AMI. Chem. 1007, 59, 305-309
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Sampling and Excitation of Refractory Solids with a Theta Pinch Discharge Designed as an Atomic Emission Source Jeffrey S.'White a n d Alexander Scheeline* School of Chemical Sciences, University of Illinois, 1209 West California Street, 79 RAL, Box 48, Urbana, Illinois 61801
Work wlth a theta plnch derlgned as an atomk emkdon w r c e for rdkb analyrh b reported. Argon at 3.5 ton pre canpared to aevwal Wed the mod htw3yle other gases. A study of ttw effects ol samp+e P0r)tknlng p r o m furlher unckratandlng of the plasma motion and plama/sampk hteredknr. W t k m of the new sample p&Monhg knowtadgo h w rmdted In d@kaftl hcreasm In analyte emtrrlon, as b demonstrated by use of sample8 of tmQsten powder, boron nmb,snd dunhun 0cercwnlcr.
sampling. This is important, as the parameter being monitored was the analyte line emiseion intensity. This assumption may not be correct, as the excitation efficiency may vary with gas composition and pressure. Deconvolution of the effects of excitation changee from t h w of sampling changes cannot rigorously be done with time-integrated data, nor unless absorption as well as emission are measured. The study of sample placement has resulted in a better understanding of the plasma motion and plasma/solid interactions that are taking place during the discharge. With this understanding, improvements in the degree of sampling have been realized. Example spectra are presented.
A theta pinch discharge uses magnetic compression to produce high-temperature and highdensity plasmas. The goal of this research is to develop a theta pinch that is capable of sampling and exciting solid materials directly, with minimal sample preparation. Of particular interest is the ability to sample bulk amounts of nonconducting materials that are not amenable to the more familiar solid sampling techniques of high voltage sparks and arc8 (I) or that are inappropriate for sampling with a laser microprobe due to heterogeneity (2). Previously, a theta pinch was designed and constructed for this purpose, and electrical and spectral characterization were begun as reported ( 3 , 4 ) . Perhaps the most interesting aspect reported in these earlier papers was the time dependence of the analyte and background emission. All ionic and continuum emission is observed only during current conduction (- 100 ps) in the main discharge. Emission from neutral species, especially the analyte, lasts much longer (-500 p s ) . Thus, the use of time discrimination is beneficial, since higher signal-to-background ratios can be obtained. The spectra presented in this paper, however, are without the benefit of time discrimination and thus were not obtained under the optimum conditions for analysis. Specifically, continuum emission and ionic emission are more prominent in the spectra shown here than they would be with optimum time gating. Theta pinches have frequently been used as spectroscopic sources, but nearly all reports involved the use of the theta pinch as an excitation source for a previously sampled material (5-8). The first reported use of a theta pinch as a specifically analytical source was by Goode and Pipes in 1981 (5). Increases in atomic emission of 100% were observed for pinched species introduced into a sealed microwave plasma tube, with little or no increase in molecular emission. Recently, Sacks reported the use of a theta pinch to magnetically tailor the plasma produced by a capacitive discharge through a graphite bundle (6). With pinching it was possible to obtain significant increases in the signal-tu-background ratios a t early times in the discharge for analytes deposited onto the graphite bundle. The authors attribute this result to increased sampling of the analyte rather than excitation effects. This paper will report on further studies of the optimum plasma fill gas composition and pressure and considerations of sample placement. These discussions will, in general, pertain to the sampling aspect of the theta pinch process. An important assumption in all following discussions is that a greater analyte emission intensity observed implies more
EXPERIMENTAL SECTION The theta pinch source used in this study, along with typical source parameters, has been described elsewhere (3). Only minor changes in the source were made. First, a 35 mm 0.d. glass or quartz tube was used as the discharge vessel, and second, an oxygen-removing filter (Catalq No. 64-1O00, Matheson Gas Producta, Joliet, IL),a dry iceacetone cold trap, and a glass gas line were added on the gas inlet side of the vacuum system. The filter was only used when hydrogen was the plasma fill gas, but ita continued use with a variety of discharge gases is anticipated. The optical setup used and the manner in which the data were collected were also described in a previous reference (4,with the following exceptions: No time-resolved data were obtained. All spectra are time integrated, and spatially resolved, using 102 mm X 127 mm Kodak Royal-X pan film (Eastman Kodak Co., Rochester, NY)as the detector (9). Kodak rapid fixer was used instead of standard Kodak acid fix, which shortened the fix processing time to 4 min. The entrance slit of the 2-m spectrograph was set to 100 pm except as noted in the figure captions. For all the spectra presented in this paper, one pinch firing was used to obtain the spectrum. The grating for the 2-m spectrograph used to collect the data is blazed at 7500 A. Therefore all of the data were collected in second or third order. Figure 5 contains emission lines observed in both second and third orders.
