Parametric Investigation of Explod ng Foil Excitation C. S. Ling and R. D. Sacks* Department of Chemistry, University of Michigan, Ann Arbor, Mich. 48 109
The dependence of analysis line and Ai foil 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 pressure and composition of the ambient gas, total available energy at a fixed tank circuit ringing frequency, and initial capacitor voltage at fixed available energy. The pressure and composition of the gas provide the greatest degree of control over analysis line to background intensity ratios with Ar in the pressure range between 10 Torr and 100 Torr providing the most favorable values. A comparisonof the pressure dependence of Ai foil matrix lines and emission lines from Cd and Ni samples deposited on the foil surface indicates a change in expioslon mechanism as the pressure is reduced. Detection limits for 18 elements obtained in Ar at 50 Torr are, in general, Considerably lower than previously reported values obtained in He at 100 Torr.
The atomic emission determination of selected trace metals in 10-pl aqueous samples using exploding foil excitation recently was reported (1).Analytical curves, which are linear over three to four decades of concentration, were obtained for several elements when samples placed on A1 foil strips were exploded using a 282-5,5-kV capacitive discharge in He at 100 Torr. While spectra were recorded in air and He at 100and 700 Torr, no detailed parametric investigation of gas composition or pressure was attempted. In addition, the effect of total discharge energy and initial capacitor voltage was not investigated. The selection of 282 J a t 5 kV and 100 Torr in He as excitation conditions was based primarily on line shapes, continuum background intensity, and experimental convenience. These preliminary studies suggested the potential application of exploding metal foils for multielement analysis of micro solution samples. A number of attractive analytical features including an extended linear dynamic range, multielement capability using micro samples, and relatively minor matrix and interelement effects provided the impetus for further studies of exploding foil excitation. Since documentation in the literature on the radiative properties of exploding metal foils is very sparse, the effect of certain easily controlled experimental parameters is, for the most part, unknown. This study was conducted to relate empirically analysis line and foil matrix line intensities, continuum background intensity, and analysis line to background intensity ratios to the compositioh and pressure of the support gas, the total discharge energy at a fixed tank circuit ringing frequency, and the initial voltage on the capacitor bank at a fixed total discharge energy. An analogous parametric study of exploding wire excitation has been reported (2). EXPERIMENTAL Apparatus. The capacitive discharge circuit and its associated
electronics and the explosion chamber have been described in detail ( I ) . In all experiments, the foil strip was oriented perpendicular to both the optical axis and the spectrograph slit and was located 28 cm from the entrance slit. No focusing optics were employed, and the spectrograph viewed the entire plasma volume. 1500
Optical Monitoring and Calibration. All spectra were recorded on Kodak SA1 plates using a 1.0-m Czerny-Turner spectrograph (Jarrell-Ash Model 78-460) with a 100-pm entrance slit and a firstorder linear reciprocal dispersion of 0.8 nm/mm. Photographic processing and calibration procedures are described in Ref. 3. Foil and Sample Preparation. Foil strips, 1.6 mm wide X 25 mm long X 0.025 mm thick were cut from 99.9995% pure A1 foil sheet (Alfa-Ventron,Beverly,Mass.) The foil strip was supported on a glass plate which was clamped between plane, parallel Cu electrodes. A 10-plaqueous sample containing 10 pg each of Cd and Ni was applied to the foil with a micro syringe as six small spots uniformly spaced along the foil strip. Working solutions were prepared daily from 1000 pg/ml stock solutions of reagent grade compounds.
