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Anal. Chem. 1986, 58,3108-3115
(13)has suggested that, in fields sufficient to cause ionization potential lowering, selection rules may also be relaxed so that lines forbidden in isolated atoms may become weak lines, and a multitude of weakly dowed transitions would appear. More complex calculations than presented here would be necessary to access this suggestion. Thus, the application of eq 1 to very dense plasmas requires additional consideration. In the previous paper (I), it was suggested that slopes of plots that were less than 0.15 were due to poor photographic intensity calibration. The current work indicates that such shallow slopes may in some instances occur. Now that it is clear that eq 1 is not a universal descriptor of the intensity distribution of spectral lines, there is a need for reexamination of the conditions under which the arsenic and iron experimental data, which appeared to support the hypothesis, were collected. Careful intensity calibration of the instrumentation and attention to the effects of data reduction algorithms will be required.
ACKNOWLEDGMENT Discussions with Sanford Asher led to the approach employed. Registry No.
HP,1333-74-0.
LITERATURE CITED (1) Scheeline, A. Anal. Chem. 1986, 5 8 , 802-807. (2) Learner, R. C. M. J . Phys. 6 1982, 15, L 8 9 L L 8 9 5 . (3) Howard, L. E.; Andrew, K . L. J . Opt. SOC. Am. 6 1985, 2 . 1032-1077. (4) Sobelman. I . I. Atomic Spectra and Radiative Transitions, Springer Series in Chemical Physics V. 1; Springer-Verlag: Berlin, Heidelberg, and New York, 1979. (5) Bethe, H. A.; Salpeter. E. E. Ouantum Mechanics of One- and TwoElectron Atoms ; Springer-Verlag: Berlin, 1957. (6) Goidwire. H. C.,Jr. Astrophys. J . Suppl. Ser. N o . 152 1968. 17, 445-457. Menzel. D. H. Astrophys. J . Suppl. Ser. 161 1969, 18, 221-246. Porter. C. E. Statistical Theories of Spectra : Fluctuations ; Academic Press: New York and London, 1965. Gradshteyn, I . S.; Ryzhik, I . M. Table of Integrals, Series, and Products; Academic Press: New York, 1980. Cowan, Robert D. The Theory of Atomic Structure and Spectra ; University of California Press: Berkeley, CA, 1981; pp 625-631. Stolarsky. K.; Scheeline. A,, work in progress. Farnsworth, P. B.; Walters, J. P. Spectrochim. Acta, Part B 1962, 376. 773-788. Learner, R. C. M. personal communication.
RECEIVEDfor review May 30,1986. Accepted August 21,1986. Financial support of the National Science Foundation (Grant CHE-81-21809) and the Office of Basic Energy Sciences, United States Department of Energy (Grant DE FG02-84ER13218), is gratefully acknowledged.
Production and Initial Characterization of an Imploding Thin-Film Plasma Source for Atomic Spectrometry Kevin P. Carney and Joel M. Goldberg*
Department of Chemistry, University of Vermont, Burlington, Vermont 05405
An Imploding thin-film plasma source for the direct atomic spectrochemkal analysis of solid samples is described. The plasma Is produced by an axially directed capacltlve electrical discharge through a conductive silver thin film that has been chemicaHy deposited on the lnterlor wall of a polycarbonate tube. Discharge Instrumentation and methods for thln-fllm depositlon are descrlbed in detail. The plasma Implodes symmetrically at a constant velocity and then fllls the dlscharge tube. Solid powder microsamples of pure vanadium and V205 are sampled by the plasma from the dlscharge tube wal. Peak power dissipation in plasmas generated by moderate energy (200-1600 J) dkcharges Is In the megawatt range with corresponding peak power densities In the megawatt per cubic centimeter range. Initial emission spectroscopic measurements are presented.
The use of pulsed discharges as atomic spectrochemical sources has been widely reported (1-3) due to their ability to deliver large amounts of energy to a small sample area in a very short period of time. The more popular pulsed discharge sources (e.g., spark and laser plasmas) are best noted for their ability to directly sample solid materials. More recently, exploding conductor plasma sources have demonstrated the capability of directly analyzing solid samples for trace metallic elemental constituents with surprisingly few matrix effects. All of these pulsed discharge sources possess very attractive features as direct solid sampling devices due to their extremely high power density capabilities. Their usefulness for the direct 0003-2700/86/0358-3 108$01.50/0
atomic spectrochemical analysis of solid materials, however, has been limited by their relatively poor excitation characteristics. While reexcitation of the atomic vapor produced by these discharges is possible, it is complicated by the rapid expansion of the resultant sample atomic vapor. Thus, direct in situ reexcitation is limited by the short residence time of analyte species in the postdischarge environment. As such, reexcitation schemes using spark and laser atom cells have typically involved transport of the sampled analyte vapor to a conventional high-frequency excitation source (e.g., an ICP or microwave-induced plasma) (4-6). The relatively long sample transit times (seconds) characteristic of these schemes, however, can result in significant condensation and/or dilution of the sampled atomic vapor before it reaches the excitation source, severely degrading its analytical characteristics. In this report, we describe initial studies of a new type of pulsed discharge source for atomic spectrochemical analysis: the imploding plasma. By producing a transient plasma through implosion, we hope to generate a spatially confined atom cell capable of very high power densities as well as long analyte atomic vapor residence times. Development of a suitable device for the production of imploding plasmas for atomic spectroscopy must rely upon the wealth of available reports of electromagnetic compression devices in the plasma physics literature. Theoretical discussions of magnetically self-pinched plasmas can be found in the literature as early as 1934 (7); however, reports of experimental investigations of imploding plasmas are not found in the open literature until the late 1950s (8). The 0 1986 American Chemical Society
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2-Pinch
8-Plnch
Figure 1. Implosion geometries.
