ANALYTICAL CHEMISTRY, VOL. 50,
NO. 13,
NOVEMBER 1978
1757
intensities larger and relative standard deviations smaller, but also the greater convenience of these materials, which are easily cut to size with a paper cutter and reused if desired, favor their use for routine determinations.
100 tor2
II Glass
H i g h hnsity
Polypropylane
Plsxigl&s
Polyethylens 700 torr
Figure 6. Post-explosion substrates showing effects of material and pressure on film vaporization for 400-A films exploded in dry air at 5 k V and 185 J
that of H D P E is about 1016 R-cm. Further, the resistivity of glass decreases to as low as 1 X lo4 R-cm as the glass is heated to near its annealing temperature (9). Thus, sufficient discharge current may be shunted through the glass, particularly near its surface where it is heated by ohmic dissipation in the film, to etch the surface and decrease the vaporization efficiency of the A1 film and sample. This effect probably is more pronounced a t 100 Torr because of the earlier transfer of discharge current from the solid or liquid film t o the gas surrounding the expanding metal vapor cloud ( 2 , 3 ) . These results suggest the use of HDPE or PP as substrate materials for analytical applications. Not only are analyte line
CONCLUSIONS The preparation of thin metal films of sufficient quality and reproducibility for use as sample cells and excitation sources for quantitative emission spectroscopy is both straightforward and inexpensive. The materials cost for film preparation is very small, less than five cents per film, and even the relatively small system described here can produce over 250 films in eight hours. Since the PP and H D P E substrates can be cut with a paper cutter, their preparation is simple and rapid. Excitation with thin films appears to be more reproducible than with metal foils. This may result from the greater uniformity of thin films together with more complete and more rapid vaporization of the film and sample. Finally, the larger surface area available with thin films of reasonable mass should permit the use of larger sample volume as well as simplify sample introduction. LITERATURE CITED (1) J. A. Holcombe and R. D. Sacks, Spectrochim. Acta, Part B , 28, 451 (1973). (2) R. D. Sacks and J. A. Holcombe, Appi'. Spectrosc., 28, 518 (1974). (3) C. S.Ling and R. D. Sacks, Anal. Chem.. 47, 2074 (1975). (4) C. S.Ling and R. D. Sacks, Anal. Chem., 48, 1500 (1976). (5) J. A . Holcombe, D. W. Brinkman, and H. D. Sacks, Anal. Chem., 47, 441 (1975). (6) L. Holland, "Vacuum Deposition of Thin Films", Wiley and Sons, New York, N.Y., 1958,pp 169-176. (7) J. Strong, "Modern Physical Laboratory Practice", Bbckie, London, 1944. (8) R. L. Sennett and G. D. Scott, J . Opt. Sac. Am.. 40, 203 (1950). (9) G. W. Morey, "The Properties of Glass", Reinhold, New York N.Y., 1954, p 558.
RECEIVED for review June 13, 1978. Accepted August 7,1978. Work supported by the National Science Foundation through grant number CHE76-11646 A01.
Parametric Investigation of Exploding Thin-Film Excitation R. D. Sacks" and D. V. Duchane' Deparment of Chemistry, University of Michigan, Ann Arbor, Michigan 48 109
The dependence of analysis line and AI thin film matrix emission line intensities as well as continuum background radiation intensity and analysis line-to-background ratios on experimental parameters has been investigated. These parameters include the film thickness, the pressure and composition of the plasma support gas, the total discharge energy at both constant charging voltage and constant capacitance, and the charging voltage at constant energy. Line intensities from analyte as well as film species increase with film thickness. Line-to-background ratios are greater in air than in Ar at all pressures. Precision is greater in air and at pressures greater than 100 Torr. Suggested analytical conditions employ 400-A thick AI films exploded In dry air at 300 Torr by 180-J, 4-kV discharges. Two explosion mechanisms are discussed, a high current density mode at high energy and pressure and a diffuse mode at low energy and pressure.