0003-2700/87/0359-0305$01 S O / O
RESULTS AND DISCUSSION The effect of the plasma fill gas composition and pressure was studied by comparing spectra produced by pinching in each of several pure gases and one mixture of gases. In earlier work only atomic gases, helium and argon, were used as the plasma fill gas. The use of molecular gases was considered for the following reasons: (1) Hydrogen was considered due to the higher plasma temperatures predicted for this gas. This prediction is based on the fact that radiative losses would be much lower for hydrogen, and thus the plasma's energy and temperature would be higher. Further, only a single level of ionization is available for hydrogen. (2) Work of similar nature with exploding thin films by Sacks (10) revealed that a 60140 argon-oxygen mixture provided the most efficient sampling of powders of refractory solids. The use of a similar mixture was attempted with the theta pinch. (3) The possibility of the chemical reactivity of some molecular gases leading to increased sampling was considered. In all, five gas combinations were tested. They include hydrogen, helium, oxygen, argon, and a mixture approximating d 1987 American Chemical Society
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ANALYTICAL CHEMISTRY. VOL. 59,NO. 2, JANUARY 1987
n2 I.00TORR
IC
II
Flour0 1. Comparative spectra of differing plasma RII gases: (a) AI 1. 3944.0058 A; (b) AI I, 3961.5200 A; (c) AI I, 3944.0058 A. and Ar 11, 3944.272 A; (d)Ca 11. 3933.663A; (e) 0 11. 3973.263 A; XI. 3875
A: X2. 4034 A.
that used in the exploding thin film work, a 50/50 oxygenargon mixture by volume. The helium and argon work is repeated from an earlier reference (4) because a new sample positioning method, to be described below, was being used. For all cases, the pressure of the gas was varied to optimize emission intensities of the analyte lines. An aluminum sheet, approximately 35 mm X 22 mm X 1.6 mm, was used as the sample. Figure 1 shows a spectrum for each case, taken a t the pressure which provided the most intern analyte emission for that gas or mixture. A discussion of each case follows. Hydrogen provided the lowest emission intensities of all gases tested. Possible explanations for this result include the following: Sputtering may be the dominant sampling mechanism. Hydrogen's low mass provides poor momentum transfer and thus little sputtering. This hypothesis is supported by previous work (4). Plasma energy losses with a molecular gas, such as vibrational, rotational, and diasociation energies, cause the plasma energy to lower significantly, and consequently reduced emission intensities are observed due to lowered sampling. Helium was observed to be the second most efficient gas of those tested. In general, analyte lines can be qualitatively recognized by their line shapes, which are not broadened as are the plasma fill gas emission lines. At present, there is no explanation for the blooming of some analyte emission lines, seen most clearly for the 3961.5-A aluminum line, line b, in the helium-excited spectrum. Line d correspondsto emission from calcium, which is not present in the aluminum samples used. This emission may be more likely ascribed to a contamination, such as chalk dust, due.to its constant presence, and ohsentation of similar emission in a blank spectrum. The aluminum emission intensity observed when oxygen is the plasma fill gas is comparable to or slightly less than that when helium is used. If sputtering is the dominant sampling mechanism, then it would be reasonable to expect increased line intensitiesfrom the increased sampling due to the heavier oxygen atoms compared to helium atoms. Possible explanations for the reduced sampling are similar to that for hydrogen;