ability of these devices to produce dense, high-temperature plasmas has led to investigations of their suitability as controlled nuclear fusion devices (9). Numerous studies have focused on plasmas created using z-pinch or theta-pinch geometries due to the relatively simple design of these systems and the high degree of control over implosion characteristics that they can afford. Plasmas generated in these geometries rely on the shock and/or electromagnetic implosion of a cylindrically symmetric plasma (see Figure 1). One characteristic that can be used to differentiate between many of the imploding plasma devices described in the literature is the mechanism used to produce the initial cylindrically symmetric plasma. Often, production of this preimplosion plasma requires low-pressure operation (IO),which diminishes its attractiveness as a routine analytical atom cell. Devices based on the implosion of plasmas formed by the rapid electrical vaporization of conducting wires, foils, or thin films (imploding conductor plasmas), however, do not require lowpressure operation and, thus, appear more amenable for investigation as routine solid sample atom cells. Imploding conductor plasmas generated using a z-pinch configuration were first investigated in the early 1960s as an extension of work then being performed on exploding wires (11-14). These studies typically utilized moderate-energy capacitive discharges (kilojoule) through internally silvered glass or quartz tubes. More recently, very high energy capacitive discharges (megajoule) have been used to generate imploding conductor plasmas suitable for the generation of pulsed X-rays. These studies have utilized cylindrical wire arrays (15)as well as cylindrical thin conducting foils (16,17) and films (18,19). While imploding conductor plasmas produced in theta-pinch configurations have also been reported (20-22),these investigations were directed toward the production of megagauss magnetic fields. As such, no spectroscopic information has been reported for these plasma devices. Certainly the previously studied imploding plasma devices are neither practical nor desirable as solid sample atom cells, but they do possess characteristics (e.g., high power density, spatial confinement, atmospheric pressure operation) that are desirable in such sources. While there have been no reports in the literature describing the use of imploding conductor plasmas as atomization or excitation sources, Sacks and coworkers have performed extensive physical and analytical studies of nonconfined or exploding conductor plasmas which clearly demonstrate their analytical utility ( 3 ) . They have successfully utilized exploding thin-film plasmas (generated by a high-voltage capacitive discharge through a planar thin conductive film) as atomization and excitation sources for the direct elemental analysis of solid powder samples (23-25). The relative freedom of these devices from matrix effects has allowed the use of straightforward calibration procedures for the analysis of even very refractory solid samples possessing complex matrices. While there have been no reports in the analytical literature on the use of imploding plasmas as atom cells, there have been a few investigations of the analytical utility of electromagnetic compression and/or confinement of spectrochemical plasmas. Goode and Pipes (26) reported observing a significant enhancement of emission from a theta-pinched microwave-induced plasma. Kamla and Scheeline (27,28) have recently
Figure 2. Discharge cassette: (A) hollow graphite electrode, (9) brass electrode block, (C) polycarbonte shield, (D) brass electrode support, (E) brass screw, (F) polycarbonate insulating block, (G) electrode contact “dimples”.
reported on the construction and initial characterization of a theta-pinch device based on the electromagnetic confinement/compression of low-pressure spark and glow discharge plasmas. Most recently, Albers et al. (29,30) presented the results of initial attempts a t electromagnetically confining exploding conductor plasmas. Whereas these initial reports clearly demonstrate the tremendous potential of electromagnetic pinch devices in atomic spectroscopy, they do not incorporate or accommodate a segregated atomization/excitation scheme. The studies presented in this report center on the generation and initial characterization of imploding thin-film plasmas as atomic spectrochemical sources for direct solid sampling. These plasmas are specifically intended as solid sample atom cells only-ultimately, in situ reexcitation (31) is planned for measurement of analyte; the atom cell has been designed with this capability in mind. This paper details the design, construction, and electrical and physical characterization of this new spectrochemical source, while a companion article (32) reports on its analytical characteristics.
EXPERIMENTAL SECTION Thin-Film Production. Conductive silver films were coated on the interior surfaces of polycarbonate tubes using a simple chemical reduction process (33).Six-foot lengths of polycarbonate tubing (AIN Plastic, Mt. Vernon, NY) were cleaned by rinsing first with dilute HN03 and then with distilled water. A silver solution was made by dissolving 6 g of AgN03 in 100 mL of distilled water, adding 30 mL of concentrated ammonia and 70 mL of 3% (w/w) NaOH, and diluting t o 500 mL with distilled water. A sucrose-reducing solution was made by dissolving 8 g of sucrose in 150 mL of distilled water, adding 1 mL of concentrated ”OB, boiling for 2 min, and adding 150 mL of methanol after cooling. One of the ends of the polycarbonate tube was capped and the tube filled with a 1 O : l mixture of the silver and sucrose solutions, respectively. The other end of the tube was then capped and the tube rotated periodically until formation of a uniform metallic silver coating was observed (typically about 15 min). The coating solution was then drained from the tube and the tube was rinsed with distilled water and air-dried. When dry, the coated tube was cut into individual tubes of the desired length. Discharge Cassette. The cassette shown in Figure 2 was designed and constructed in order to facilitate the insertion and removal of discharge tubes from the discharge circuit. Besides providing electrical connections to the main discharge circuit, the cassette also serves as a physical anchor for the discharge tube and allows convenient observation of the plasma. Electrical connection to the silver thin film is made through the use of hollow graphite electrodes (marked A), which press-fit into each end of the discharge tube. These electrodes press-fit into brass electrode blocks (marked B) and are further secured by polycarbonate shields (marked C). The polycarbonate shields provide additional electrical insulation as well as some support for the discharge tube. Each electrode block is anchored to and makes electrical con-
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986 Discharge Circuit
1-i
E LE C T RONlC S
1 Mon oc hr orna to r D,
R3
Flgure 4. Apparatus block diagram
QL
Figure 3. Simplified discharge circuit schematic. Component values are as follows: D, 12 kV, 400 mA (InternationalRectifier 50MV120); T,, 0-120 V, 2.25 A, variable transformer; T,, 15 kV, 30 mA neon sign transformer (Jefferson Electric); R, 100 MR, 5 W, 1%, 22.5 kV (Dale Electronics ROX-3); R,,,, 100 kn, 100 W (ClarostatVK-1004);R,, 250 MR, 5 W, 1%, 22.5 kV (Dale Electronics ROX-3); R, 2.49 kR, 1%; S,, gravity-operated spark-gap switch; S,, gravltydperatedsafety relay; DVM, 200-mV full-scale digital panel meter (Simpson 24500); DT, plasma discharge tube; C, capacitor bank.