'Present address, Los Alamos Scientific Laboratory, P.O. Box 1663, Los Alamos, N.M. 87544. 0003-2700/78/0350-1757$01.00/0
The radiative properties of wires and foils exploded by capacitive discharge largely can be controlled through manipulation of certain parameters including discharge energy, wire or foil mass, and surrounding gas pressure and composition ( I , 2 ) . A change from 100 Torr Ar to 50 Torr He reduced detection limits by as much as an order of magnitude when exploding-foil excitation was used for the atomic emission determination of trace elements in 1O-pL aqueous samples ( 3 ) . A companion report ( 4 ) has shown that greater precision, more complete sample consumption, significantly lower background intensity, and greater convenience of film preparation and sample introduction may render exploding thin-film excitation more useful than wire or foil excitation for micro solution and solid samples. Since exploding wires and foils have found many engineering as well as military applications, a considerable literature exists on their electrical and radiative properties (5-8). This provided a useful starting point in the development of analytical procedures and applications using these systems. Very little literature, however, is available for exploding thin 1978 American Chemical Society
1758
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
films. This, coupled with significant differences in mass, surface area, and electrical resistance of films relative to wires and foils provided the impetus for a detailed investigation of their radiative properties. Such studies are needed in choosing experimental conditions for specific analytical problems and may provide insight into the vaporization, plasma formation, and current conduction processes responsible for the observed radiation patterns. Unlike wires and foils, thin film mass is easily controlled during production and thus is a principal variable in this study. Experiments were conducted to relate empirically analysis line and film matrix line intensities, continuum background intensities, analysis line-to-background ratios and analytical precision to the film mass, the composition and pressure of the support gas, the total discharge energy a t both fixed capacitance and fixed charging voltage, and the charging voltage at a fixed total discharge energy.
EXPERIMENTAL Apparatus. The capacitive discharge circuit and its associated electronics, the explosion chamber, and the thin film production apparatus are described in a companion report ( 4 ) . In all experiments the film was oriented perpendicular to both the optical axis and spectrograph slit. A 5-cm diameter, 15-cm focal length quartz lens was used to image the entire plasma volume onto the collimator mirror of a 1.0-m Czerny-Turner spectrograph (Jarrell-Ash Model 78-460) using a 35-rm entrance slit and having a first-order linear reciprocal dispersion of 0.8 nm/mm. Current waveforms from the capacitive discharge circuit were recorded on a Nicolet Model 1090A digital storage oscilloscope, which has a 2-MHz bandwidth and a sampling interval of 0.5 k s per point. All spectra were recorded on Kodak SA1 photographic plates. Photographic processing and calibration procedures are described in Reference 9. Film Preparation. All films were prepared from 99.999% pure A1 lumps (Alfa-Ventron, Beverly, Mass.). High density polyethylene was used as a substrate material. Substrate blanks were cleaned by soaking for 24 h in reagent grade isopropyl alcohol. Each film had a tensile bar shape with a 0.95-cm2region on each end for electrical contact and a 1.9 cm X 0.32 cm center region on which the sample solution was applied. Prior to its explosion, the resistance of each film was measured along its long dimension using a digital multimeter. Sample Preparation. A 1O-gL aqueous sample containing 200 ng each of Mn and Pd was applied to the film with a microsyringe as five small spots uniformly spaced along the narrow portion of the film. A heat lamp was used to evaporate the water leaving a solid, microcrystalline residue. Working solutions were prepared daily from 1000 pg/mL stock solutions of Mn as Mn(N0J2 and Pd as Pd(N0J2.
RESULTS AND DISCUSSION Manganese and palladium were chosen as test elements because of their very different vaporization temperatures. The nitrate salts were selected because A1 is somewhat resistant to attack by acidic nitrate solution. The Pd solution was quite acidic since it was prepared by dissolving P d metal in "OB. Upon heating, Mn(N0J2 decomposes to M n 0 2 , which volatilizes after further decomposition a t about 2300 K (10). The P d ( N 0 J 2 is converted to Pd(OH)*and then to elemental P d , which does not vaporize until a temperature of about 4250 K is attained (11). Aluminum metal vaporizes a t about 2720 K (12). It should be noted that the thermochemistry of these materials may be somewhat altered with the extremely rapid heating and vaporization occurring with exploding thin-film excitation. Oscilloscopic monitoring of discharge current waveforms indicated that vaporization is complete and dielectric breakdown of the support gas occurs in less than 1.0 FS.