RESULTS AND DISCUSSION Voltage-Capacitance Combinations at Fixed Energy. Explosions were conducted in air and He a t 50 and 700 Torr for three voltage-capacitance combinations, each resulting in 282 J initially stored on the capacitor bank. Figure 1shows plots of relative intensity of the background continuum in a line-free region near 286.4 nm as functions of initial voltage on the capacitor bank. Each point represents the mean intensity from five explosions. Over the voltage range from 4.3 to 8.7 kV there is very little change in background intensity with changing capacitor voltage. The effect of gas composition and pressure is very dramatic, however. The background intensity is lower at 50 Torr than at 700 Torr in both gases and is lower in He at both pressures. The background intensity in He at 50 Torr is nearly a factor of 400 lower than in air at 700 Torr. Figure 2 shows similar plots for the 288.15-nm ion line and the 283.79-nm neutral-atom line from the A1 foil matrix. Here a general trend toward lower intensity values is observed for both A1 species with increasing capacitor voltage. This trend is more obvious at 700 Torr than a t 50 Torr in both gases and is relatively insignificant for Al(1) in both gases a t 50 Torr. The range of intensity values observed for the four gaspressure combinations for any capacitor voltage value is considerably greater for the Al(I1) line radiation. The decrease in intensity at higher initial capacitor voltage may be the result of reduced foil vaporization. When dielectric breakdown occurs through the metal vapor or surrounding gas following the choking off of current conduction by the solid or liquid metal during vaporization, continued vaporization by joule heating of the bulk metal terminates. A t higher capacitor voltage, dielectric breakdown of the gas or metal vapor may occur at a slightly earlier time resulting in reduced vaporization of the foil. Similar results have been observed with exploding wires (2). The similarity between the Al(I1) and background intensity dispersion for the three voltage-capacitance combinations suggests that the continuum background originates from processes associated with the high population of Al(I1) species such as free-free or free-bound electron transitions. Figure 3 shows relative intensity vs. capacitor voltage plots for the Cd(1) 346.6-nm line and Cd(I1) 274.85-nm line. Again, a trend toward lower intensity values is observed a t higher voltage values, especially for explosions conducted at 700 Torr. The intensity dispersion for the four gas-pressure combinations is quite similar for Cd(1) and Al(1). For Cd(II), however, the range of intensity values is much smaller, and, in addition, air a t 700 Torr results in the lowest intensity at all capacitor
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
-Air-700
Cd
(E) He-SO
Air-SO
-
He 700 Air -700
Air-50
.z 5000 Cd (I)
5 2000 Air-700 He 700
-
4
5
6 7 8 Capacitor V o l t o ~ .kV-
2od b
Air-50
9
IO01
Figure 1. Plots of the relative intensity of the continuum background near 286.4 nm as functions of initial capacitor voltage for foils exploded at 282 J
50
4‘4y o AI
5000
(II)
i
20 IOL 4
\ I
5
He-SO
I
6
7
8
Capacitor Voltage, kV
9 --C
Flgure 3. Plots of the relative intensity of the Cd(l) 346.6-nm line and the Cd(ll) 274.85-nm line as functions of initial capacitor voltage for foils exploded at 282 J
Air-700
He-700 Air-50
‘7 too
:{-100
Air-700 He -700
Air-50
\
so, 4
P r e s s u r e . Torr~
He-SO
Figure 4. Plots of the relative intensity of the continuum background near 286.4 nm as functions of ambient gas pressure for explosions conducted at 5 kV and 282 J
I
5
6 7 8 9 Capacitor Voltage. kV-D
Figure 2. Plots of the relative intensity of the AI(I) 283.79-nm line and the AI(II) 288.15-nm line as functions of initial capacitor voltage for foils exploded at 282 J
voltage values. This suggests that the Cd(I1) species may not be significantly associated with current conduction processes in the discharge. Analogous plots for the Ni(1) 300.36-nm line and Ni(I1) 286.37-nm line obtained very similar results to those shown for Cd in Figure 3. The decrease in line radiation intensity for analytical species such as Cd and Ni with increasing initial capacitor voltage coupled with the relative invariance of the background radiation intensity suggests the use of low voltage with high capacitance to obtain a higher analysis line to background ratio, particularly for explosions conducted at higher pressures near 700 Torr. For example, an increase in line to background ratio of more than a factor of four can be obtained at 4.3 kV relative to 8.7 kV for the Cd(1) line in air at 700 Torr. However, this advantage is largely reduced a t lower pressure, particularly for lines from ionic species, Ambient Gas Pressure Control of Relative Intensities. Figure 4 shows plots of relative intensity of the continuum background near 286.4 nm as functions of gas pressure for foils exploded in air, Ar, and He over the pressure range from 10 Torr to 700 Torr. All explosions were conducted a t 5 kV with 282 J initially stored on the capacitor bank.