nection with a brass support electrode (marked D) via two brass screws (marked E). Each electrode is electrically insulated from the other by two polycarbonate blocks (marked F), which also provide structural support for the cassette. Electrical connection to the capacitive discharge circuit is made by three hemispherical "dimples" (marked G ) machined into the bottom of the brass support electrodes. These "dimples" mate with three raised brass hemispherical electrodes located in the discharge chamber. The heights of these electrodes are adjustable and are also used for crude alignment of the cassette within the discharge chamber. Discharge Chamber. The discharge chamber was designed and constructed so as to provide electrical, atmospheric, and acoustic isolation of the discharge. The chamber is similar to that described previously by Suh (34). The main chamber body is a nylon tube (17.8 cm o.d., 14.0 cm i.d., 20.3 cm length) that is 0-ring-sealed to a fixed acrylic base plate and a removable acrylic cover plate (both 22.9 cm in diameter and 1.9 cm thick). Two 50.8-mm-diameter, 6.35-mm-thick observation windows (SI-UV Quartz, ESCO Products, Inc., Oak Ridge, NJ) are mounted on nylon extension tubes along the discharge tube axis. Two gas-flow ports are provided for introduction and exhaust of filler gas, and two electrode connectors on the bottom of the chamber are provided for connection to the capacitive discharge source. The entire chamber is mounted on an adjustable stage, which allows continuous vertical adjustment of the chamber. Discharge Circuit. The capacitive discharge circuit is shown in Figure 3 and is similar to the exploding thin-film discharge circuit described by Suh, Collins, and Sacks (35). Two capacitor banks, C, are available for use with this system. The first bank consists of four 200-pF, 3000-V capacitors (Sprague Electric Co., Waltham, MA). Each of two pairs of these capacitors are hardwired in series (100-MR bias resistors are wired in parallel with the capacitors) so as to effectively provide two 100-pF capacitances capable of 6000-V charging potentials. These capacitor pairs can also be connected in series with each other, providing a 50-pF, 12000-V configuration. The second bank consists of two 12-pF, 10000-V capacitors (Axel Electronics, Jamaica, NY). These capacitors were used either individually (at their rated values) or in parallel (at 24 pF, 10000 V). Switching of the discharge was accomplished by using a simple gravity-operated spark-gap switch (35). Electrical connection of the capacitor bank and discharge chamber to the discharge circuit was accomplished with 10-gauge solid copper wire and high-current connectors (Hampden Engineering Corp., East Longmeadow, MA). The wire was insulated with heavy-walled Tygon tubing. Connecting wires were kept as short as possible in order to minimize the residual circuit inductance. Optical and Electrical Monitoring. A block diagram of the apparatus is shown in Figure 4. Electrical properties of the discharge were monitored with high-voltage and high-current probes: discharge current waveforms were measared by using a
L
Hornontol
SilCQ
Y
Vertical SllCQ
Flgure 5. Spatially resolving optics. Component values are as follows: M,, spherical concave, 50 mm diameter, 500 mm focal length (Melles Griot); S,,500-km external slit (at tangential image); S,, 100-pm monochromator entrance slit (at sagittal image). M, and M, are 261
mm apart (along the optical axis) with a 10' angle of off-axis illumination resulting in a 30.6-mm separation between the tangential and sagittal foci. Rogowski coil (Pearson, Model 1025); plasma voltage waveforms were obtained by subtracting voltage waveforms measured at each chamber electrode with a high-voltage probe (Tektronix, Model P1099). Observation of emission from the plasma was always made along the axis of the discharge tube. Spatially and temporally integrated spectra were obtained with a 1-m plane-grating Czerny-Turner spectrograph (Jarrell-Ash, Model 75-150) having a first-order reciprocal linear dispersion of 0.8 nm/mm and recorded using 10.2 cm X 25.4 cm Kodak SA-1 photographic plates. The plates were developed according to the manufacturer's instructions. A microphotometer (Jarrell-Ash, Model 2100-A), modified to allow scanning, was used to observe and record the photographically obtained spectra. Spatially integrated single-wavelengthemission profiles were obtained using a 0.64-m Czerny-Turner monochromator (Instruments SA, Model HR-640) having a first-order reciprocal linear dispersion of 0.5 nm/mm. Photoelectric detection was accomplished by using a Hammamatsu 1P28A photomultiplier tube biased by a Fluke Model 410B regulated high-voltage power supply. For all spatially integrated experiments, the discharge chamber was placed on the optical rail at a distance suitable for ensuring that the spectrograph/monochromator observed emission from the entire plasma volume. Temporally and spatially resolved single-wavelength emission profiles were obtained by use of the optical system shown in Figure 5. A neutral density filter was typically placed just before the monochromator entrance slit in order to prevent saturation of the photomultiplier tube detector by the intense light pulse generated by the plasma. This optical system provides spatial zone discrimination by using the astigmatism generated by an over-and-under mirror configuration and is described in detail by Salmon and Holcombe (36). An external slit (Sl) is placed at the tangential focus of the mirror pair, thus allowing selection of a horizontal slice of the emission from the plasma-the location of the slice selected is determined by vertical translation of either the discharge chamber or external slit S1. The entrance slit of
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
Table I. Discharge Tube Properties and Discharge Conditions parameter
description Discharge Tube Properties
substrate thin film dimensions, cm film mass, mg film thickness," pm resistance, Q
polycarbonate silver 3.8 (length) X 1.3 (0.d.) X 0.95 (i.d.) 3.2 0.28 1-2
Discharge Conditions
charging voltage, kV capacitance, p F inductance,*pH energy, J support gas
pressure
4-8 12, 24, 50 5.6 96-1600
argon atmospheric
Calculated from bulk density. Early studies used longer cables. resulting in 15-uH residual inductance. ~
