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(4) Sacks, R. D.; Goldberg, J. M.; Collins, R. J.; Suh, S. Y. Frog. Anal. At. Specfrosc. 1982, 5 (2-3), 111-154. (5) Goldberg, J. M.; Sacks, R. D. Anal. Chem. 1982, 54, 2179-2186. (6) Shohet, J. L. “The Plasma State”; Academic Press: New York, 1971. (7) Post, R. F. Rev. Mod. fhys. 1958. 2 8 , 338-362. (8) McKenna, K. F.; Bartsch. R. R.; Commisso, R. J.; Ekdahl, C. A.; Qulnn, W. E.; Siemon, R. E. fhys. Flu& 1980, 2 3 , 1443-1462. (9) Grlem, H. R.; Kolb, A. C.; Lupton, W. H.; Phllllps, D. T. Nucl. Fuslon 1982, (part 2), 543-551. (10) Boyer, K.; Elmore, W. C.; Little, E. M.; Quinn, W. E.; Tuck, J. L. fhys, Rev. 1980, 119, 831-843. (11) Ekdahl, C. A.; Commlsso, R. J.; McKenna, K. F. J . Appl. fhys. 1981, 5 2 , 3245-3248. (12) Kamla, G. J.; Scheeline, A. Anal. Chem., following paper in this Issue. (13) Wright, J. K. “Shock Tubes”: Wiley: New York, 1961. (14) Bond, J. W. fhys. Rev. 1957, 705, 1683-1694. (15) Kolb, A. C. fhys. Rev. 1957, 107, 345-350. (16) Kolb, A. C. Rev. Mod. fhys. 1980, 3 2 , 748-757. (17) Oberauskas, J.; Serapinas, P.; Salkauskas, J.; Svedas, V. Spectrochim. Acta, Part B 1981, 3 8 8 , 799-807. (18) Scheellne, A.; Kamla, G. J.; Zoellner, M. J. Spectrochim. Acta, Part B 1984, 3 9 8 , 677-691. (19) McKenna, K. F.; York. T. M. fhys. Flujds 1977, 2 0 , 1556-1565. (20) Commisso, R. J.; Grlem, H. R. fhys. Flulds 1977, 20, 44-50. (21) Panarelia, E. Can. J . fhys. 1980, 5 8 , 983-999. (22) Bodin, H. A.; Green, T. S.; Newton, A. A.; Niblett, G. B. F.; Reynolds, J. A. Plasma fhys . Controlled Nucl. Fusion Res. 1968, No. 1 , 192. (23) Jordan, H. L. Nucl. Fusion 1982, (part 2), 589-593. (24) Kolb, A. C.; Griem, H. R.; Lupton, W. H.; Phllllps, D. T.; Ramsden, S. A.; McLean, E. A.; Faust, W. R.; Swartz, M. Nucl. Fuslon 1982, (part 2) 553-559. (25) Ebihara, K. J . fhys. SOC.Jpn. 1980, 48, 958-964. (26) Commlsso, R. J.; Ekdahl, C. A.; Freese, K. B.; McKenna, K. F.; Qulnn, W. E. fhys. Rev. Lett. 1977, 3 9 , 137-139. (27) Costa, S.; DeAngelis, R.; Ortolani, S.; Puiattl, M. E. Nucl. Fusion 1982, 2 2 , 1301-1311. (28) Lee, S. Aust. J . fhys. 1983, 3 6 , 891-895. (29) Simard, P. A. Can. J . fhys. 1982, 60, 820-824. (30) Koppendorfer, W. J . Nucl. Mater. 1978, 76 & 77, 418-421. (31) Smith, D. L.; Krlstiansen, M.; Hagler, M. 0. J . Appl. Phys. 1977, 48, 4521-4527. (32) Malone, R. C.; Morse, R. L. fhys. Rev. Lett. 1977, 3 9 , 134-139. (33) DeSilva, A. W. Plasma fhys. 1979, 2 1 , 873-883. (34) Commlsso, R. J.; Bartsch, R. R.; Ekdahl, C. A.; McKenna, K. F.; Siemon, R. E. fhys. Rev. Lett. 1979, 4 3 , 442-445. (35) Ebihara, K. Jpn. J . Appl. fhys. 1981, 20, 1135-1144. (36) W e , S. R.; Plpes, D. T. Spectrochim. Acta, Part B 1981, 3 6 8 , 925-929.