since oxygen is a molecular gas, the additional enegy required for vibrational and rotational excitation and for molecular dissociations lowers the plasma energy significantly. Another passibility is that oxygen may passivate the aluminum sample through the formation of aluminum oxide and, thus, limit sampling. In any event, chemical reactivity of the plasma fill gas leading to increased sampling is not observed for oxygen or hydrogen. The oxygen/argon combination provides lower emission intensities than those for either pure component. The 3961.5-A AI I line is barely observable. This mixture of gases was found to be effective for sampling of refractory solids in previous work with exploding thin films (10). The reason for the decrease in emission intensity with the theta pinch for this mixture of gases is not understood. Welding grade argon is clearly the most efficient gas for sampling ofthcae tested. The 3961.5-A AI I line appears very bright and narrow (AX < 0.1 A). With the use of time discrimination (4), it appears that favorable conditions for analysis are passible with this gas. A significant characteristic of argon is that it is the highest atomic weight nonmolecular gas tested. This suggests the use of krypton or xenon as the plasma fll gas would be beneficial,although these gases would be prohibitively costly for routine use. Since all atomic gases performed better than molecular gases, it appears that the plasma energy losses for molecular processes are a more significant lowering factor in emission intensity than the positive factor from increased sampling with heavier atoms for the range of atomic weights of the gas species used. For the majority of the earlier characterization work, aluminum sheets, approximately 35 mm X 20 mm X 1.6 mm, were used as the samples. In most cases the sheets were placed in the discharge vessel and thus would he touching the vessel walls. This is an undesirable sample position for several reasons. When the main discharge is fired, a cylindrical sheet of current is formed in the plasma along the outer edge of the discharge vessel. A solid lying across the discharge vessel cylinder would prevent the complete formation of this current sheet, therefore limiting plasma compression. This is important as the formation of, and the radial compression of, this current sheet, leading to its subsequent contact with the sample, is considered the primary sampling mechanism. Also, a characteristic of the theta pinch plasma is that the plasma temperature increases as the plasma is compressed. Thus better sampling conditions may result with a sample position more along the central axis of the discharge vessel, since a hotter plasma will be interacting with the solid sample. A sample positioning method was devised in earlier work which allowed the sample to be coaxially supported in the center of the discharge vessel using a support rod. Samples used for this previous work were rods of aluminum, approximately 6.4 mm in diameter, which is also the diameter of the support rod. Only small amounts of analyte emission were seen for these samples In fact, emission was weaker from rod samples than from sheets placed in the discharge vessel, even though it was expected the sheet placement was a poorer sample position. A pceaibility whicb could account for this observation was that the plasma current sheet was not being compressed sufficiently to contact the aample during the pinch process. The support mechanism was then modified to hold sheets of aluminum instead of rods. Figure 2 shows two different width sheets that were both exposed to 21 pinch firings in the new sample position (i.e., suspended along the central axis of the discharge vessel). The 22-mm sheet shows the effect of sampling, with evident sample damage apparently due to the plasma current sheet striking the sheet. Intense aluminum emission was observed for these pinches. Although weak aluminum emission was visible, the 8-mm sheet does not
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8 mm Appearance of dmerenl widm a h h m sheet samples alter exposure to 21 pinch firings. FIQMO2.