the monochromator (S2) is placed at the sagittal focus of the mirror pair and selects a vertical slice of the plasma emission. Since horizontal discrimination is determined by the spectrometer entrance slit and both the discharge chamber and spectrometer were fixed horizontally, the spectrometer observed a horizontal emission component only from the center of the plasma discharge. All temporally resolved waveforms (i.e., discharge current, plasma voltage, emission intensity) were monitored with a Nicolet Model 4094-2 digital storage oscilloscope and recorded on floppy disk media using a Nicolet Model F-43 floppy disk recorder. All waveforms were acquired at a sampling rate of 2 MS/s. Discharge current waveforms were acquired simultaneously with all other waveforms as a time reference. All emission profiles presented are averages of four replicate acquisitions. Experimental Procedures. Table I lists the properties of the imploding thin-film discharge tubes as well as typical discharge conditions. Solid powder samples were prepared as suspensions in isopropyl alcohol and deposited on the interior walls of the discharge tubes using a variable micropipette. The alcohol was then evaporated by placing the discharge tubes in a drying oven for a few minutes. Materials and Reagents. All graphite electrodes were machined from spectroscopic grade graphite rods (Ultra-Carbon,Bay City, MI). All reagents and powder samples were reagent grade. R E S U L T S AND DISCUSSION Thin-Film Production. Initiation of an imploding thinf i plasma requires a thin conductive film coated on the inner surface of a nonconducting substrate. The thin film should be of low mass and high purity so that it will produce a well-characterized spectral background of minimum intensity. The substrate should be easily machinable, have a very high electrical resistance, possess a smooth inner surface, and be able to withstand the intense shock waves generated during an implosion discharge. Our initial studies centered around the production and evaluation of silver thin films. Silver was chosen as a thin-film material for a number of reasons. First, many of the imploding thin-film studies reported previously utilized silver thin films (11,13,14);second, the production of thin conducting silver films is easily accomplished by a simple chemical reduction procedure (33); and last, silver thin films have been wellcharacterized in the spectrochemical application of exploding thin-film plasmas (23-25, 37-39). A number of substrate materials were investigated with respect to their utility as discharge tubes. Since the chemical coating procedure for silver is most commonly used on glass, glass tubes were the first substrates investigated; acrylic, nylon, Teflon, and polycarbonate tubes were evaluated as well. As expected, the silver coating procedure produced a very uni-
3111
form, mirrorlike silver film on the glass substrates. Silver films also formed on both the polycarbonate and acrylic substrates, but neither the Teflon or nylon substrate was coated with a silver film when treated with the coating solution. The films formed on the acrylic substrates adhered only very weakly and would flake off when rinsed with water. Both the glass and polycarbonate substrates were deemed suitable for further study as imploding silver thin-film plasma cells. Although the glass tubes were coated with what appeared to be a more uniform silver thin film than were the polycarbonate tubes, polycarbonate was the obvious substrate of choice due to its greater electrical insulating properties as well as superior physical strength and machinability. In spite of the obvious choice, discharges using glass substrates were initially characterized along with discharges using polycarbonate substrates. Although the production of silver thin-film discharge tubes using the chemical reduction method is reasonably straightforward, it results in relatively thick silver films of variable uniformity. The typical film thickness required to produce a uniform conducting film of silver is approximately 0.3 pm. This corresponds to a thin-film mass of about 3 mg for a 0.95-cm-i.d. X 3.8-cm-long discharge tube-this is about an order of magnitude more massive than films produced by vacuum-deposition techniques. Furthermore, it is difficult to accurately control the batch-to-batch uniformity of the thickness of the thin films produced by this method. Attempts to produce thin conducting films using other methods (e.g., electroless plating techniques (40) or thermal decomposition of metalloorganic resins (41)),however, were not successful. The simple chemical reduction of silver process produced the highest quality conductive films and was used for all of the studies reported in this paper. Initial Discharge Characterization: Physical Observations. After the discharge tube was secured in the cassette and the cassette was placed into the chamber, argon gas was allowed to flush (ca. 3 L/min) the chamber for at least 2 min. The capacitor bank was then charged to the desired voltage and the discharge initiated. The discharge produces a sharp retort when confined within the discharge chamber. Accompanying the retort is an extremely bright flash of light from the imploding plasma generated in the discharge tube. The cassette remains physically unaffected, despite the energy dissipated by the discharge. Discharge tubes using silver thin films coated on both polycarbonate and glass substrates were initially investigated with regard to the physical changes induced by discharges of increasing total energy. Discharges using the 50-kF capacitor bank configuration were used to generate imploding thin-film plasmas at initial capacitor bank voltages from 4000 to 8000 V (400-1600 J total energy). These studies utilized both glass and polycarbonate tubes having a 9.5 mm i.d. and a 1.6 mm wall thickness. The silver thin film was completely scavenged by even the lowest energy discharge, leaving a residue only where the electrodes made contact. Surprisingly, the glass discharge tubes were able to survive all but the most energetic of the discharges (at which point the tube shattered during the discharge). The fragile nature of the glass tubes, however, was evident by observation of significant cracking patterns on all tubes subjected to the capacitive discharge. The silver thin film was also completely scavenged off of the polycarbonate discharge tubes. The polycarbonate tubes easily withstood even the most energetic discharge with minimal effects. In fact, it was possible to clean, recoat, and use the tubes over again-this was found not to be practical as it was observed that a small ring of polycarbonate was etched from the tube near the ends of each electrode by each discharge. This slight change in the surface of the tube often
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$? 53 + a
0 ~
look --A Flgure 7. Microphotometer trace of a spatially and temporally integrated spectrum from an 8-kV, 50-pF discharge with a 1-gg sample of pure vanadium powder: (A) Ag(I), 328.07 nm; (6)V(I), 318.34 nm, 318.40 nm, 318.54 nm; (C) V(II), 309.31 nm, 310.23 nm, 311.07 nm,
311.84 nm.