(37) Waiters, J. P. I n “Contemporary Topics in Analytical and Clinlcal Chemistry”; Hercules, D. M., Hleftje, G. M., Synder, L. R., Evenson. M. A., Eds.; Plenum Press: New York, 1978; Val. 3. (38) Oberg, E.; Jones, F. D.; Horton, H. L. “Machinery’s Handbook”, 21st ed.; Industrial Press: New York, 1980. (39) Bulletin 807R, Maxwell Laboratories, fnc.: San Diego, CA. (40) Casper, D. Maxwell Laboratorles, Inc.: San Diego, CA, personal communication. (41) Engineering Bulletin No. 1007, Maxwell Laboratories, Inc.: San Dlego, CA. (42) White, J. S.; Scheellne, A., School of Chemical Sciences, University of Illinois, Urbana-Champaign, IL, unpublished research. (43) Mathews, S. E.; Ekimoff, D.; Walters, J. P. Appl. Spectrosc. 1982, 3 6 , 617-626. (44) Tektronix Model P6015 Instruction Manual, Tektronix, Inc.: Beaverton, OR. (45) Coleman, D. M. Ph.D. Thesis, University of Wisconsin, Madison, 1976 (University Microfllms no. 77-3393). (46) Horowltz, P.; Hill, W. “The Art of Electronics”; Cambridge University Press: Cambridge, 1980. (47) Coleman, D. M.; Walters, J. P. Spectrochim. Acta, Part 8 1976, 3 18, 547-587. (48) Thang, T.; Scheeline, A. Appl. Spectrosc. 1981, 3 5 , 536-540. (49) Morrison, R. “Grounding and Shielding Techniques in Instrumentation”, 2nd ed.; Wlley: New York, 1977. (50) Pressel, P. I . Bendix Corp., personal communication. (51) Scheeline, A. Appl. Specfrosc. 1984, 3 8 , 124-135. (52) Purcell, E. M. “Electricity and Magnetism”; McGraw-Hill: New York, 1965; Berkeley Physics Course Volume 2. (53) Spiegel, M. R. “Applled Dlfferential Equations”, 3rd ed.; Prentice-Hall: Englewood Cliffs, NJ, 1981. (54) Scheellne, A. Appl. Spechosc. 1981, 3 5 , 70-77. (55) Albers, D.; Johnson, E.; Tisack, M.; Sacks, R. Appl. Spectrosc. 1986, 40. 60-70.
RECEIVED for review September 13,1985. Accepted December 4, 1985. This work was supported in part by the National Science Foundation (Grant CHE-81-21809)and the Office of Basic Energy Sciences, US. Department of Energy (Grant DEFG02-84-ER13218). Fellowship support (G.J.K.) by Dow Chemical Co., Phillip Petroleum Co., and the University of Illinois are appreciated. Portions of this work by G.J.K. were completed in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry.
Theta Pinch Discharge Designed for Emission Spectrochemical Analysis: Spectral Characterization Gregory J. Kamla and Alexander Scheeline*
School of Chemical Sciences, University of Illinois, 1209 West California Avenue, 79 RAL, Box 48, Urbana, Illinois 61801
The hlgh-temperature plasma generated by a theta plnch discharge was studled as an emlsslon source for solid samples. Both sample geometry and the discharge fill gas affect plasma tormatlon and subsequent plasma/sample Interactlon. Ablation and emlsslon from solld samples have been demonstrated for alumlnum, tungsten, and stalnless steel samples exposed to the plasma. Tlme-resolved emlsslon data show neutral emlsslon occurring well afler cessation of Ion and contlnuum emission. Time resolutlon therefore removes the llmkatlons of hlgh backgroundand lon line Interference, whlch are associated with hlgh energy dlscharges.
A theta pinch discharge has been designed for use as a source for emission spectrochemicalanalysis. These discharges use magnetic compression to create high temperature plasmas.
Originally designed for fusion research, theta pinch discharges have been used to generate plasmas with temperatures and electron densities in excess of 1 keV and 1017 ~ m - ~In. fusion-related studies on these high-temperature plasmas, ablation has been observed from solid materials exposed to the plasmas ( 1 , 2 ) . These studies suggested that this discharge could be employed as an analybical emission source, where the discharge should be capable of sampling solid materials directly with little or no sample preparation. Ideally, a solid sample would be placed in the discharge vessel, the theta pinch would be fired, and the emission intensity from the sample measured. In the preceding paper (3),the design and electrical characterization of this discharge were discussed. In this paper, spectroscopic studies on the theta pinch plasma will be described. Spectral data have been acquired from the theta pinch discharge using both cylindrical and helical coils ( 3 ) .
