. Each fiber bundle contained 4.0 pg of Cr as Na,Cr04. (a) 18 mm from plasma axis: (b) 9 mm from phsma axis; (c)at plasma
axis. graphite fiber bundle was treated with 4.0 pg of Cr as an aqueous solution of Na2Cr04. All traces were corrected for continuum background by subtracting corresponding traces obtained at the same wavelength but from bare graphite fiber bundles. Again, the solid-line traces were obtained with the magnetic field, and the broken-line traces were obtained without the magnetic field. Unlike the continuum background, the Cr neutral-atom radiation along the plasma axis (c) is significantly more intense with the magnetic field present. This is particularly apparent during the first 2 ms of the discharge. While some intensity oscillations are observed, they are more erratic than for the continuum background. After about 3 ms, the intensity is quite low, and there is relatively little intensity difference with and without the magnetic field. At 9 mm from the plasma axis, the radiation vs. time profiles are more similar with and without the field, but during the first 3 ms, the intensity is still somewhat greater with the field present. Later in time, the intensity decays somewhat more slowly than along the plasma axis. At 18 mm from the plasma axis, the situation is quite different, and the Cr neutral-atom radiation intensity is much greater in the absence of the magnetic field. In addition, radiation persists at moderate intensity for a considerably longer time after the start of the discharge. Qualitatively similar results were obtained for Cr ion radiation. The intensity profiles, however, show more pronounced oscillations with the discharge current, and the radiation intensity at 18 mm from the plasma axis is considerably lower than that at 9 mm from the axis. Plasma Formation Processes. Prior to the kink in the discharge current waveform (see Figure 5), no line radiation is detected, and presumably, the discharge current is conducted entirely by the solid graphite fibers. After the kink in the current waveform, the discharge voltage and resistance abruptly drop, and line radiation from the plasma is observed. This appears to mark the onset of dielectric breakdown and plasma formation.
Flgure 9. Microdensitometer traces from spacially integrated, timegated photographic spectra of 8-kV, 9604 discharges: (a) 5 ps after the kink in the current waveform; (b) 20 ps after the kink in the current waveform; (c) 40 ps after the kink in the current waveform. Line assignments: A, C 11, 426.7 nm; B, Ar 11, 433.1 nm; C, Ar 11, 434.5 nm; D, Mn I, 423.5 nm; E, Mn I, 436.5 nm; F, Mn I, 437.1 nm; G, Mn I, 438.2 nm.
Figure 9 shows microdensitometer traces from time-gated spectra obtained at three different time gates following the kink in the current waveform. In all cases, the time-gate width was about 2.5 ps and 8-kV, 1700-pH discharges were used in an Ar-02 atmosphere. The magnetic field was not present. Each graphite fiber bundle was treated with 4.0 pg of Mn as an aqueous solution of MnS04. Spectrum a was obtained 5 ps after the kink in the discharge current waveform. This is about the earliest time that radiation intensity is large enough to obtain a single-shot spectrum. The only spectral features observed are a number of broad, low-intensity ion lines of C and Ar. Spectrum b was obtained about 20 ps after the kink in the current waveform. This is about the time of maximum intensity from atmospheric species (C, labeled A; Ar, labeled B and C). Also, this is the earliest time at which lines from the Mn sample are observed. Spectrum c was obtained 20 ps after spectrum b. The Mn neutral-atom lines are the dominant spectral features, and the C and Ar ion lines have become much less intense. The lines marked D-G are from Mn neutral-atom species. By 50 ps after the kink in the current waveform, the C and Ar ion lines are below detectability. They do not appear again. The presence of the magnetic field results in a significant decrease in the intensities of the C and Ar ion lines, but no qualitative changes in the spectra are observed. The trends observed in Figure 9 are similar to those reported in ref 8 for the plasmas produced by the capacitive discharge vaporization of thin Ag films. Plasma formation probably occurs via the dielectric breakdown of the surrounding gas and traces of graphite vapor. The hot graphite should release copious quantities of thermionic electrons. These electrons are accelerated by the large electric field (about lo3V/cm) that is present just prior to dielectric breakdown. The “hot” electrons in the high-energy tail of the kinetic-energy distribution are probably responsible for collisional ionization of the gas. Continued vaporization of the graphite and the Mn sample then occurs from ablation by the hot plasma gases. As impurities of lower ionization potential and sample vaporize,
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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. (7) Albers, D.;Sacks, R. Anal. Chem. 1987, 59, 593-597. (8) Tisack, M.; Sacks, R. Spectrochim. Acta, Parf 8 . in press. (9) Kamla, G. J.; Scheeline, A. Anal. Chem. 1888, 58, 923-932. (10) Kamla, G. J.; Scheeline, A. Anal. Chem. 1986, 5 4 , 932-939. (11) 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: 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