W
u i
0
30
60
90
I
1
120
1%
Time, ps Flgure 6. Current, voltage, and power waveforms for a 6-kV, 24-wF
discharge. created an electrical discontinuity in the thin film, causing arcing in the tube when the capacitor bank was charged. Based on the results of these initial studies, fresh polycarbonate substrates were used for all further studies. Electrical Characteristics. The electrical waveforms recorded for a typical imploding plasma discharge (6 kV, 24 wF) are shown in Figure 6. The current waveform is characteristic of an underdamped RCL tank circuit and is similar in amplitude and duration to current waveforms reported for exploding thin-film plasmas (3). A slight “ k i n k in the waveform is observable very early in the discharge. Temporally coincident with this “kink” is a large spike in the voltage waveform. These features are probably due to temporary disruption of current flow as the rapidly vaporized silver film undergoes dielectric breakdown-accordingly, they mark the time of plasma formation. The waveform of the power dissipated in the plasma was obtained by multiplication of the current and voltage waveforms. The maximum power dissipation is observed at a time corresponding to the first current maximum and is typically in the megawatt range. If we assume that the plasma occupies the entire volume of the discharge tube at this point in the discharge (a “worst-case” scenario), then the power densities produced by these plasmas are in the megawatt per cubic centimeter range-thus, we would expect very good solid sampling conditions. Temporally Integrated Spectra. Initial characterization of the optical emission properties of the imploding plasma source involved recording spatially and temporally integrated spectra. Our immediate concern was whether a powder sample deposited inside the discharge tube prior to initiation of the discharge would be sampled by the imploding plasma. Vanadium was chosen as a test sample due to its ready availability in a number of fairly refractory compound forms that would be difficult to analyze directly with most commonly used atom cells.
Shown in Figure 7 is a microphotometer trace of a portion of the spectrum recorded for an 8-kV, 50-wF discharge with 1pg of pure vanadium powder deposited inside the discharge tube. The vanadium ion lines around 310 nm (marked C) are clearly visible as are vanadium neutral atom emission lines around 318 nm (marked B), indicating that the vanadium powder has been sampled by the plasma. Also shown is the complete self-reversal of one of the silver neutral resonance lines at 328.07 nm (marked A); this is expected due to the large mass of silver vaporized during the discharge. It is also clear that the detectability of vanadium is quite poor due to the extremely high continuum background emission. It is interesting to note that no emission from either argon ion or neutral atom species is observed in the spectral region shown in Figure 7 as well as in the entire spectral region between 240 and 440 nm, whereas emission from numerous silver ion and neutral atom species is evident. This indicates that the plasma formed is composed entirely of thin-film material (as well as analyte). This is contrasted with observations of exploding thin-film plasmas in which argon ion lines are readily observed under similar discharge conditions ( 3 ) . We expect, then, that dielectric breakdown and, hence, plasma formation occur solely through the thin-film vapor and not through the argon support gas. This suggests that the support gas composition has very little direct influence on the characteristics of imploding thin-film plasmas. This was investigated by recording spectra from discharges initiated using helium as a filler gas. Qualitatively, these spectra are quite similar to those obtained in an argon atmosphere with the exception that a lower continuum background emission is observed with the helium atmosphere. Emission intensities from discharges in helium also appear to be less reproducible than argon discharge emission intensities. Temporally and spatially integrated spectra were also recorded for discharges performed with glass substrates. The spectra obtained were qualitatively similar to those obtained with polycarbonate discharge tubes with one exception: no cyanogen banding was observed with the glass discharge tubes. One would not expect to observe cyanogen bands with either the glass or polycarbonate discharge tubes as the discharges are performed in a nitrogen-free atmosphere; however, cyanogen banding (around 385 nm) was observed with the polycarbonate tubes. The absence of these bands with the glass tubes implies that the cyanogen is formed as a result of the plasma sampling the walls of the polycarbonate discharge tube (nitrogen is present in polycarbonate as a solvent impurity from the polymerization process). This is indirect evidence for expansion of the plasma within the discharge tube after implosion of the plasma. Temporally Resolved Spectra. In order to better understand how the plasma emission varies with time as well as obtain quantitative emission intensity information, emission intensity vs. time waveforms for a number of different species
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0
10
20
30
40
50
Time, p s
Flgure 9. Time-resolved continuum emission profiles measured at 275.0 nm during the first current half-cycle at various displacements
from the center of the discharge tube for an 8-kV, 12-pF discharge: (a) 3 mm (outer edge of discharge tube), (b) 2 mm, (c) 1 mm, (d) 0 mm (center of discharge tube).