0 1986 Amerlcan Chemical Society 0003-2700/86/0358-0932$01.50/0
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Table I. Spectroscopic Detection System Components 2 m Spectrometer System Components
over-under pair, first surface parabolas, aluminized with MgF2 overcoat, 50.8 mm diameter, fi = 600 mm, f i = 602 mm, separation along optical axis = 272.55 mm, entrance angle = 13.137', Three B Optical Co., Gibsonia, PA six-faceted, optical polygon, f = m, Lincoln Laser Co., Phoenix, AZ rotating mirror side-by-side pair, first surface parabolas, aluminized with MgFz overcoat, 76.2 mm diameter, f s = 1002 mirror 3-mirror 4 mm, f4 = 1001 mm, separation along optical axis = 399.54 mm, entrance angle = 9.267', Three B Optical Co., Gibsonia, PA spectrograph/monochromator Minuteman SMP-20, Czerny-Turner mounting, 2 m focal length, 102 mm X 128 mm grating with 1200 grooves/mm ruling, f/15.5 in zero order, Minuteman Laboratories, Inc., Acton, MA
mirror 1-mirror 2
Photoelectric Detection Instrument Parameters detector amplifier transient digitizer
oscilloscope XY recorder
-
9781R side window photomultiplier tube, Thorn EM1 Gencom, Inc., Fairfield, NJ; Model 205A-01R high-voltage power supply, operated at -750 V, Bertran Associates, Inc., Syosset, NY Model 2A44 video amplifier, input and output impedance = 50 R, constant gain = 100 up to 50 MHz, Pacific Precision Instruments, Concord, CA; Model 902 f15 V power supply, Analog Devices, Inc., Norwood, MA Biomation Model 8100; channel A, + input connected to 2A44 amplifier output, dc coupled, f 2 V range, +1 V offset; channel B, off; arm, 0.09 delay, input mode, external source, channel A, + slope, 0.00 level, dc coupled; trigger, 5 VFS - 50 R input connected to timing circuit main discharge trigger, 1.96 delay, input mode, external source, channel A, + slope, +0.10 level, dc coupled; time base, 0.05 rs sample interval, internal source; record mode; pretrigger; rear panel: NOT Z OUT connected to oscilloscope A horizontal time base trigger, Y OUT connected to oscilloscope left vertical amplifier, PLOT connected to XY recorder Y input; Gould, Inc., Biomation Division, Santa Clara, CA Tektronix 7854 oscilloscope; vertical amplifier, Model 7A13, + input, dc coupled; horizontal time base, Model 7B85, external trigger, 100 fis per division; Tektronix, Inc., Beaverton, OR Plotmatic 715: Allen DataaraDh, Inc., Salem, NH
Discussed in this paper are t h e effects of sample form, and position within the discharge vessel, along with the effects of type a n d pressure of t h e discharge fill gas. Time-resolved studies of sample and fill gas emission are also described.
EXPERIMENTAL SECTION The theta pinch source used in this study, along with typical source operating parameters, was described in the preceding paper (3). The optical systems assembled in our laboratory use a rider/rail bed system, modeled after the systems first designed by Walters (4,for alignment of optical components. The spectrometric system was originally designed to perform studies on the analytical high-voltage spark discharge (5). This system was set up to acquire time-resolved, space-resolved photographic data and is based on the designs of Klueppel et al. (6) as modified by Coleman (7), further altered to use a 2-m spectrograph. The system components are listed in Table I. This optical system was used to observe the emission from the theta pinch discharge side-on, through the center slot in the theta coil. Observation of plasma emission is made through the discharge vessel central glass, or quartz, tube. The mechanical time-resolution capabilities of the optical system were not used. No intermediate 50-rm slit was employed. A sine bar was used to align and secure the hexagonal mirror. Time-integrated, space-resolved data were acquired with this system on 102 mm X 127 mm Kodak Royal-X pan film (Eastman Kodak Co., Rochester, NY)(8). The f i i was processed as follows: (1)develop, Kodak D-19, 12 min, 20 "C; (2) stop, SB-la', 30 s, vigorous agitation; (3) fii, Kodak acid fix, 8 min. To acquire these data, the spectrograph entrance slit was set to 100 fim. With this f/15.5 optical system, reasonable photographic densities from the helical coil pinch were obtained from single-shot exposures. The emission was collected from the entire discharge sequence; the spectrograph shutter was opened, the preionizer and main discharges were fired, and then the shutter was closed. The preionizer emission did not interfere with the main discharge emission. Observable preionizer emission required exposure times of 60 s or greater. The 2-m spectrometer system was also used to collect timeresolved, space-integrated photoelectric data on the discharge. A block diagram of this detection system is illustrated in Figure 1. The detection components, along with their operating parameters, are listed in Table I. The emission signal was detected by the photomultiplier, amplified, and stored by the transient digitizer. All data are raw signals without benefit of background subtraction. The monochromator exit slit width was fixed at 100
2 METER SPECTROMETER SYSTEM
THETA PINCH DISCHARGE
PHOTOMULTIPLIER
XY RECORDER
CONTROL ELECTRONICS I
TRANSIENT DIGITIZER
I
I
OSClLLOSCOPE
AMPLIFIER
I
Figure 1. Photoelectric detection system block diagram.
fim. The transient digitizer was triggered by the main discharge trigger pulse from the control electronics and was monitored by an oscilloscope and XY recorder. The monochromator entrance slit width was varied between 100 pm and 20 gm to optimize the amplitude of the video amplifier signal, as derived from the photomultiplier, with respect to the digitizer resolution. The resulting digitized signals were then normalized to a constant instrument throughput for a 100-pm entrance slit width by division of the digitized signal by its relative slit width. The emission intensity is linearly related to the slit width for emission line widths narrower than the spectral window (9).