Table I summarizes information regarding the spectral lines monitored in this study. One neutral-atom and one ion line were selected for each test element as well as from the A1 film
Table I. Emission Lines Used in Parametric Study excitation potential relative to species ionization wavelength, ground potential, species nm state, eV eV A W 305.0 7.63 5.96 Al(I1) 266.9 4.62 18.75 322.8 5.93 7.40 Mn(I) Mn(1I) 289.8 8.37 15.57 340.4 4.44 8.30 PdP) Pd(I1) 285.4 8.31 19.34 material. Continuum background intensity was measured in a line free region near 323.0 nm. Film Thickness. T o investigate the effect of film thickness on the intensity of continuum background and selected emission lines, 250, 400, and 550 A thick A1 films were exploded in dry air a t various pressures using 180-5, 4-kV discharges. Visual examination of the substrates after the explosions revealed no noticeable differences as a function of film thickness. As was found in all parts of this study, residual A1 remained on the substrates for explosions at pressures less than 300 Torr. However, this occurred for all film thicknesses. Figure 1 shows the relative intensity of the continuum background, Al(1) line, and Al(I1) line as functions of pressure for the three film thicknesses. Each point in this and in subsequent figures is based on an average of five explosions a t the indicated condition. Error bars for estimates of standard deviations from the five measurements are indicated for representative points. The intensities, in general, decrease with decreasing pressure for all film thicknesses with the continuum intensity decreasing most significantly. I t is noteworthy that the rate of decrease with pressure is greatest a t higher pressure for the continuum radiation and greatest a t lower pressure for both A1 species. This is in contrast with previous studies with A1 foils, which showed that the continuum radiation as well as that from Al species decreases with pressure more precipitously a t lower pressure ( 3 ) . The Al(I1) intensity is relatively pressure independent above about 300 Torr but does show indication of passing through a broad maximum a t 500 Torr for the thicker films. The higher intensities from the A1 species for the thicker films a t nearly all pressures is easily explained by the greater available mass of Al. It is not clear, however, why the continuum intensity a t all pressures is lowest for the 400-A films. This suggests that the continuum radiation is not associated directly with the quantity of A1 vapor produced. Figure 2 shows similar plots for Mn(1) and Mn(I1) lines. Again, a trend toward lower intensity a t lower pressure is observed for both species. The rate of decrease with pressure is greater for the thicker films. Significantly these analyte intensities are greater with the thicker films in nearly all cases. This is surprising since unlike the Al, the Mn was present a t constant concentration. Moreover, the range of intensity values for Mn(1) for the different film thicknesses is even greater than that observed for Al(1) at pressures greater than 100 Torr. These trends may be related to the stronger shock wave produced by the rapid vaporization of the more massive films (13). Very similar results were obtained for Pd(1) and Pd(I1) lines. Table I1 lists the line-to-background intensity ratios for these emission lines a t several different pressures for the three film thicknesses. Note that for the Al(1) line, the line-tobackground ratio consistently increases with increasing film thickness. For the Al(I1) line, however, the behavior of the ratio is somewhat erratic. I t peaks at the intermediate film thickness for the 700- and 300-Torr explosions but is lowest
ness, A 250
r-
120-
80
40
-
AI ( 1 1 )
01
1
I
1
J
line, nm Al(1) 305.0 AI(I1) 266.9 Mn(1) 322.8 Mn(1I) 289.8 Pd(1) 340.4 Pd(I1) 285.4
400
Al(1)
550
Al(I1) 266.9 Mn(1) 322.8 Mn(I1) 289.8 Pd(1) 340.4 Pd(I1) 285.4 Al(1) 305.0 Al(I1) 266.9 Mn(1) 322.8 Mn(I1) 289.8 Pd(1) 340.4
305.0
700 40.3 5.9 2.2 10.1
12.0 7.2 65.8 9.4 13.7 34.7 56.0 15.9 69.4 7.2
300 84.1 18.7 2.0
27.4 27.4 24.7 145.7 30.2
18.9 70.6
111.4 35.0 165.2 22.5
100
141.8 35.1 1.7 23.1 36.0 15.7 196.2 27.6 19.0 87.7 139.6 54.1
349.6 89.6
4.5
8.1
10.7
13.4 22.4
32.9 50.8
72.7 97.4
1760
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
- 1
/
.O
Ar
0
--m u I
I-
fressure, torr
Ar
Ar
Flgure 3. Postexplosion substrates showing effect of pressure on film vaporization for 400-A films exploded in air at 4 kV and 180 J