Time. usec
Figure 8. Time-resolved emission profiles from a 6-kV, 50-pF dis-
charge. Net line (solid),continuum background (dashed),and line-tobackground ratio (dotted)profiles are recorded at (a) 318.39 nm, V(I), and (b) 310.23 nm, V(I1). A 1-wg sample of V as V,O, was deposited in discharge tubes for net line profiles. were recorded. Figure 8 shows emission profiles (line minus background) from vanadium neutral atom (Figure 8a) and ion (Figure 8b) lines. These waveforms were obtained by using a 6-kV, 50-pF discharge with 1-pg samples of vanadium as pure Vz06powder. Corresponding background emission profiles (obtained at the same wavelength without any sample deposited) and line-to-background ratios are shown as well. As expected, both net line and continuum background emission profiles oscillate with the discharge current (Figure 8c). Curiously, the background emission during the first current half-cycle is not greater than during the second half-cycle. In fact, the background emission measured a t the vanadium ion line wavelength is double-peaked during the first current half-cycle. This behavior is only observed with more energetic discharges (it is not observed, for example, with a 4-kV, 50-pF discharge) and is probably related to the implosion process. It is also interesting to note that there is a net negative line emission (i.e., absorption) early in time during the discharge for both atomic and ionic species. This suggests that there are significant populations of unexcited neutral atom and ionic analyte species a t the ends of the discharge tube. The selfreversal of analyte lines early in time during the discharge results in a net lowering of overall analyte line-to-background ratios. For both atomic and neutral atom species, very low line-to-background ratios are observed when the radiation is integrated over the entire discharge. From the line-to-background ratio waveforms for both species it is apparent that time-gating late in the discharge should result in an improvement in powers of detection. We were also interested in what effect the variability in thin-film thickness had on the reproducibility of the emission from the plasma. For this study, four emission profiles were obtained with no analyte (i.e., only continuum background
emission) and were integrated over the entire discharge. The relative standard deviation in this integrated background signal was typically 5%-this is comparable to the precision reported for integrated background signals with exploding thin-film sources that utilized vacuum-deposited thin films. In addition, comparison of replicate background emission profiles revealed very consistent temporal emission characteristics in spite of the use of the chemical thin-film deposition procedure. Implosion Processes: Qualitative Observations. In order to better understand the dynamics of the implosion process, a series of spatially and temporally resolved spectroscopic investigations were performed. Our initial concerns regarded verification that we were, in fact, investigating an imploding plasma. For these studies, then, temporally resolved continuum emission intensity profiles were measured a t a number of vertical locations in the discharge tube. Temporal differences between the first appearance of radiation at each vertical position were recorded and correlated with displacement of the observation zone from the center of the discharge tube. Initial studies monitored continuum emission profiles a t seven locations across the entire diameter of the discharge tube. Plots of continuum emission appearance times vs. displacement from the center axis of the discharge tube indicated that the plasma implodes symmetrically with a constant implosion velocity. Furthermore, the implosion occurs quite early in time, typically early in the first current half-cycle of the discharge. Due to the symmetry of the implosion process, all further studies monitored emission from vertical zones between the center of the discharge tube and one edge only. Figure 9 shows continuum emission profiles measured at four vertical positions in the discharge tube using an 8-kV, 12-pF discharge. Each profile shows continuum emission characteristics recorded during the first current half-cycle for a 500-pm horizontal slice of the plasma. Each zone is displaced from the central axis of the discharge tube in 1-mm increments, where 0 mm corresponds to the center of the discharge tube and 3 mm the outer edge of the tube. Propagation of the plasma from the outer edge of the discharge tube toward the center is clearly demonstrated by the increase in emission appearance time as the displacement from the center of the discharge tube is decreased. A linear plot is obtained if the appearance time is plotted vs. vertical poistion, corresponding to a constant implosion velocity of about 300 m/s. The plasma, however, does not implode as a narrow cylinder but, rather, seems to fill the discharge tube. If the plasma were imploding as a narrow cylindrical shell, one would expect
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
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Time, p s
Figure 10. Time-resolved continuum emission profiles measured over the entire discharge at various displacements from the center of the discharge tube. Discharge condRions and spatial locationsare identical to Figure 9.
to observe a decrease in emission intensity as the plasma shell passes through the observation zone. If the plasma shell was much narrower than the 500-pm observation aperture, then one would expect the plasma to pass through each zone in approximately 2 fis-all of the emission profiles, then, would be narrow peaks with intensities governed by the current at that point in time. This behavior is not observed with any of the emission profiles in Figure 9. Instead of narrow single-peaked bands, broad multiple-peaked emission bands are observed at all four zones. This indicates that the imploding plasma shell is wider than the 500-pm observation window. Further interpretation of the implosion process is complicated, however, by the effects of the varying current flow through the plasma. Since the continuum emission intensity varies with the discharge current, changes in emission intensity are not related solely to plasma propagation characteristics. Thus, detection of the trailing edge of the implosion is semiquantitative at best and must involve interpretation with respect to the discharge current waveform. The trailing edge of the plasma is most clearly observed in the emission profile from the outer edge (waveform a) of the plasma in Figure 9. Emission from this region reaches a maximum before the current maximum, indicating movement of the trailing edge of the plasma out of the zone. The trailing edge of the plasma is more difficult to discern in the emission from the next region (waveform b) due to the temporal coincidence of both the current and emission peaks, but the location of the trailing edge can be approximated. Based on these two observations, the implosion velocity of the trailing edge of the plasma is estimated to be 100 m/s. From these observations of the leading and trailing edges of the imploding cylindrical plasma, we estimate the width of the plasma shell at the time of implosion t o be approximately equal to the 3-mm radius of the discharge tube itself. This results in a very turbulent implosion in which the aftershock from the implosion of the plasma's leading edge can hinder the propagation of the rest of the plasma shell toward the center of the discharge tube. Thus, after the leading edge of the plasma implodes, the emission profiles a t all spatial locations become quite similar both qualitatively and quantitatively, indicating that the plasma fills the entire discharge tube volume. This behavior is seen in Figure 9 at times greater than about 30 p s . The long-term postimplosion characteristics of the plasma