RESULTS AND DISCUSSION Preliminary studies of t h e effect of preionizer/main discharge timing showed t h e most efficient coupling occurred when the main discharge was fired at t h e time of peak spark preionizer current. T h e ion population of the preionizer plasma is expected to be greatest at the time of peak current. Formation of t h e plasma current sheet, and plasma compression, by the main discharge should be most efficient with a plasma having a high ion population, and a corresponding low plasma resistance. In opposition t o this, if the main discharge is fired well after t h e preionizer, when t h e ion population is low, t h e resistivity of the plasma is high, and thus energy dissipation by t h e plasma should increase. To examine t h e effect of ion population on plasma heating by the theta pinch, the delay of the main discharge trigger pulse
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1988
was varied relative to the preionizer, and the resulting emiasion intensity was monitored. Little change in the emission intensity was observed for delay settings from 0.1 to 20 pa. A slight decrease in intensity was observed at 50 0s. and the lowest intensity occurred a t 100 pa. The intensity then progressively increased for 200 and 500 ps delays. The emission intensity remained relatively constant for delays from 500 pa up to 3 s. A fall-off in intansity was not observed until delays of 10 s or greater. These experiments on the ion lifetime were performed with the cylindrical theta coil. The field generated by the main discharge circuit was not sufficient to break down the discharge vessel gas, and coupling from this circuit was only observed with a preionized plasma. The field from the main discharge circuit with the helical coil does caw gas breakdown and plasma formation without a preionizer, and therefore the above ion lifetimes could only be verified by using the cylindrical coil. From these data, coupling appears most efficient for shorter delay times, but the delay did not critically affect coupling within the range of 0-50 pa after preionizer peak current. Two features in these data were unexpected. The first was the existanceof a significant ion population seeonds after the last spark fired, significant in that the ion density was still high enough to couple with the main discharge. The existence of ions a t times well after discharge cessation has also been observed in atmospheric pressure spark discharges (10-22). Ground-stateions have also been determined to exist in spatial regions far from the plasma torch in inductively coupled plasmas (23). At the present time, an explanation for these apparently related phenomena is not known. The second feature of these data was the discontinuity of the intensity a t the delays of 100 and 200 ps. One would expect a progressive decrease in intensity with increases in main discharge delay. The reason for this observed discontinuity is unclear. The sampling efficiency of the theta pinch was initially studied by using aluminum foil samples. The foil samples were cut into 19 m a squares, and cleaned with methanol. These foil squares were placed within the discharge vessel, centered with respect to the theta coil center slot, and rested on the surface of the discharge tube. The inner diameter of the discharge tube is 22 m a , and the foil sample rested below the discharge tube center, approximately 7 m a from the bottom of the tube. For efficient coupling between the main discharge circuit and the plasma, the sample geometry must allow formation of the plasma current sheet. For example, placement of a larger foil square within the discharge vessel, such that it bisected the center of the tube, restricted formation of the current sheet and subsequent plasma compression. When the 19 mm square aluminum foil sample was placed in the discharge vessel, fving the helical coil theta pinch c a d the foil to bend in half. Closer inspection of the foil revealed that the two edges of the foil that were bent together had fused. Small aluminum depcaits were also observed on the surface of the dischargevessel glasa tube. These observations suggest that melting of the sample occurred from the theta pinch plasma. This preliminary experiment illustrated the substantial energy depoaition from the main discharge circuit into the sample with the helical theta coil. The effects of fill gas atomic number and presaure were studied. Data were collected from argon discharges at 0.14, 0.45, 1.5,and 3.8 torr, and from helium dinchslges at 0.13,0.42, and 1.4 torr. These experiments were run using the spark discharge as preionizer. The range of pressures used was limited on the low end by the discharge vessel vacuum pump efficiency and on the high end by the stability of the spark discharge. Due to helium’s high ionization potential, difficulty in spark breakdown was O L W N ~ for helium discharges at 0.13
ARGON FILL GAS
I
0.14
TORR
I AI
Flguuro 2. Helical coll meta pinch spectra for 0.14-3.8 ton argon. Wavelength increases horn rlgM to left AI 1. 3961.520 A. 3p2P,*-4s2s,,2.
torr. To mure stahle breakdown under all fa gas conditions, the spark source capacitor voltage was set to produce stahle breakdown with the helium discharge a t 0.13 torr, and this voltage was then kept constant for the entire experiment. At this capacitor voltage of 4.5 kV. the peak spark current varied from 50 to 57 A for the argon discharges and from 50 to 53 A for the helium discharges, with current increasing with increasing pressure. Previoua experiments had shown no dependence of plasma emission intensity on the spark peak current, and therefore no effect was expected from the small differences in the peak currents resulting from changes in gas pressure. To determine the effect of fill gas on sampling, the sample probe with an insulated aluminum sample was used. The insulated sample was chosen to prevent interference from sampling by the preionizer discharge. The spectra acquired from theta pinch discharges in argon are shown in Figure 2. These spectra from argon discharges with the insulated aluminum sample show an increase in the continuum intensity as the pressure is increased. Both argon line intensity and number increase with preasure up to 1.5 torr and then decrease slightly a t 3.8 torr. Line broadening is apparent a t 3.8 torr. Aluminum emission, from the Al I line a t 3961.520 A, was observed only for the discharge at 3.8 torr. Although the maximum emission intensity from the argon fa gas was observed a t 1.5 torr, suggesting that the plasma temperature was highest at this pressure, aluminum emisaion was only observed at 3.8 torr and suggests that not just plasma temperature but also plasma density is an important variable for sampling. Unlike the discharges in argon, no continuum emission was observed from helium discharges. The intensity of helium emission increased with increasing pressure, while the intensity of impurity emission from nitrogen, oxygen, and carbon decreased with increasing pressure. No aluminum emission was observed in any of the helium spectra. Coupling between the main discharge circuit and the p h was more efficient with argon than with helium. The presence of aluminum emission only a t the highest argon pressure suggested poasihle improvements could be attained a t pressures above 3.8 torr. At discharge veasel pressures greater than 3.8 torr, the spark discharge becomes unstable, so a glow discharge was used instead. The glow discharge produced a stable, uniform plasma up to the vacuum system upper pressure limit of 12 torr.