exhibit the degree of sampling damage as observed for the 22-mm sheet. Thus the pinched current sheet must not be collapsing to the &mm width, which explains the low emission levels observed when the 6.4-mm aluminum rod was used as the sample in the earlier experiments. Also, the emission intensity was significantly greater for the coaxially supported sheets than for sheets placed touching the vessel walls. The conclusion is that current sheet formation and compression are occurring, but the compression is not to the degree sought The desired degree of compression is to a diameter of 8-10 mm. Compression to diameters smaller than this is undesirable since sampling of the support rod, used to coaxially suspend the samp1e;would begin to occur. It was also concluded that positioning the sample such that it does not touch the discharge vessel walls provides improved sampling, as proposed, and it is now considered necessary to suspend the sample within the discharge vessel. It is also necessary that some portion of the sample be approximately 10 mm away from the central axis of the vessel. Work on increasing the plasma compression should ease this requirement. The spots on the 22-mm sheet were originally thought to be similar to cathode spots seen with high-voltage sparks. Obervation under a microscope revealed them to be droplets of aluminum that had been melted off the edge of the sheet and then were thrown back onto the sheet‘s surface. This ablation of material from the sheet’s edge was also observed for stainless steel sheet samples, although the drops of molten material were rarely seen. A curious observation is that these drops appear at both edges, but only on the ‘top” of the sheet, regardless of how the sheet is oriented within the discharge vessel. The proposed explanation is that gravity causes material to be deposited only at the edge for which the circular motion of the plasma current sheet carries the material above the sheet. A t the other edge the material is initially swept below the sheet and m o t overcome gravity to hit the bottom of the sheet, or simply does not stick if it does strike the sheet. As a result of the oscillatory nature of the main discharge current waveform, the circular motion of the induced plasma current alternates in direction, which a n explain spots at both edges, but only on one sample surface. The main discharge coil presently used in the theta pinch experiments is of a double helical design. This design was adopted to ensure that tbere would be no axial plasma currents induced from the axial component of the main discharge current. Such plasma currents could be injected into the preionizer circuit, potentially damaging this circuit. It is expected, however, that the induced current, being a second-order effect, will not exceed the capabilities of the preionizer circuit. Therefore, a single helical coil, i.e., a solenoid, which does not prevent the axial coupling, is being designed. This should increase the magnetic field strength relative to the dual helical coil, as is illustrated in Figure 3. Intensities were computed by use of the equations in ref 11. Field is increased in the solenoid as current is not split into the two half coils of the dual helical design: current, and therefore field intensity, does not double because of higher inductance in the solenoid compared to the dual helical design. The field strength is expected to increase from 25 to 35 kG at the sample paition, ignoring effects due to the introduction
HELICAL
O
COIL END
COIL END
L
AXIAL POSITION Flgure 3. Comparlsm of expected magnetic field intenstties for two different types of meta coils.
A
B
i
I
I
l
I
a
b
c d e
f
‘I 1
I
l
l
FIgm 4. Speara winparkg the pmvbus and new sampb posilbning using a tungsten powder sample: spectrum A, previously obtained spectrum; spectrum B. spectrum wiih new sample posnbn; enhance sli. 50 fim; (a) Ar 11. 4237.223 A; (b) W I, 4241.448 A; (c) W I. 4259.357 A (d) W 1. 4263.315 A: (e) W I, 4269.392 A; (f) Ar It, 4277.55 A.
of the sample and the presence of the plasma. The small remaining local minimum at the discharge center results from reduced coil winding density to allow for an optical port.. The larger magnetic field intensities should lead to increased plasma compression, especially a t the sample position, which may ease the sample placement requirement mentioned in a previous section. As mentioned, considerable improvements in sampling have resulted from the positioning of the sample away from the discharge vessel’s walls, and over 10 mm off axis. To demonstrate this point, a comparison of the sampling difference between the previous and this “new” sample positioning was made. A spectrum was taken with a sample of tungsten powder placed on a suspended aluminum sheet and compared to an earlier spectrum with the same sample for which the sheet touched the vessel walls. In the older spectrum, 27 tungsten lines were identified over the approximately 150-A range available in the spectrum, while for the newer spectrum 73 analyte lines were identifiable. Figure 4 shows a portion
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ANALYTICAL CMMISTRY. VOL. 59, NO. 2, JANUARY 1987
A
I
I
II
a bc d
-
gun 5. spectra mng barnem$sion frm a baron sample: spehnnA, blank; specbum E. sample: (a) 11. 3737.893 A; (b) B I. 2496.778 A; (c) B 1. 2497.733 A; (d) Ar 11. 3751.521 A.
Q
D
C
F(gure 8. Speclrs showing aluminum emission from ttw ceramlc ML-60: S p e h m A. blank; specbum E. sample: (a) AI 1. 3944.0058 A. and Ar 11. 3944.272 A; (b) Ai I, 3961.5200 A; (c)Ar 11. 3979.356
A.