In all three gases, a very dramatic decrease in continuum background intensity is observed as the pressure is reduced. This decrease is most dramatic in Ar where the intensity decreases by more than three orders of magnitude over the pressure range investigated. Both the continuum intensity and its pressure dependence are very similar in air and Ar a t pressures greater than 100 Torr. Below 100 Torr, however, the intensity in Ar decreases more rapidly with decreasing pressure. A t all pressures greater than 10 Torr, the background intensity in He is significantly lower than in air or Ar with the relative difference between He and the other gases greatest in the pressure range between 100 and 200 Torr. A t pressures below 200 Torr, the decrease in intensity with decreasing pressure is significantly less extreme in He relative to the other gases. Figure 5 shows similar plots for Al(1) and Al(I1) lines from the foil matrix. The plots for Al(I1) are very similar to those in Figure 4.In fact, for pressures of 50 Torr and greater, the Al(I1) points for the spectra obtained in air are nearly superimposable with the corresponding background points in air. Again, this suggests that the continuum background radiation is associated with high populations of ionized aluminum species. The plots for Al(1) are considerably different than those for Al(I1). While the relative intensity is lower in He than in air
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
1501
I If
fz:
no
0
1000500
K
loWt 500
IOSOKX)
N
I
2ootf L
200
300
XK)
200
300
500 P r a r o u r a . Torr-
I Too
Figure 7. Plots of the relatlve Intensity of the NI(l) 300.36-nm line and the Ni(ll) 286.37-nm line as functions of ambient gas pressure for explosions conducted at 5 kV and 282 J
100
1050100
-
I
Too
P r a s s u r a , Torr-
Flgure 5. Plots of the relative intensity of the AI(I) 283.79-nm line and the AI(II) 288.15-nm line as functions of ambient gas pressure for explosions conducted at 5 kV and 282 J Cd
(PI ne Alr Ar
200 100
Ar Alr
ne
'1i 5
201;&
Id
o;
2bo
A
I
500
roo
P r e s s u r e . Torr-
Figure 6. Plots of the relative intensity of the Cd(l) 346.6-nm line and the Cd(ll) 274.85-nm line as functions of ambient gas pressure for explosions conducted at 5 kV and 282 J
or Ar a t all pressures greater than 100 Torr, the difference between the three gases is smaller for Al(1). In addition, the decrease in relative intensity when the pressure is reduced from 700 Torr to 10 Torr is no greater than about a factor of 30 for Al(I), while it is about a factor of 400 for Al(I1) in air and a factor of lO3for the background in Ar. A very different situation is observed in Figure 6 where relative intensities of Cd(1) and Cd(I1) are plotted as functions of pressure. For Cd(II), a well defined minimum occurs a t about 200 Torr in all three gases. At lower pressure, maxima occur in He and Ar near 100 Torr and near 50 Torr, respectively; while in air, the relative intensity continues to increase with decreasing pressure down to a t least 10 Torr. It is also interesting to note that, at all pressures greater than 50 Torr, the relative intensity is greatest in He and lowest in Ar. This 1502
is in strong contrast with the plots for Al(I), Al(II), and background intensity. The increasing intensity with decreasing pressure at pressures below 200 Torr results in a much smaller range of intensity values in any gas, and, in fact, the range of intensity values for Cd(I1) is less than an order of magnitude over the entire pressure range in any of the support gases. For Cd(I), the intensity minimum at 200 Torr is observed only in Ar where a maximum also is observed near 50 Torr as with Cd(I1). Also, the relative intensity is lower in He than in air or Ar a t all pressures. This is consistent with the plots for A1 and continuum background intensity in Figures 4 and 5. Figure 7 shows similar plots for Ni(1) and Ni(I1) lines. Again, a minimum is observed in the plots for Ni(I1) a t a pressure of about 200 Torr in all three gases. However, these minima are much less pronounced than with Cd(I1) in Figure 6. As the pressure is reduced from 200 Torr, maxima are observed in He near 100 Torr and in Ar and air near 50 Torr. These results are all qualitatively similar to those obtained for Cd(I1). However, the range of intensity values observed for Ni(I1) over the pressure range investigated in each of the support gases is greater than that observed with Cd(I1). The plots for Ni(1) show no maxima at any pressure and, in general, are very similar to the plots observed for Al(1) in Figure 5. The only significant difference between the plots for Ni(1) and Al(1) is that the relative differences in intensity between the He explosions and those conducted in the other gases are greater for Ni(1) than for Al(1) at all pressures. This same trend is observed when comparing Al(1) and Cd(1) in Figures 5 and 6, respectively. The very extreme differences in pressure dependence of radiation from continuum background and ionic species of the foil matrix relative to that from ionic species of the analyte deposited a t relatively low concentration on the foil surface suggest a change in the relative extent of participation in current conduction processes of sample and foil matrix materials as the pressure is reduced. The general decrease in intensity of all species with decreasing pressure at pressures between 700 and 200 Torr may reflect either a reduced vaporization efficiency at reduced pressure due to earlier dielectric breakdown of the support gas or the metal vapor produced during the early stages of vaporization or a reduced discharge current density caused by the greater plasma vol-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