ANALYTICAL CMMISTRY. VOL. 58. NO. 4, APRIL I986
ARGON FILL GAS
835
AI SAMPLE
3.8 TORR
I
1 I
IIII
A 12
TORR
I AI Flgure 3. Helical coil theta pinch spectra for 1.5-12 torr argon. Wavelength increases from right to left Ai 1. 3961.520 A. 3P2PO,,,-4S2S,,,.
By use of the glow discharge as the preionizer, spectra were recorded from argon discharges a t 1.5.3.8, 6.8, and 12 torr. These spectra were shown in Figure 3. The argon line intensity was greatest at 1.5 torr, decreasing as the pressure was increased. Weak emission was observed from the discharge at 12 torr, suggesting poor coupling a t this higher pressure and verifiig an upper limit for optimum argon pressure. The background continuum emission increased with pressure from 1.5 to 3.8 torr and was approximately the same at 3.8 and 6.8 torr. Aluminum emission, again from Al I a t 3961.520 A, was observed only a t 3.8 and 6.8 torr. From these data, the argon pressure of 3.8 was determined best for the insulated aluminum sample; the aluminum intensity was slightly greater at 3.8 than at 6.8torr,and the emission line width at the lower pressure was narrower. Although sampling by the theta pinch was now observed for the insulated sample placed in the center of the discharge vessel, the emission intensity from the centered aluminum rod was not as great as was observed in the initial firings with the aluminum foil sample. Sampling by the theta pinch was greater near the outer surface of the discharge vessel rather than in the center. For this reason, 0.8 mm thick aluminum sheets were tried as samples, and aluminum emission was observed from these thicker samples. The aluminum sheet remained rigid; it was not deformed by plasma compression as was observed with the foil sample. Small structures, which resemble cathode spots, were localized near the edges of the sheet, which rested against the discharge vessel. At a discharge vessel pressure of 3.8 torr, where most efficient sampling by the theta pinch plasma was observed, both the glow discharge and the spark discharge produce stable, spatially uniform discharges. To determine whether or not major differences in theta pinch plasma emission could be observed for spark or glow discharge preionization, spectrom p i c data were collect4 from theta pinch plasmas produced with each preionizer discharge. The effect of changing the preionizer source from spark to glow discharge was minimal. Although differences in the spatial structure of the emission were observed, there was no significant difference in the aluminum emission intensity between the two preionizers. Due to the simplicity in the number of components and in the required timing, the glow discharge was used as the
BCDE
Flguun 4. Me I1 specbe fmm the helical coil theta pinch wlth an aluminum sheet sample. Upper inset is blank spectrum; lower inset Is sample specbum: (A) Ai 11.2816.179 A. 3p’P - 4 ~ ~ 5(6) , ; Mg 11. 2802.704 A. 3s*~,,fip2po,,; (c) 11.2797.998 3~?9~-3d2D ; (0) Mg 11. 2795.528 A. 3s2S,,,-3p2Po,,,; (E) Mg 11. 2790.776y. 3p2Po,,,-3d2D,,,.
A.
preionizer for all subsequent experiments. The aluminum sheet used as the sample for the preionizer comparison was type 5052 alloy. Along with aluminum, the 5052 alloy also contains 2.5% magnesium and 0.25% chromium. Spectra were obtained from the theta pinch in the wavelength region of magnesium and chromium emission. The spectrograph wavelength counter was set to 8500 A, second order for Cr I emission near 4275 A and third order for Mg I and I1 emission near 2800 A. A quartz tube was used as the discharge vessel for transmission of the magnesium emission. These data were acquired from dischargesin argon a t 1.5 and 3.8 torr. Spectra from the theta pinch discharge a t 1.5 torr are presented in Figure 4. In the sample spectrum, no emission from the chromium was O ~ W N ~Emission . from magnesium ion was observed. A1 I1 emission a t 2816.179 A was also observed in this wavelength region. Emission from the magnesium neutral transition at 2852.127 A could not be verified due to the spectral overlap of the intense AI I1 line a t 4277.524A. The spectrum from the discharge a t 3.8 torr contained the same magnesium ion emission as shown in Figure 4 for the 1.5 torr discharge. Emission from Cr I or Mg I was not observed a t 3.8 torr either. To determine the ability of this theta pinch to sample high melting materials, tungsten and stainless steel samples, metals which have higher melting points than aluminum, were also exposed to the theta pinch plasma. The tungaten sample was in the form of a subl0-runpowder (Anderson Physics Laboratories, Inc., Urbana, IL). To introduce this sample into the discharge vessel, the tungsten powder was placed on a sheet of aluminum, and the aluminum sheet was then centered within the discharge vessel. This aluminum sheet had the same dimensions as those used previously to study aluminum emission. Data with this powder sample were acquired from dischargea in argon at both 1.5 and 3.8 torr. After the theta pinch discharge was fired, inspection of the dischage vessel revealed not only a scattering of the tungsten powder but also tungsten that had been vapor deposited on the surface of the discharge vessel. The spectra from discharges a t 1.5 torr are given in Figure 5. Emission from the tungsten sample can be seen in the bottom spectrum of Figure 5. In this spectrum 27 tungsten lines have been identified (14). The two most intense tungsten lines, which are labeled in the figure, are from neutral species. Similar
936 ANALYTICAL CHEMISTRY. VOL. 58. NO. 4, APRIL 1986
W SAMPLE
II
AB
FIpm 5. W I speclra fromum hem1cdltheta phch Malungsten powder sample. Upper In& is bhnk spectrum; lower Inset I8 sample spectrum: (A) W I. 4302.108 A; (8)W I. 4294.814 A.