may be observed in the spatially resolved continuum emission profiles shown in Figure 10. Here, emission profiles are presented for the entire time span of the discharge for the identical experiment presented in Figure 9. After the plasma implosion (early in the first current half-cycle), there is very little difference between the spatially resolved emission profiles. Qualitatively, emission intensities from all locations oscillate with the discharge current. The only significant quantitative differences are observed a t times corresponding to peaks in the discharge current-no conclusive trends are observed. I t appears, then, that the plasma simply fills the discharge tube after the initial implosion. This is both beneficial and necessary for efficient atomization of solid samples deposited on the tube wall. Implosion Mechanism. The implosion characteristics of these plasmas may be attributed to two processes: electromagnetic compression (EMC) of the initial thin-film plasma (z-pinch) and shock implosion due to the rapid vaporization of the thin silver film. If EMC were the predominant mechanism, then one would expect the imploding plasma to accelerate toward the center of the discharge tube (16, 17). Our studies show the leading edge of the imploding plasma traveling at a constant velocity-no acceleration was observed. This was investigated further by measuring implosion velocities using three different diameter discharge tubes. Identical implosion velocities were measured for all three tubes (9.5 mm, 12.7 mm, and 15.9 mm diameters) when imploded using identical electrical discharge conditions. Furthermore, EMC would require the generation of a significant magnetic field due to the current flow through the initial hollow thinfilm plasma. The relatively small current passing through the plasma early in the discharge would not produce a significant magnetic pinch pressure. The current required to electromagnetically confine (Bennett pinch condition) a 5000 K plasma having an ion density of 1017 cm-2 is about 4000 A (9)-for the implosion experiments shown in Figures 9 and 10, the plasma is observed to have already begun imploding well before this current is attained. Shock implosion, then, is the most likely mechanism for the implosion of these plasmas. In fact, the implosion velocities measured for these plasmas (ca. 300 m/s) and those reported by Collins and Sacks (39)for unconfined (exploding) thin-film plasmas are very similar. The propagation velocity For exploding thin-film plasmas is closely associated with the initial dI/dt of the discharge current; a similar correlation has been observed with imploding thin-film plasmas (42).
CONCLUSIONS The studies reported here demonstrate the ability of imploding thin-film plasmas to rapidly atomize solid powder samples from the inner wall of the discharge tube. The exceptionally high power densities (in the megawatt per cubic centimeter range), however, result in far from efficient excitation of the analyte atomic vapor. Additionally, initial emission spectroscopic studies show the plasmas to be very heterogeneous in both time and space, suggesting that further information on the physical and analytical characteristics of these plasmas must be extracted usng both temporally and spatially resolved spectroscopic techniques. ACKNOWLEDGMENT We thank Walter Weir and Norman Rosberg for their help with the design and construction of the apparatus. We also thank Victoria Smith for her work on developing methods for conductive thin-film deposition. Registry No. Ag, 7440-22-4; V, 7440-62-2; V'. 14782-33-3. LITERATURE CITED (1) Laqua, K. In Analylical Laser Spectroscopy; Omenetto, N., Ed : Wtley: New York. 1979
Anal. Chem. 1966, 58,3115-3121 Waiters, J. P. Science (Washington, D . C . ) 1977, 198, 787-797. Sacks, R. D.; Goldberg, J. M.; Collins, R. J.; Suh, S. Y. Prog. Anal. At. Spectrosc. 1982,5 , 111-154. Scott, R. H. Spectrochim. Acta, Part 8 1978,338, 123-125. Ishizuka, T.; Uwamino, Y. Spectrochim. Acta, Part 8 1983, 388, 519-527. Ishizuka. T.; Uwamino, Y. Anal. Chem. 1980,52, 125-129. Bennett, W. H. Phys. Rev. 1934,4 5 , 890-897. Anderson, 0. A.; Baker, W. R.; Colgate, S. A,; Ise, J., Jr.; Pyie, R. V. Phys.Rev.1958, 110,1375-1387. Chen, F. F. Introduction to Plasma Physics; Plenum: New York, 1974. Shiioh. J.; Fisher, A.; Rostoker, N. Phys. Rev. Lett. 1978, 4 0 , 515-518. Dennen, R. S.;Wilson, L. N. I n Exploding Wires; Chase, W. G., Moore, H. K.. Eds.; Plenum: New York, 1962; Vol. 2. Vitkovsky, I.M.; Bey, P. P.; Faust, W. R.; Fulper. R.. Jr.; Leavitt, G. E.; Shipman, J. D., Jr. I n Exploding Wires; Chase, W. G . , Moore, H. K., Eds.; Plenum: New York, 1962; Vol. 2. Nash, C. P.; DeSieno, R. P.;Olsen, C. W. I n Exploding Wires; Chase, W. G., Moore. H. K., Eds.; Plenum: New York, 1964; Vol. 3. Schenk, G.; Linhart, J. G. I n Exploding Wires; Chase, W. G., Moore, H. K., Eds.; Plenum: New York, 1964; Vol. 3. Stallings. C.; Nieisen, K.; Schneider, R. Appi. Phys. Lett. 1978, 2 9 , 404-406. Turchi, P. J.; Baker, W. L. J . Appi. Phys. 1973,4 4 , 4936-4945. Baker, W. L.; Clark, M. C.; Degnan, J. H.; Kiuttu, G. F.; McClenahan, C. R.; Reinovsky, R. E. J . Appi. Phys. 1978. 4 9 , 4694-4706. Lau, J. H.; Gupta, R. P.; Kekez, M. M.; Lougheed, G. D. I n Proceedings of the 1982 I€€€ International Conference on Plasma Science; IEEE: New York, 1982. Kekez. M. M.; Gupta, R. P.; Lau, J. H.; Lougheed, G. D. I n Proceedings of the 1982 I€€€ International Conference on Plasma Science; IEEE: New York, 1982. Cnare, E. C. J . Appl. Phys. 1966,3 7 , 3812-3816. Freeman, J. R.; Cnare. E. C.; Waag, R. C. Appi. Phys. Lett. 1967, IO, 111-113. Kachiiia, D.; Herlach, F.; Erber, T. Rev. Sci. Instrum. 1970,4 1 , 1-7. Clark, E. M.; Sacks, R. D. Spectrochim. Acta, Part 8 1980, 358, 471-488. Goldberg, J.; Sacks, R. Anal. Chem. 1982,5 4 , 2179-2188. Swan, J. M.; Sacks, R. D. Anal. Chem. 1985,57, 1261-1264.