ume at reduced pressure. Time-integrated photographs of A1 foils exploded under different experimental conditions have clearly shown that a larger volume, lower luminosity plasma is obtained a t reduced pressure. This has also been observed for exploding wire plasmas (4). The substantial increase in intensity of Cd(I1) and Ni(I1) radiation coupled with the dramatic decrease in intensity of Al(I1) radiation as the pressure is reduced from 200 Torr indicates that the sample material is carrying an increased fraction of the discharge current relative to the foil matrix material at reduced pressure. Two explanations seem plausible, both of which involve the time and pressure dependent dielectric breakdown of the gas or metal vapor. Analogous studies with exploding wires (2)have shown that dielectric breakdown occurs earlier in time with subsequent reduced wire vaporization efficiency at reduced pressure. This is consistent with longer electron mean free paths and correspondingly greater probability of impact ionization of the gas or metal vapor at reduced pressure ( 5 ) . Since the sample material is present in small quantity on the foil surface, it may vaporize completely before the increasing resistance of the foil promotes breakdown of the gas or metal vapor. Thus, at lower pressure, the amount of sample vapor relative to the amount of foil matrix vapor may increase to the point where the sample vapor is forced to carry a larger fraction of the discharge current. This differential vaporization of the sample and foil matrix materials would be enhanced by the lower boiling points of the Cd and Ni chlorides used here relative to that of the A1 foil (1). An alternative explanation can be inferred from time- and spacially-resolved spectra of exploding foils recently obtained using a rotating mirror streaking technique similar to one described by Walters and Malmstadt (6). Spectra obtained 10 mm above the foil, when the foil was exploded in 100 Torr Ar at 5 kV and 282 J, showed intense radiation from Ar ion lines originating from states over 23 eV above the neutralatom ground state for the first 5 ys of the discharge. No radiation from A1 or the Ni or Cd sample was observed until after the peak in the Ar ion line intensity. Once radiation appeared from the sample and foil matrix, radiation from Ar rapidly fell to below detectability. Foils exploded in Ar at 700 Torr showed no trace of Ar radiation. This provides strong evidence for a change in explosion mechanism as the pressure is reduced. At 700 Torr, dielectric breakdown seems to occur through the metal vapor; while a t 100 Torr, it occurs through the Ar. The increase in line radiation from the sample elements with the simultaneous decrease in radiation from the Ar may indicate excitation and ionization of the Ni and Cd through collisions of the second kind (7) with ionized Ar, possibly involving direct charge transfer. Since the Cd and Ni salts boil at a much lower temperature than the A1 foil matrix and since these salts initially are located on the foil surface, the Cd and Ni vapor should be localized near the leading edge of the expanding metal vapor cloud. This would increase the collisional interaction of these elements with the Ar ion discharge relative to the A1 matrix vapor. This, in turn, would result in increased excitation with a subsequent increase in line intensity from Cd and Ni as the pressure is reduced below the value required for dielectric breakdown through the Ar support gas. At pressures less than about 50 Torr in Ar, greatly reduced vaporization efficiency caused by the very early shunting of the current through the Ar coupled with volume dilution from the more rapid expansion of the metal vapor plasma could explain the rapid decrease in both neutral-atom and ion line intensities with decreasing pressure for Cd and Ni as well as for Al. The analytical implications of this are significant. Since the
500-
-
200
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10050 -
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, / 7 0 0
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Figure 8. Plots of the relative intensity of the continuum background near 286.4 nm as functions of energy for foils exploded at 22.5yF in Ar and He
Table I. Normalized Line to Background Ratios in Ar Pressure, Torr
Cd(I) Cd(I1) Ni( I) Ni(I1) AUI) Al(I1)
10
50
200
700
99.0 1430 24.0 36.4 38.5 9.52
69.4 321 13.0 82.5 5.40 2.50
1.27 1.97 1.55 1.20