spectra were also obtained from discharges at 3.8 torr. The stainless steel sample, 302 alloy, was in the form of a thin sheet, 25 pm thick by 19 mm wide. This sheet was positioned in the discharge vessel the same as were the aluminum sheet samples. With this sheet sample and discharge pressures of 1.5, 3.8, and 6.8 torr, no sample emission was observed from either iron, present a t 70%, or chromium, present at 18%. Although no emiasion was recorded from this sample, upon inspection of the sample after exposure to the theta pinch plasma, small, light-colored structure were ohserved on the top surface of the sample sheet. These structures, which were also observed in a greater abundance with the aluminum sheet samples, resembled cathode spots and suggest that some sampling of the stainless steel sheet did occur, but apparently, not to the extent that significant emission intensity resulted. In later experiments hy White (15), stainless steel turnings were used as a sample rather than the stainless steel sheet, and iron emission was ohserved. The observation of sample emission from the tungsten powder and the stainless steel turnings, along with the absence of observed emission from the stainless steel sheet, suggests that the amount of material eroded by the theta pinch plasma is dependent on sample form. Increasing the surface area of the sample causes an increase in the mass of sample material eroded by the plasma. This finding is consistent with observations by others, where ablation of the sample was limited to a surface layer on the order of 1 pm thick ( I , 2). Timereaolved, spaeeintegated photoelectric emission data were collected from discharges in argon a t pressures of 1.5 and 3.8 torr. Emission data were acquired from neutral and ionized argon and aluminum species and from the continuum background. Argon and continuum emission measurements were taken from discharges with no sample present, assuring no interference from sample emission. Aluminum emission measurements were taken with aluminum sheet samples positioned in the discharge vessel center as previously described. For all of the emission data, the signal peak that occurred a t zero time was from RFI generated by the TG-40 spark gap driver, not from plasma emission. The current waveform employed was that of Figure 8 8 in the accompanying paper (3). Continuum background emission was recorded at 3965 A, the region between the monitored line emission of AI I a t 3961.5200 A and Ar I1 at 3968.360 A. Timeresolved continuum emission from discharges a t 1.5 and 3.8 torr are plotted in Figure 6, insets A and B. Oscillations in the emission a t both pressures follow the Oscillations in discharge current. At
1.5 torr, the maximum signal corresponded to the second oscillation half-cycle of the diecharge. The emission intensity a t 3.8 torr was greater than a t 1.5 torr, and the maximum signal occurred later in time, at the third oscillation half-cycle. This increase in continuum emission at 3.8 torr was also observed in the time-integrated photographic spectra of the plasma. Emission a t 3.8 torr decays somewhat more slowly than a t 1.5 torr, with low level emission still evident a t 80 ps after firing the discharge. Emission profiles from Ar I a t 4259.3617 A are illustrated in Figure 6, insets C and D. Emission intensity again follows the Oscillations of the dkharge current. At 1.5 torr, maximum emission occurred at the second current half-cycle. At 3.8 torr, this maximum oecurred at the fourth current half-cycle, and the emission intensity was greater a t 3.8 torr than a t 1.5 torr. The emission decay for Ar I was much slower than that observed for the continuum. The arrows in insets C and D a t 95 ps mark the end of the main discharge. It can be seen in both figures that emission occurs after the cessation of discharge current. Figure 7 shows the Ar I emission on a reduced time scale. Again, the arrows mark the time of current cessation. A t 1.5 torr, emission from Ar I was still evident 240 ps after the theta pinch was fired, 145 ps longer than the duration of measured discharge current. At 3.8 torr, the emission signal ended abruptly at 105 pa, but emission pulses then were observed later in time. Two, 14 pa wide pulses were observed starting at 183 and 203 pa. A weaker emission signal was also observed a t times greater than 240 @. The emission pulses observed were reproducible a t 3.8 torr but were not observed a t 1.5 torr. At the present time, an explanation for their existence is lacking. Ar I1 emission data are given in Figure 6, insets E and F. The intensity of the Ar I1 line emission was greater than any of the other lines monitored in this experiment. This was expected from observation of the time-integrated spectra. Time-integrated emission from Ar I1 was dominant both in intensity and in number of lines observed. For both 1.5 and 3.8 torr, the maximum emission intensity oeeurred on the third discharge current half-cycle, with the maximum a t 1.5 torr only slightly greater than a t 3.8 torr. The oscillations in the emission signal were greater a t 1.5 torr also, following the current oscillations to a greater degree. The emission signal decayed for both discharges before cessation of discharge current. The emission signal does not continue after discharge current cessation as was observed for the neutral species. Ar I11 emission data are given in Figure 6, insets G and H. The emission intensity is greatest for both pressures a t the second discharge current half-cycle, one half-cycle before the maximum that was observed for the Ar I1 emission. From the main discharge circuit model proposed in the accompanying paper (3), the peak plasma current was calculated to occur on the second current