resistance of Al films (-5.5 Q ) relative to Al foils (-0.017 0 ) . Under such conditions, the explosions are of little analytical utility. Figure 3 shows a photograph of several post-explosion substrates obtained a t different pressures in dry air using 180-5, 4-kV discharges. There is no visible residue a t 700 or 300 Torr. At 100 Torr, unvaporized material is observed only where the five sample droplets were applied to the film. It was not determined if this residue is unvaporized A1 film or sample or both. At 50 Torr, the residue at the location of the sample droplets is more pronounced. At 10 Torr, much of the entire film remains unvaporized. The greater persistence of the residue a t the sample droplet locations may result from an increase in electrical resistance in these areas due to attack of the film surface by the dilute HN03 used in preparing the analyte test solutions. While HNOBhas a passivating action on the A1 surface (15),even a relatively small corrosion depth would be significant with a 400-A film. The more complete vaporization at higher pressure probably results from a longer period of metallic current conduction prior to dielectric breakdown of the surrounding gas together with greater ablation of the substrate surface in the higher density gas plasma after breakdown. Studies of the effects of pressure on emission intensities were carried out a t discharge energies of 180 J (22.5 pF at 4 kV) and 281 J (22.5 WFa t 5 kV) in both air and Ar. Figure 4 shows results for continuum background, Al(1) and Al(I1). The continuum intensity shows a nearly linear decrease with decreasing pressure in all four cases. The continuum is significantly more intense in Ar a t both energies as well as being more intense a t higher energy in both gases. The decrease in continuum intensity with pressure may reflect a lower current density plasma resulting from reduced inertial confinement of the plasma at reduced pressure. The effect of discharge energy also may be explained in terms of current density, since the peak current in the first current half-cycle after vaporization increases from 7.9 X lo3 A a t 4 kV to 1.0 x lo4 A a t 5 kV. The greater continuum intensity in Ar relative to air is consistent with the more intense ion-electron recombination continuum in Ar reported for other plasma sources (16). T h e Al(1) intensities are seen to decrease with decreasing pressure under all conditions of energy and atmospheric
Air Air
AI (11) 0 0
d 1
I
I
1
200
400
600
800
Prorruro, t o r r Figure 4. Plots of relative intensity of continuum background, the Al(1) line and the AI(I1) line as functions of pressure for 400-A AI films exploded in air and Ar at 180 (broken lines) and 281 (solid lines) J
composition. However, they are much less dependent on discharge energy than the continuum background intensities, particularly in dry air. Also, the rate of decrease of Al(1) intensity with decreasing pressure is seen to accelerate at pressures less than 100 Torr. This is even more apparent for Al(II), where the line intensities show only modest decreases
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
“t
/
T..
a c
c L
Ar
9
> .-
1761
Table 111. Line-to-Background Intensity Ratios in Air and Ar at Several Pressures p:ressure, Torr emission atmosline,nm phere 700 500 300 100 50 Al(1) 305.0 dry air 65.7 85.2 146 196 194 28.3 34.1 43.9 72.8 68.2 argon 9.4 14.6 30.2 29.1 17.5 Al(I1) 266.9 dry air argon 3.4 4.5 7.1 13.5 13.0 5.3 7.3 11.0 15.6 7.6 Mn(1) 322.8 dry air argon 3.1 3.1 3.9 2.2 3.0 Mn(I1) 289.8 dry air 5.6 7.9 12.4 20.7 21.9 argon 4.4 4.3 5.2 10.0 10.1 Pd(1) 340.4 dry air 26.5 32.5 57.1 1 1 3 144 argon 12.8 12.4 18.3 28.6 29.7 3.6 6.0 11.3 17.0 Pd(I1) 285.4 dry air 9.8 areon 2.3 2.5 3.7 9.6 7.3 Table IV. Relative Standard Deviations (%) in Air and Ar a t Several Pressures pressure, Torr emission atmosline, nm phere 700 500 300 100 Al(1) 305.0 dry air 6 1 4 9 9 argon 14 7 10 29 Al(I1) 266.9 dry air 6 10 7 16 argon 16 5 7 22 Mn(1) 322.8 dry air 20 4 10 29 argon 21 25 33 48 Mn(I1) 289.8 dry air 25 13 8 20 argon 37 21 27 4 Pd(1) 340.4 dry air 7 9 7 8 argon 35 7 17 8 Pd(I1) 285.4 dry air 18 13 8 23 argon 19 16 16 15 ~
c
2 350
Ar
9
a
.i
c
4 Air
II IO 9 t
Mn (11) 0
200
400
600
800
Pressure, t o r r Figure 5. Plots of relative intensity of the Mn(1) and Mn(I1) lines as functions of pressure for 400-8, AI films exploded in air and Ar at 180 (broken lines) and 281 (solid lines) J
until t h e pressure is reduced t o less than 100 Torr for explosions a t 281 J and modest or no decrease until the pressure is reduced to less than 300 Torr for explosions a t 180 J. It is a t pressures below 300 Torr that incomplete vaporization was found to occur. This suggests that, a t pressures less than 300 Torr, the intensities of the A1 emission lines may be limited by inefficient vaporization of t h e film material. A change in the plasma current conduction mechanism also may be involved as the pressure is reduced below 300 Torr. Figure 5 shows similar plots for t h e Mn(1) and Mn(I1) analyte lines. As for the A1 lines, the Mn line intensities are relatively independent of discharge energy a t all pressures in air and are relatively pressure independent in air at higher pressure. In Ar, however, the Mn(I1) intensity is seen to increase more significantly with increasing pressure a t high pressure relative to Al(I1). Again, very similar results were obtained for Pd(1) and Pd(I1) lines. The qualitatively similar trend toward lower intensity a t lower pressure for continuum background, lines from the A1 film material and analyte lines is in contrast with previous results for wires (14) and foils (3)which indicated that analyte
50 8
43 26 81
27 50 20 39 5 32 22 38