1.00 1.00 1.00 1.00 1.00 1.00
1.68
0.758
Cd and Ni radiation reach their maximum intensity in Ar in the pressure range between 200 and 10 Torr where continuum background radiation is decreasing very rapidly with decreasing pressure, maximum analysis line to background ratios should be obtained in Ar at pressures less than 100 Torr. This is confirmed in Table I where line to background ratios are presented for four pressures in Ar normalized to the values obtained at 700 Torr. This strongly suggests using Ar at 10 or 50 Torr to reduce detection limits. Even though 10 Torr Ar obtained greater analysis line to background ratios for three of the four analysis lines, 50 Torr is suggested for analytical work, since the intensity maxima observed for Cd(I), Cd(II), and Ni(I1) lines near 50 Torr should result in less stringent pressure control requirements. Energy Dependence of Line and Background Radiation Intensity. When A1 foil strips were exploded a t a fixed capacitance of 22.5 yF but with the total explosion energy varying from 45 to 405 J, the plots shown in Figure 8 for background intensity were obtained. Experiments were conducted in air, Ar, and He at 50,200, and 700 Torr. Since there were no definitive differences between the plots obtained in air and Ar, only the He and Ar results are presented.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
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Table 11. Detection Limits for Exploding-Foil Excitation Wavelength, 200 Torr
Element
nm
SO Torr
Ag AS
328.1 218.0 267.6 306.8 346.6 345.4 358.8 253.7 323.3 285.2 279.8 352.4 283.3 283.0 252.9 284.0 377.6 328.2
AU
Bi Cd
co Cr
Hg
Li Mg
Mn Ni
-:2 0 0 0--
.
=
Pb Pt
/-
700 Torr
1000- He
Sb
Sn T1 Zn
soo200-
Detection limit. ng In 100 In 50 Torr He Torr Ar
1.7 80 25 5 4.0
12 1.7
15 20 0.40 0.40 2.0 16 100 45 30 1.0
16
0.49 10 5.0 10 0.32 2.0 0.33 3.2 6.0 0.030 0.031 0.25 5.0 12 8.0 20 2.0 2.0
10050-
20 -
-
0
d
loo
200 xx, Enorey. J-
400
I
Flgure 9. Plots of the relative intensity of the AI(I) 283.79-nm line as functions of energy for foils exploded at 22.5 I.LFin Ar and He
In both gases, a very significant increase in background intensity is observed as the energy is increased from 45 J. The increase is largest at low energy values, and at these low energy values, the increase is larger in Ar than in He a t all three pressures. In Ar at 200 and at 700 Torr, the intensities become nearly energy independent at energies greater than about 300 J. In Ar a t 50 Torr, the background intensity increases with increasing energy at least up to 405 J. In He at all three pressures, the background intensity increases continuously with increasing explosion energy over the entire energy range investigated. However, the increase is much more dramatic at 700 Torr. Energy plots for Al(I1) are, in general, very similar to the background plots in Figure 8. The only two significant differences are the smaller increase in Al(I1) line radiation intensity when the energy is increased from 45 to 101J in Ar a t 200 and 700 Torr and the much larger increase in intensity with increasing energy in He at 50 Torr. Figure 9 shows intensity vs. energy plots for Al(1) in Ar and He. Again, a very significant increase in intensity with increasing energy is observed at low energy values, and, in fact, the intensity increase in the energy range from 45 to 101J in Ar a t all three pressures is greater than that observed for the background or the Al(I1) in either gas. However, at higher energies, the relative intensity becomes quite independent of energy in both gases with indication of an intensity maximum near 282 J in both gases at 200 Torr and in He at 700 Torr. The range of intensity values observed for Al(1) in He is much smaller than the range of values observed for Al(I1) and the continuum background. Similar plots for Ni(I), Ni(II), Cd(I), and Cd(I1) showed no definitive differences from their A1 analogues. The very significant increase in relative intensity with increasing energy at low energy values, which is observed for all species as well as for the continuum background is hardly surprising since 39.6 J are required for the thermodynamically reversible vaporization of the 2.75 mg A1 foil strips used here 1504
( I ) . Thus, the 45 J available for the lowest energy explosions would, under the most favorable circumstances, leave only 5.4 J for excitation. However, since considerable energy is dissipated in the 0.14 residual resistance of the discharge tank circuit and additional energy is carried away from the explosion as hydrodynamic energy of the strong shock wave produced during the initial expansion phase of the explosion (8, 9),it is quite unlikely that complete vaporization occurred a t 45 J. This is further supported by the observation of luminous streamers from large incandescent particles for both wire (2) and foil explosions conducted at low energy. Thus, the increase in intensity observed with increasing explosion energy at low energy values probably results from more complete foil and sample vaporization as well as from the greater energy