half-cycle. This would suggest that the maximum plasma compression, and therefore maximum plasma temperature, occurred on the second discharge half-cycle. Thus, the population of excited Ar III levels would be greatest during the higher temperatures of the second dischargehalf-cycle. Comparison of Ar 111emission a t 1.5 and 3.8 torr shows a significant increase in emission a t the lower pressure. This increase in emission is consistent with the higher plasma temperatures expected a t the lower discharge pressure. The emission decay of Ar 111was similar to that for AI 11, with the emission signal intensity negligible by the end of the main discharge. For the three argon levels investigated, the emission signal followed the main discharge current; oscillations in the emission signal were observed a t each discharge current half-cycle. The temporal emission profiles of Ar I1 and Ar 111were similar. This ion emission occurred only during the
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4,APRIL 1986
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main discharge. Emission from Ar I on the other hand, continued after cessation of discharge current, and emission pulses were observed at 3.8 torr. The slower decay observed for Ar I emission is consistent with a recombination mechanism, where lower lying excited energy states are populated by radiative cascade of species from higher energy states. Time-resolved emission data from aluminum sheet samples were obtained in the same manner as the argon fill gas data. The emission signals from the aluminum samples were not as reproducible as were the argon fill gas emission signals. For
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example, for four consecutive firings of the theta pinch at 1.5 torr, the relative standard deviations of the integrated Ar I1 and AI I emission intensities were 0.03 and 0.1, respectively. Emission profiles for AI I, 11, and I11 are compared in Figure 8. As observed for the argon fill gas, the emission signal from the ionized sample species follows the oscillations in the discharge current, with minimal emission observed after cessation of discharge current. Unlike the time-integrated data in which the AI I lines were most intense, the emission signal from the AI I1 line showed the maximum intensity. At.1.5
ANALYTICAL CHEMISTRY, VOL. 58, NO.
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torr, the A1 I1 emission intensity was greatest on the second discharge half-cycle. A1 I11 emission was most intense on the fourth discharge half-cycle, but similar intensities were observed for the second through sixth half-cycles. At 3.8 torr, similar temporal profiles were observed for A1 I1 and A1 111. Maximum emission occurred on the second and third halfcycles. The A1 I1 emission intensity was greater at 3.8 torr than at 1.5 torr, but for A1 I11 the emission intensity appeared greater at 1.5 torr than at 3.8 torr. The greater emission intensity for A1 I11 at 1.5 torr is consistent with a higher temperature plasma at this lower pressure and was also observed in time-integrated photographic spectral data. Figure 8, insets E and F, contain emission profiles from A1 I. At times during the discharge, emission from the neutral aluminum was similar to the emission from ionized aluminum. The emission signal oscillated, following the oscillations in the discharge current. Unlike the emission from the ionized species, the emission from the neutral species continues after
the main discharge has ended. The slower emission decay was also observed for neutral argon. But unlike the neutral argon emission, which peaked during the first few half cycles of the main discharge and then slowly decayed, neutral aluminum emission increased near the end of current conduction. In Figure 9, neutral aluminum emission is presented on a reduced time scale. The arrows again indicate the end of the main discharge current. The emission signal at 3.8 torr was greater than the signal at 1.5 torr. A t 1.5 torr, two emission peaks were observed well after the time of peak discharge current. Although the reproducibility in the time at which these peaks appeared was poor, two emission peaks were consistently observed. Data collected using an even slower time scale showed emission from A1 I lasting for greater than 400 ps. At 3.8 torr, three emission peaks were observed that were not associated with the oscillatory main discharge current. Of these three peaks, the emission intensity was greatest on the third peak. Appreciable emission signals were observed for
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
times greater than 600 ps. Although the peak intensity of the A1 I1 line was greatest during the discharge, the increase in AI I emission after discharge cessation accounts for the increased integrated Al I intensity observed photographically. Emission from the ionized aluminum species was similar to that observed for the ionized argon fill gas. The reasons for the observed differences between the neutral emission and the ion emission are unclear, as is the difference between the neutral argon and aluminum emission. If one considers a recombination mechanism, followed by radiative cascade, the emission observed from the aluminum line would be expected to last longer than the argon emission since this aluminum transition is to the ground state, and with its lower energy level one would expect a slower decay. Although a mechanism describing the prolonged emission from the transition to the aluminum neutral ground state has not yet been determined, analytical advantages in neutral emission signal-to-background can be envisioned by detection of plasma emission after cessation of the main discharge current. After cessation