line intensities decrease with decreasing pressure t o a point but then often increase as the pressure is reduced to 200 Torr and below. With wires and foils, differential vaporization, which favors an increased ratio of sample-to-matrix element concentration in the pIasma a t low pressure, was postulated. No such differential vaporization is apparent with thin films. In this respect, it should be noted that, the wires and foils were on the order of lo4 times more massive than the analyte material placed on them, while the thin film mass is only about 50 times greater than the 200 ng of sample element used in these studies. Thus, inefficient vaporization of the wire or foil might be expected to increase the current-carrying burden on the vaporized sample material to a far greater extent than inefficient vaporization of a film. Although the intensities of the emission lines examined decrease as the gas pressure is reduced, the continuum background intensity decreases to a greater extent under the same conditions. Table I11 lists the line-to-background ratios for explosions a t five pressures in both Ar and air. These values are based on average intensities from five explosions a t the indicated conditions. For every pair listed, the ratio IS greater in dry air than in Ar. In most cases, the air values are a factor of two to three greater than the Ar values. There appears t o be no significant difference between film and sample species or between neutral-atom and ion lines in Table 111. When pressures are compared, it is observed t h a t lineto-background ratios increase with decreasing pressure in both gases with maximum values usually occurring at 100 or at 500 Torr. Analyte lines show relatively little change in line-tobackground ratios in Ar in the rang€’from 700 to 500 Torr. In air, however, a two- to threefold increase is observed as the pressure is reduced to 500 Torr. The maximum observed ratios a t 100 or 50 Torr typically are about a factor of three greater than a t 700 Torr in both gases and for both film and
1762
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
Table V. Discharge Parameters for Energy Studies ringing charging capacipeak voltage, tance, energy, current, frequency, J kA kHz kV !.lF 2.0
3.0 4.0 5.0 5.8 5.0
5.0 5.0 5.0
22.5 22.5 22.5 22.5 22.5 7.5 15 22.5 30
45
4.0
15.5
101
6.1
15.5
180 281 378
7.9 10.0 11.8
15.5 15.5 15.5
5.8
26.0
8.3 10.0
18.9
11.5
13.4
94 188 281 315
15.5
analyte species. Qualitatively similar results were reported for line-to-background ratios in Ar with exploding-foil excitation ( 3 ) . Table IV lists relative standard deviations for the line intensities in Table 111. For the 20 pairs of values for the four analyte species, only in three cases are the dry air values greater than the corresponding Ar values. This greater precision coupled with the consistently higher line-to-background ratios strongly suggests the use of dry air for further analytical studies. When the five pressures are compared, a trend toward larger relative standard deviations at lower pressure is observed in both gases. None of the six values listed for dry air a t 300 Torr exceed & l o % . These values are significantly better than those previously reported for exploding-foil excitation, where relative standard deviations in the f 1 5 to f 3 0 % range were obtained for analyte line intensities without an internal reference technique (2). Discharge Energy. The capacitive discharge circuit used here is an underdamped tank and thus produces a damped, sinusoidal current waveform. Time and spacially resolved spectra of exploding A1 films (17) have indicated that most of the analytically useful radiation as well as continuum background is produced during the first current halfcycle and in the discharge current channel volume within 5 mm of the substrate surface. Thus, the current density during this first half-cycle should be important in controlling the radiative properties of exploding thin films. Since peak current density should bear some continuous relationship to peak current, manipulation of the latter through changes in either circuit capacitance or charging voltage should be illustrative. The peak current i, in the first current half-cycle ap(2E/L)"', where i, is in A, the proximately is given by i, discharge energy E in J, and the circuit inductance L in H. The discharge energy increases linearly with capacitance and as the square of the charging voltage. Increasing capacitance increases the time from film vaporization t o peak current and thus may result in reduced analyte and film material in the current channel zone a t the time of peak current. Increasing charging voltage should decrease the time for dielectric breakdown of the gas or metal vapor relative to the start of the discharge as well as affecting the nature of current conduction processes in the plasma (14). Table V lists the discharge parameters for two sets of experiments, one in which the discharge voltage is varied a t a constant capacitance of 22.5 pF and the other in which the capacitance is varied a t a constant charging voltage of 5 kV. Peak current was measured with a shorting bar in place of the thin film. Figure 6 shows plots of background, Al(I), and Al(1I) line intensities as functions of discharge energy at constant capacitance for five pressures in dry air. In this and in all subsequent figures, broken lines indicate explosions conducted at 500 and 300 Torr. The continuum background decreases steadily with decreasing energy at all pressures, but the extent of the decrease is greater a t the higher pressures.