available for excitation following vaporization. The relative invariance in the intensity of Al(I), Al(II), and continuum background with respect to increasing energy in Ar at pressures of 200 Torr and greater a t energies of 282 J or greater suggests that complete vaporization occurs under these conditions. A t 50 Torr in Ar, dielectric breakdown of the Ar may occur so early with subsequent reduced vaporization efficiency that complete vaporization requires an energy greater than 405 J. The invariance of Al(1) relative intensity with respect to increasing energy in He at all pressures and at energies greater than 282 J also indicates complete vaporization at these higher energy values. The continued increase in relative intensity with increasing energy for Al(I1) and continuum background even at higher energy values in He may suggest a change in excitation conditions to favor a greater relative population of ionic A1 as the energy is increased. Again the qualitative similarity in the plots for Al(I1) and continuum background is noteworthy and suggests that the background is, at least in part, associated with a high population of ionized Al. Extension of Detection Limits. Figures 8 and 9 indicate that analysis line to background ratios are not strongly dependent on total available energy. However, explosions conducted at 282 and 405 J were, in general, more reproducible than those conducted at lower energy. This is consistent with previously reported results for exploding wire excitation (IO), where adequate reproducibility was obtained only when the total available energy was much greater than the energy required for the thermodynamically reversible vaporization of the wire. The analysis line to background ratios presented in Table I coupled with the greater reproducibility obtained at 50 Torr
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
relative to 10 Torr strongly suggest the use of Ar at 50 Torr for analytical work. Table I1 compares absolute detection limits for 18 elements in Ar a t 50 Torr with previously reported values obtained in He at 100 Torr ( I ) . Since these previous values were obtained using 282 J a t 22.5 pF, new values were obtained using these same conditions to provide a more direct comparison. These detection limits are defined as the minimum amount of the element required to produce a line intensity equal to three times the intensity equivalent of the root-mean-square noise on the microdensitometer trace in a nearby wavelength region of continuum background. Of these 18 elements, only Bi obtained a poorer detection limit in Ar. The improvement for the other 17 elements ranged from a factor of 1.5 for Sn to a factor of 13 for Mn and Mg.
CONCLUSIONS Since the pressure range which produced the lowest detection limits also is the range where the pressure dependencies of the A1 foil matrix line radiation and of the radiation from the Ni and Cd samples are quite different, the use of the A1 foil as a built-in internal reference is not suggested despite its convenience. This is consistent with reproducibility data presented in Ref. 1where the use of the A1 foil as an internal reference provided no significant improvement in precision; while an added internal reference was, in most cases, very effective. The detection limits presented here are not the lowest attainable values but reflect realistic values obtained on a slow photographic emulsion a t an ambient gas pressure and explosion energy which resulted in reproducibility consistent with previously reported values (I).Suggested operating parameters include Ar support gas a t 50 Torr and a 5-kV discharge a t 22.5 KF. Since the exploding foil plasma must be very heterogeneous in both time and space, time- and spacial-resolution techniques should obtain additional improvements in analysis line to background ratios. This approach has been quite successful with laser microprobe excitation (I1,12).Studies of this type are in progress. Based on quantum efficiency alone, photoelectric detection
should result in a considerable reduction in detection limits. An increase in precision also should be realized with photoelectric detection. However, the shot-to-shot fluctuation in continuum background intensity will require a background correction for each explosion. A recently described dualchannel monochromator attachment (13) in conjunction with a gated dual-channel integrator will be used in future work to facilitate this background correction. The greater sensitivity of photoelectric detection also will permit studies of analysis line to background intensity ratios a t very low ambient pressures and with very thin foils where continuum background intensity is below detection from single explosions record on photographic emulsions ( I ) . It is anticipated that the lower detection limits and greater speed and convenience which should be obtained with photoelectric detection coupled with the simplicity and low cost of exploding foil excitation may provide a useful analytical system for the determination of trace metals in micro solution samples.