of discharge current, the continuum and ionic emission decay to minimal levels, and with time-gated detection, background emission and spectral interferences from ionic emission can be eliminated. Thus, the energy of this plasma source is no longer limited by the increases in continuum background emission and in spectral interf‘erences typically associated with increases in plasma energy. CONCLUSION Studies of the time-resolved emission from the argon fill gas and an aluminum sample showed that the continuum and ionic emission follow the discharge current waveform. Emission from neutral species also showed a dependence on the discharge current, but neutral emission was also observed after cessation of the discharge current. This continuation of neutral emission after cessation of continuum and ionic emission leads to improved signal-to-background ratios for neutral emission monitored after the cessation of discharge current. Not only lower background emission levels are achieved after the discharge but also spectral interferences associated with emission from ionic states are eliminated. Because the neutral sample emission can be monitored after the current discharge, the energy delivered to the plasma is no longer limited by the level of plasma continuum and ionic emission. Higher energy discharges could be assembled, increasing the energy delivered to the plasma. Increases in plasma energy would improve the sampling capability of the discharge, and the associated increase in continuum and ionic emission could be discriminated against. This work on the theta pinch discharge has demonstrated the ability of this discharge to sample conductive solid materials. For this discharge to become a viable analytical emission source, improvements in the sampling efficiency are necessary. Improving the sampling efficiency will require not only increasing plasma/sample interactions but also better characterization of this magnetically constricted plasma. In future experiments, attempts will be made to increase the plasma temperature by using hydrogen rather than argon or helium as the fill gas. The maximum plasma kinetic temperature for discharges in argon is limited by the distribution of energy among the available excitation and ionization degrees of freedom of the argon species. With a hydrogen discharge, energy deposited by the main discharge causes dissociation
939
and ionization of the hydrogen, but once the hydrogen has been singly ionized, all additional energy delivered to the plasma increases the plasma kinetic temperature. The higher ionization potentials of helium prevented complete ionization of the fill gas in the helium discharges. Along with experiments on increasing the plasma energy, future work on this source must also include further characterization of the magnetically constricted plasma. Time and space resolved emission data would lead to an improved understanding of the theta pinch plasma dynamics. These data would allow observation of both plasma formation and plasma/sample interactions. For example, the plasma temperature during the initial compression stage can be inferred from the implosion velocity of the plasma, and observation of the sample emission as a function of both time and space would increase the understanding of the sampling and emission processes. Nevertheless the data presented herein show promise that solid samples may be rapidly and directly sampled and excited using a theta pinch. Time resolution appears to be the key to obtaining adequate selection of analytical signal while suppressing background and ion line interference. ACKNOWLEDGMENT The authors wish to thank the following: R. Harrison, M. Funkhouser, E. Lash, and N. Vassos for assistance in construction; J. Wehmer for loan of the transient digitizer; R. Vogel for use of the densitometer; and P. W. Bohn for helpful comments. Registry No. Al, 7429-90-5;Mg, 7439-95-4;W, 7440-33-7;Ar, 7440-37-1; stainless steel, 12597-68-1. LITERATURE CITED Smith, D. L.; Krlstlansen, M.; Hagier, M. 0. J . Appl. Phys. 1977, 48, 4521-4527. DeSilva, A. W. Plasma Phys. 1979, 21, 873-883. Kamla, G. J.; Scheeline, A. Anal. Chem ., preceding paper in this issue. Walters, J. P. I n “Contemporary Topics in Analytical and Clinical Chemistry”; Hercules, D. M., Hieftje, G. M., Snyder, L. R., Evenson, M. A., Eds.; Plenum Press: New York, 1978; Voi. 3. Scheeline, A.; Kamla, G. J.; Zoellner, M. J. Spectrochim. Acta, Part B 1984, 398, 677-691. Klueppel, R. J.; Coleman, D. M.; Eaton, W. S.; Goldstein, S. A,; Sacks, R. D.; Waiters, J. P. Spectrochim. Acta, Part B 1978, 338, 1-30. Coleman, D. M.; Sainz, M. A.; Butler, H. T. Anal. Chem. 1980, 52, 746-753. Sacks, R. D.; Lln, C. S. Appl. Spectrosc. 1979, 3 3 , 258-268. Goode, S . R.; Crouch, S. R. Anal. Chem. 1974, 4 6 , 181-182. Coleman, D. M.; Walters, J. P. J . Appl. Phys. 1977, 48, 3297-3300. Arakl, T.; Walters, J. P. Spectrochim. Acta, Part B 1979, 348, 371-303. Scheeline, A.; Travis, J. C.; DeVoe, J. R.; Waiters, J. P. Spectrochim. Acta, Parte 1981, 3 6 8 , 153-161. Gilison, G.; Horlick, G. 27th Rocky Mountain Conference, Paper 57, Denver, CO, July 1985. Harrison, G. R. “M.I.T. Wavelength Tables”, M.I.T. Press: Cambrldge, MA, 1962. White, J. S.; Scheeiine, A. School of Chemical Sciences, University of Illinols, Urbana-Champaign, IL, unpublished research.
RECENED for review September 13,1985. Accepted December 4, 1985. This work was supported in part by the Office of Basic Energy Sciences, Department of Energy (Grant DEFG02-84-ER13218). Fellowship support (G.J.K.) by Dow Chemical Co., Phillips Petroleum Co., and the University of Illinois are appreciated. Portions of this work by G.J.K. were completed in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry.