I
100 torr
500
16
300
100
50
1
I
I
1
040
I
E 500 torr
i20r
1
140
I
240
1
340
I 440
Energy, J Figure 6. Plots of
relative intensity of continuum background, the AI(1) line and the AI(I1) line as functions of energy for 400-A AI films exploded in air at 22.5 pF
The plots for Al(1) show a pronounced minimum at 281 J for the 500- and 300-Torr explosions. The intensities then increase somewhat a t 180 J followed by a rapid decrease a t still lower energies. The trends are quite different a t 100 and 50 Torr. Here, no minimum is observed a t 281 J and, in fact, there is indication of a maximum at this energy for the 50-Torr explosions. A t energies less than 180 J, this line was undetected a t the two lowest pressures. This change in energy dependence between 300 and 100 Torr is even more pro-
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
r
1
I
I
1763
T
1
torr
~
0
C
-
C w
0 90
I70
250
330
410
Enrrgy, J
Figure 7. Plots of rehtive intensity of the AI(1) and AI(I1) lines as functions of energy for 400-A AI films exploded in air at 5 kV
nounced for the Al(I1) line. I t is interesting to note that for both Al(1) and Al(II), the 700-Torr plots in the energy range from 101 to 281 J are qualitatively similar to the 100-Torr plots at higher energy. This also applies to the 500-Torr (low energy) and 50-Torr (high energy) plots. These trends suggest two different explosion mechanisms, one at high pressure and energies greater than 281 J and the other over the entire energy range at low pressure as well as a t lower energies for the higher pressure explosions. Corroborating evidence for a change in explosion mechanism when the pressure is reduced from 300 to 100 Torr is provided in Figure 7 which shows plots of Al(1) and Al(I1) intensity as a function of energy a t constant charging voltage. A very pronounced minimum is observed a t 281 J for both species a t pressures of 300 Torr and above. Maxima are observed a t this energy for the Al(I1) line a t pressures of 100 and 50 Torr. The Al(1) line intensity varies almost linearly with energy over the entire range at pressures of 100 and 50 Torr. Again it is observed that the Al(I1) plots a t low pressure show qualitatively similar trends to the lower energy portions (less than 281 J) of the Al(I1) plots a t pressures of 300 Torr and greater. Decreasing either discharge energy or gas pressure should result in a decrease in current density in the discharge current channel. Decreasing energy results in a corresponding decrease in peak current and thus should decrease current density. Decreasing pressure results in reduced inertial confinement and thus more rapid expansion of the current channel (18). At high pressure and high energy, the plasma current density may be sufficient to result in a significant magnetohydrodynamic pinch (19). This would further increase current density with a corresponding increase in continuum background as well as line radiation from the film. At lower energy and lower pressure, where current density is insufficient to obtain a significant pinch, the plasma may operate in a more diffuse mode, where current density decreases rapidly with decreasing pressure and energy. While this explanation is highly speculative, it may provide a framework for further study.
40
,
a 120-
A 40
I 40
240
340
4 40
E n o r ~ y ,J Figure 8. Plots of r e h t i e intensity of the Pd(1) and Pd(I1) lines as function of energy for 400-A AI films exploded in air at 22.5 pF
Figure 8 shows plots of Pd(1) and I’d(I1) line intensities as functions of discharge energy a t constant capacitance. The trends observed here are very different from those for the A1 film species. The minima observed for Al(1) and Al(I1) a t 281 J for the higher pressure explosions are completely absent. The Pd(1) line intensities show broad maxima at about 180 J for pressures of 300 Torr and greater. The intensities then drop very rapidly at lower energies. The Pd(I1) intensities generally show a rather gradual decrease with decreasing energy at high energy and a more precipitous decrease at lower energy. Very similar results were obtained for Mn( [) and Mn(I1) lines with two notable exceptions. First, the maxima observed a t 180 J for the Pd(1) line at higher pressure is more pronounced for Mn(1). Second, the plots for Mn(I1) showed minima a t 281 J a t the higher pressures and were qualitatively similar to the corresponding Al(I1) plots. The constant voltage plots for the four analyte species also were similar to their A1 analogues. From an analytical viewpoint, emission line-to-background ratios appear to be quite dependent on discharge energy or
t
8.0 I2
4a
-
‘
‘\*
/m’om
+ * ---- - - - 4 3 0 0 c
4
+-------*300
*.
e I
---
- .