Table 1. Percentages of Major Components of Three Types of Aluminum Ceramics MI.-60 AM%
Si02
60.00 36.22
MG80 P
AL
80.00
99.7 0.11
4
Not available.
of each spectrum, which reveals the greater visibility and number of tungsten lines with the new sample position. The emission lines are 20 mm tall for spectra of this type. The exposure was the same for both except that the entrance slit to the spectrograph was set a t 50 am for the new spectrum, rather than a t 100 am for the previous spectrum. With the new sample pmitioning knowledge, an attempt to sample a refractory nonconductor was made, as the direct excitation of such samples is the main goal of this research. The first sample chosen was solid boron nitride, a refractory ceramic. The sample was in the shape of a disk 18 mm in diameter and 2 mm thick. I t was supported at the end of a narrow aluminum sheet in the center of the discharge vessel. The resulting spectra confirming the emission from boron are shown in Figure 5. Along with this neutral atom emission, boron ion emission was also visible. Further spectra were taken with three different types of aluminum oxide ceramic samples (Bolt Technical Ceramica, Conroe, TX). The ceramic types and the percentages of the major components are listed in Table I. As the aluminum oxide concentration increases, the ceramic becomes denser and becomes more difficult to sample. This is evidenced by the decreasing aluminum emission intensity even with increasing aluminum concentration. Figure 6 shows the aluminum emission for the ML-60ceramic. In order to obtain this spectrum, the sample was broken into small pieces, ranging in diameter from approximately 0.1 to 3 mm, and placed on a copper sheet in the same manner as was the powder in the tungsten experiments. This spectrum was obtained with one sampling. To obtain spectra with comparable aluminum emission from the other two ceramics,
Flgwa 7. Spectra showing s l m n emission frm the Wamk ML-80: spectrum A, blank; spectrum E, sample: (a) Si 1. 3905.5227 A.
multiple samplings (up to four) were required. Even so, the emission intensity was often not as intense as for the single sampling of the ML-60. From the same spectrum shown in Figure 6, it was possible to observe emission from silicon from the silicon oxide in the ceramic. Figure 7 shows the portion of the spectrum with the silicon emission. The blank spectrum shows that no silicon emission is observed from the glass that makes up the discharge vessel. This is attributed to the sample position of the ceramic relative to the walls of the discharge vessel.
CONCLUSIONS The capability of the theta pinch source to sample and excite refractory solids. both conductors and nonconductors, has been demonstrated. Analyte emission intensities have been significantly increased by spatial positioning of the sample within the discharge vessel. The present sample position is a coaxially supported sample. Further compression of the plasma to a
Anal. Chem. 1087, 50,309-312
diameter of 8-10 mm has been shown to be desirable.
ACKNOWLEDGMENT The authors wish to thank the following: Bolt Technical Ceramics for the donation of the ceramics used as samples; the glass services shop and the photographic services shop of the School of Chemical Sciences at the University of Illinois for the construction of the glass gas line used in these experiments and for aid in preparation of the figures, respectively; the Eric Oldfield research group at the University of Illinois for the donation of the boron nitride sample. LITERATURE CITED (1) Walters, J. P. Science 1977, 198, 787-797. (2) Carr, J. "4.; Horuck, G. Spectrochim. Acta, Part B 1980, 378(1), 1-15. (3) Kamla, 0. J.; Scheeline, A. Anal. Chem. 1986, 58, 923-932. (4) Kamla, G. J.; Scheellne, A. Anal. Chem. 1986, 58, 932-939.
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M e , S. R.; Pipes, D. T. Spectrochim. Acta, Part B 1981, 368, 925-929. Albers, D.; Johnson, E.; Tlsack, M.; Sacks, R. D. Appl. Spectrosc. 1986, 4 0 , 60-70. Boyer, K.; Elmore, W. C.; Lktle, E. M.; Qulnn, W. E.; Tuck, J. L.; Phys. Rev. 1980, 119, 831-843. Ekdahl, C. A,; Commlsso, R. J.; McKenna, K. F. J. Appl. Phys. 1981, 52, 3245-3248. Sacks, R. D.; Lln. C. S . Appl. Spectrosc. 1979, 33, 258-268. Clark, E. M.; Sacks, R. D. Spectrochlm. Acta, Part 8 1980, 358, 47 1-488. Purcell, E. M. Necfricify and Magnetism; McGraw-Hill: New York, 1965; Vol. 2, p 203.