LITERATURE CITED (1) C. S. Ling and R. D. Sacks, Anal. Chern., 47, 2074 (1975). (2) R. D. Sacks and J. A. Holcombe, Appl. Spectrosc., 28, 518 (1974). (3) J. A. Holcornbe, D. W. Brinkman, and R. D.Sacks, Anal. Chern., 47,441 (1975). (4) J. A. Holcombe, Ph.D. Thesis, Department of Chemistry, University of Michigan, 1974. (5) W. G. Chace, R. L. Morgan, and K. R. Saari, in "Exploding Wires", W. G. Chace and H. K. Moore, Ed., Plenum, New York, 1959, Vol. 1, p 59. 16\ J. P. Walters and H.V. Malmstadt. Anal. Chem.. 37. 1484 (1965). i7j D. Cobine, "Gaseous Conductors", Dover, New York, 1958. (8) F. D. Bennett, Phys. Fluids, 1, 515 (1958). (9) F. D. Bennett, in "Exploding Wires", W. G. Chace and H. K. Moore, Ed., Plenum, New York, 1959, Vol. 1, p 211. (IO) J. A. Holcornbe and R. D. Sacks, Spectrochirn. Acta, Part B, 28, 451 (1973). (1 1) W. J. Treytl, J. B. Orenberg, K. W. Marich, and D. Glick, Appl. Spectrosc., 25,376 (1971). (12) W. J. Treytl K. W. Marich, and D. Glick, Anal. Chern., 47, 1275 (1975). (13) D. W. Brinkrnan and R. D. Sacks, Anal. Chern., 47, 1723 (1975).
2.
RECEIVEDfor review February 20,1976. Accepted May 19, 1976. The authors acknowledge support of this study by the National Science Foundation through grant number MPS72-05099.
Laser Microprobe Spectrometry of Single-Crystal Metals and Alloys R. Kirchheim," U. Magorny, K. Maser, and G.T61g Max-Planck-lnstitut fur Metallforschung, Labor fur Reinststoffe am Insthut fur Werkstoffwissenschafn, Stuttgart, Germany
Single-crystal metals and alloys of high purity and different orlentation were used as specimens for laser microprobe emission spectrometry, in order to study the matrix effects of this analytical tool. Low-Index planes were prepared from the fcc metals Au, Cu and AI; the bcc metals Fe and Nb; and the hcp metal CO. Density of characteristic lines In photographically recorded spectrograms and reproducibility of lines revealed a strong dependence on crystal orientation; as a rule, density of lines increases with the density of atoms within a crystal plane. Physical reasons for this anisotropic effect and its Influence on quantitative analysis are discussed. Theoretical conslderations were checked by measuring momentum transfer durlng the evaporation process, and by performing quantitative analysis of single-crystal Cu-AI alloys. The an-
isotropic effect is canceled in these alloys by using a matrix line as an internal standard. Over an AI concentration range from the detection limit of about 300 ppm up to 7 wt %, !he calibration curve is a straight line, and the AI content of the samples could be determined with a precision better than 10%.
The laser microprobe has been used for sampling and sometimes also for excitation, in optical emission spectroscopy and mass spectroscopy (1-3). Ruby or neodymium laserlight can be focused down to a spot size of about 5-10 w, which is the lower limit of lateral resolution in local analysis (2, 4 ) . By varying the laser energy, the amount of evaporated material,
ANALYTICAL CHEMISTRY, VOL. 48,
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SEPTEMBER 1976
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