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(26) Goode, S. R.; Pipes, D. T. Spectrochim. Acta, Part 8 1981,368, 925-929. (27) Kamia, 0. J.; Scheeline, A. Anal. Chem. 1986,5 8 , 923-932. (28) Kamla, G. J.; Scheeline, A. Anal. Chem. 1986,58, 932-939. (29) Albers, D.; Johnson, E.: Tisack, M.; Sacks, R . Appi. Spectrosc. 1986, 4 0 , 60-70. (30) Albers, D.; Sacks, R. Spectrochim. Acta, Part 8 1986, 4 1 8 , 39 1-402. (31) Coleman, D. A.; Sainz, M. A.; Butler, H. T. Anal. Chem. 1980,52, 746-753. (32) Carney, K. P.; Goidberg, J. M. Anal. Chem., following paper in this issue. (33) Sugden, S. J . Chem. SOC.1933,768-776. (34) Suh, S.Y. Ph.D. Thesis, University of Michigan, Ann Arbor, MI, 1980. (35) Suh. S. Y.; Collins, R. J.; Sacks, R. D. Appi. Spectrosc. 1981, 35, 42-52. (36) Salmon, S.G.; Hoicombe, J. A. Anal. Chem. 1978,5 0 , 1714-1716. (37) Swan, J. M.; Sacks, R. D. Spectrochim. Acta, Part 8 1985, 4 0 8 , 1239-1254. (38) Suh, S. Y.; Sacks, R. D. Spectrochim. Acta, Part 8 1981, 368, 1081-1 098. (39) Collins, R. J.; Sacks, R. D. Anal. Chem. 1983,55, 2036-2043. (40) Pearlstein. F. I n Modern Electroplating, 3rd ed.; Lowenheim, F. A,, Ed.; Wiley: New York, 1974. (41) Englehard Data Sheet Pure Metal Resinates; Englehard Corp.: East Newark, NJ. (42) Carney, K. P.; Goidberg, J. M., Department of Chemistry, University of Vermont, Burlington, VT, unpublished research.
RECEIVED for review June 25,1986. Accepted August 7,1986. We gratefully acknowledge financial support from a Research Corporation Cottrell Research Grant, the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the University of Vermont Committee on Research and Scholarship. We also are grateful to BASF Wyandotte Corporation for donation of the microphotometer and to IBM for donation of the spectrograph.
Analytical Characterization of an Imploding Thin-Film Plasma Using Spatially and Temporally Resolved Spectrometry Kevin P. Carney and Joel M. Goldberg* Department of Chemistry, Uniuersity of Vermont, Burlington, Vermont 05405 The analytlcal utility of emission from an Imploding thin-fllm plasma atom cell for the dlrect atomlc spectrochemlcal analysls of solid samples Is evaluated. Time-resolved emlsslon profiles of spatlally Integrated radiation from the plasma reveal a very intense continuum background as well as reversal of analyte Ion and neutral atom llnes early in time. Timegating late in the discharge Improves analyte Ilne-tobackground ratlos. Spatially resolved spectrographic detection of plasma emission shows that the hlgh continuum background and analyte line reversal are localized In the center of the discharge tube, while vapor expelled from the tube produces a well-defined analyte spectrum In regions outslde of the confined plasma. Line-to-background ratios obtained by temporally resolved observation of emlsslon from outside of the plasma discharge tube were much improved over spatially Integrated values; however, the IrreproduclMlIty and lack of mass dependence of the analyte emisslon from thls outslde region preclude any analytical use. Temporally resolved measurement of the degree of analyte line reversal In the center of the plasma discharge tube is found to be the most analytically useful of all of the measurement techniques investigated wlth this Imploding plasma source.
Interest in direct solid sampling for atomic spectrochemical analysis has stimulated the investigation of novel high power
density plasmas as potential solid sample atom cells (1-7). In a companion report (8),we have presented the results of initial studies of the characteristics of an imploding thin silver film plasma suitable for direct solid sampling methods. Peak power densities in the megawatt per cubic centimeter range were reported, and sampling of solid powders deposited inside the tube prior to initiation of the plasma discharge was demonstrated. Initial spectroscopic studies, however, demonstrated poor powers of detection due to an extremely intense continuum background emission as well as significant selfabsorption of analyte emission lines. The spatial and temporal heterogeneity of emission from the plasma, though, indicate that powers of detection may be enhanced through the use of time-gating and spatial-masking techniques. This report, then, presents the results of a series of spectroscopic investigations of the analytical utility of emission from imploding thin-film plasma atom cells. Both spatially and temporally resolved spectroscopic techniques were used in order to better understand the emission characteristics of these plasmas.
EXPERIMENTAL SECTION All experimental conditions were as described in the accompanying paper (8) with the exception of the following. Optical and Electrical Monitoring. For the acquisition of spatially resolved but temporally integrated spectra, an overand-under mirror optical system was used to image the plasma
0003-2700/86/0358-3115$01.50/00 1986 American Chemical Society