IVED for review August 1,1986. Accepted September 19, 1986. This work was supported by the Office of Basic Energy Sciences, US. Department of Energy (Grant DE FG0284ER13218). Portions of this work were performed by J.S.W. in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry.
Evaluation of a Voigt Effect Coherent Forward Scattering Atomic Spectrometer L. A. Davis and J. D. Winefordner* Department of Chemistry, University of Florida, Gainesville, Florida 32611
Coherent forward scatterlng (CFS) atomlc spectrometry Involves the rotation of pdsrlzed llght by atoms located wlthln a transverse or a kngnudlnal magnetic fMd, the Volgt and the Faraday effect, respectlvely. A contlnuum source Is used to exclte slmultaneously all atomlc transltlons. A set of polarlzers Is used before and after the atom reservolr to polarlze Inltlally the Incident light and detect the rotated light, respectlvely. Thls study employed both an air-acetylene flame and a graphlte furnace as atom reservoirs. A dc transverse magnetlc field Is employed around the atom reservoir. A monochromator, a photomuttlpller, and a lock-in ampllfler form the detection system. The analytical figures of merlt for several elements were determined by uslng the system. While the lhlts of detectlon (LODs) for the flame were poor, the absolute LODs for the graphite furnace showed some promise In view of the multVelement capabllltles of CFS In analytical chemistry.
The magnetooptic rotation of polarized light by atoms located within a magnetic field was first observed by Hanle in 1924 (I). Church and Hadeishi (2) first suggested the use of coherent forward scattering (CFS) as an analytical technique. Since that time, multielement CFS spectrometry (3-6) has been evaluated by using both transverse and longitudinal magnetic fields, the Voigt effect and the Faraday effect, respectively. Our system employs a transverse magnetic field, the Voigt effect, surrounding either an air-acetylene flame or a graphite furnace as an atom reservoir. One calcite prism polarizer is used to polarize the incident light prior to the atom reservoir, while the second polarizer, the analyzer, is placed after the atom reservoir to detect the incident light that is rotated by the atoms. The coherent forward scatter signal is then detected by sequentially slew-scanningthe spectrometer to the 0003-2700/87/0359-0309$01.50/0
wavelength of the atomic transitions of interest. When atoms are placed in a magnetic field, the atomic transitions experience Zeeman splitting (7,8).In a normal Zeeman splitting pattern, the atomic absorption line splits into three components, one A and two u components. The A component remains at the resonance wavelength and interacts with polarized light whose electric vector is parallel to the magnetic field, while the u components split outside of the resonance wavelength and interact with polarized light whose electric vector is perpendicular to the magndtic field. Many elements do not have the simple normal splitting pattern but rather an anomalous Zeeman pattern in which the ground state and excited states are both split unequally, resulting in more than three components (1-10). In CFS, the first polarizer, placed prior to the atom reservoir; is oriented at a 45O angle to the magnetic field so as to populate equally the ?r and u components (9,lO). The CFS signal is composed of both a dichroic component and a birefringent component (9-1 7). Dichroism is the difference between the absorption of the A and u components, while birefringence is the difference in the refractive indixes of the A and u components. The CFS signal produced is directly proportional to the incident intensity of the source. When a continuum source is used as the excitation source, a CFS signal is produced across the entire profile of the split (normal or anomalous Zeeman splitting) atomic line as well as providing simultaneous excitation of all the atomic transitions; in the case of the line source, the split components may be outside of the source line profile. The quadratic dependence of the CFS signal on the concentration of the analyte (9-17) is measured when the second polarizer, the analyzer, is oriented orthogonallyto the fist polarizer. Davis, Krupa, and Winefordner (9)have given a discussion of the principles of CFS.
EXPERIMENTAL SECTION A block diagram of the experimental setup is given in Figure 1. A 300-W Eimac xenon arc lamp was used as a continuum 0 1987 American Chemical Society