.
,O
300
ry
pressures of 300 Torr and greater. Not only do the intensities fall off drastically a t lower pressure b u t also, the intensity increase at 3.46 kV relative to 4.0 kV is greater at the two lower pressures. Again, this suggests a change in explosion mechanism as the pressure is reduced below 300 Torr. While the indication of a mechanism change is less obvious for the Al(1) plots, a change in current conduction piocesses might be expected to affect Al(I1) more than Al(1) since the former species may be a principal carrier of the discharge current. T h e general trend toward lower intensity at higher capa-
for-all pressures. The intensities drop off gradually a t higher voltages and more rapidly a t the lower voltage. While the reasons for these trends are not clear. they are analytically significant, since three of the four analb-te species investigated showed maximum intensity a t 4 kV for nearly all pressures and the continuum background intensity indicated minimum values a t this voltage. CONCLUSIONS Over the range of values studied, each parameter resulted in changes in analyte line-to-background ratios by typically a factor of 3 to 5 . Their combined effects, however, reflect an increase of 1 to 2 decades in these ratios and should obtain
AI
(11)
J m- - -
c c c
e
loo 50
5.0
&o
of detection. The response of all measured line intensities and continuum intensity to the parameters discussed here is much smaller with exploding films relative to wires and foils. T h e plots shown in this report all employed a linear intensity scale with intensity changes seldom greater than one decade for a given spectral element. For analogous studies with wires ( 1 4 ) and foils (3)covering similar parameter ranges, intensity changes often spanned 3 to 4 decades and required a log intensity scale. Complete film vaporization over a wider range of parameter values together with the much smaller metal vapor mass may
7.0
I
peak current. Increasing the energy above about 180 J results in a rapid increase in continuum background intensity as well as a significant decrease in neutral-atom line intensities from t h e analyte species. Below 180 J, analyte line intensities fall off with decreasing energy more rapidly than the background intensity. Thus, about 180 J results in the most favorable line-to-background ratios for analyte species. Voltage-Capacitance Combination. In the final part of this parametric study, the discharge energy was held constant at 180 J and t h e voltage-capacitance combination varied. Here, t h e peak current is constant a t about 7.9 kA, but the time of peak current relative to sample vaporization increases with increasing capacitance. Figure 9 shows plots for continuum background, Al(1) and Al(I1). At all pressures, a minimum is observed a t 4 kV and 22.5 MF.The intensities increase gradually at higher voltages and more rapidly a t the lower voltage. I t is noteworthy that t h e plots for Al(I1) are relatively pressure independent at
L_I
mass control is important since, for the range of thickness values studied, a 1%change in film thickness tk-pically results in about a 1% change in line intensities from both film material and analyte species. The response of the AI line intensities to the various parameters often differs from the response of the analyte species line intensities. This suggests that the A1 film material may not be useful as an internal reference. However, the results reported here suggest t h a t internal referencing may he counterproductive with photographic detection since photometric error rather than explosion reproducibility may be the limiting factor in determining analytical precision. The explanations provided for the observed intensity trends are in many cases very speculative. Since exploding conductor plasmas are very heterogeneous in both space and time (14,28), temporally and spacially resolved intensity measurements will be required to obtain a more definitive description of exploding thin-film plasmas.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
LITERATURE CITED J. A. Holcombe and R. D. Sacks, Specfrochlm., Acta, Part B , 28, 451 (1973). C. S. Ling and R. D. Sacks, Anal. Chem., 47, 2074 (1975). C. S. Cling and R. D. Sacks, Anal. Chem.. 48, 1500 (1976). D. V. Duchane and R. D. Sacks, Anal. Chem., preceding paper in this issue. (5) "Exploding Wires", W. G. Chace, and H. K. Moore, Ed., Plenum, New York, N.Y., Vol 1, 1959. Ref. 5, Vol. 2, 1962. Ref. 5, Vol. 3, 1964. Ref. 5, Vol. 4, 1968. J. A. Holcombe, D. W. Brinkman, and R. D. Sacks, Anal. Chem., 47, 441 (1975). F. A. Cotton and G. Wilkinson. "Advanced Inorganic Chemistry", Interscience, New York, N.Y., 1972, p 852. Ref. 10, pp 990-1000. 0. Kubaschewski and B. E. Hopkins, "Oxidation of Metals and Alloys", Academic Press, London, 1962, pp 102, 103, and 230.
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(13) R . D. Sacks, "Shock Tubes, Exploding Conductors and Flashlamps". in Anarnica Uses of Plasmas", R. M. Barnes, Ed., Wiley-Interscience, New York, N.Y., in press. (14) R. D. Sacks and J. A. Holcombe, Appl. Specfrosc., 28, 518 (1974). (15) U. R. Evans, "Metallic Corrosion, Passivity and Protection", E. Arnold & Co., London, 1946. (16) S. Greenfield, Metron 3 , 224 (1971). (17) D. V. Duchane, "The Application of Expldng Thin Films to Trace Metals Analyses", Ph.D. dissertation, University of Michigan, Ann Arbor, Mich., 1978. (18) P. Thomas and R. D. Sacks, Anal. Chem., 50, 1084 (1978). (19) L. A. Arzimovich. "Elementary Plasma Physics", Blaisdell, New York, N.Y., 1965.
RECEIVED for review June 13, 1978. Accepted August 7, 1978. Work supported by the National Science Foundation through grant number CHE76-11646 A01.
Atomic Emission Determination of Selected Trace Elements in Micro Samples with Exploding Thin-Film Excitation D. V. Duchane' and R. D. Sacks* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48 109
The highly luminous metal-vapor plasma obtained from the electrical explosion of 400-A thick, 13-,ug AI thin films has been used as a free-atom generator and excitation source for the analysis of selected trace elements in aqueous media. Ten-microliter samples of metal salts are applied to the film surface. The film strips are exploded in 300 Torr dry air with a 4-kV, 180-5 capacitive discharge. Analytical curves and reproducibility data are presented for Mn, Sn, Cd, Pb, and Pd, both with and without an internal reference technique. The relative standard deviations for these elements at the 100-ng level are 5, 11, 8, 9, and 5 % , respectively. Detection limits, usually in the low to sub-ng range, are presented for 14 elements. Concomitant effects are found to be relatively minor if the total solute content is less than about 0.01 M.
When furnace techniques are used for the volatilization of micro solid or solution residue samples, the relatively low rate of furnace heating results in reducing analyte free atom number densities ( 1 ) as well as significant concomitant effects (2). Since noncapacitive power sources generally are used for furnace heating, the heating rate is limited by laboratory wiring as well as the heat capacity of t h e relatively massive furnace. Peak power dissipation in the furnace is usually no more than a few kW. If a high-voltage capacitive discharge power source is used to heat a thin metal film of low mass and heat capacity, explosive vaporization of the film and a sample deposited on its surface probably is complete in about 1 FS. When an A1 film of 1.3 X mm2 cross sectional area is vaporized by a 4-kV, 22.5-wF discharge, a current of 3.7 kA is measured 1 1 s after the start of the discharge. If the film material still exists 'Present address, Los Alamos Scientific Laboratory, P.O. Box 1663, Los Alamos. N.M. 87544. 0003-2700/78/0350-1765$01 .OO/O
as a metallic conductor a t this time, it would carry a current density of over 10' A/mm2 and dissipate over 10 MW, assuming room temperature resistance. This probably is conservative. If the film is exploded from a polymeric substrate such as polyethylene, complete scavenging of the film and sample is observed (3). Since a fresh film is used each time, memory effects and furnace deterioration problems are obviated. Since only a few micrograms of high-purity metal are vaporized from a hydrocarbon substrate, the analytical blank is minimal for most elements. If dielectric breakdown of the gas surrounding the film occurs during or after film vaporization, a high current density plasma is formed which excites and to some extent ionizes the analyte vapor. Since the plasma is in close proximity to the substrate surface, ablation may augment film and sample atomization. This report considers exploding thin films for volatilization and excitation of trace elements in micro-volume solution samples. Companion reports consid'er thin film preparation and control (3) and the effects of various parameters on exploding thin-film excitation ( 4 ) .
EXPERIMENTAL Apparatus. The capacitive discharge circuit used for the explosive vaporization of thin films has; been described (3). The thin-film production apparatus and ancillary hardware also are considered. All spectra were recorded on Kodak SA1 photographic plates. Emulsion processing and calibration have been discussed ( 5 ) . A 1.0-m Czerny-Turner spectrograph (Jarrell-Ash model 78-460) with a first-order reciprocal linear dispersion of about 0.8 nm/mm and using a 35-wm entrance slit was used throughout this study. Entrance slit illumination is the same as i n Reference 4. Procedures. Aluminum films prepaired by vacuum deposition on high-density polyethylene substratles were used exclusively. Film preparation and control is described in Reference 3. Each film had a tensile bar shape with a 0.95-cm2region on each end for electrical contact and a 1.9 cm X 0.32 cm center region on which a 10-pL aqueous solution sample was placed as five small, evenly C 1978 American Chemical Society
