Atomic emission determination of selected trace metals in micro

Atomic Emission Determination of Selected Trace Metals in Micro Samples Using Exploding-Foil Excitation C. S. Ling and R. D. Sacks’ Department of Chemistry, University of Michigan, Ann Arbor, Mich. 48 104

The highly luminous metal-vapor plasma obtained from the electrical explosion of metal foils has been used as an excitation source for the analysis of selected trace metals in aqueous media. Microdensitometer traces of spectra obtained from the explosion of Ai and Ag foils at several pressures in air and He are presented. Current waveforms and time-resolved background spectra are discussed with respect to the choice of experimental conditions suitable for analytical applications. Ten-pi aqueous samples of metal salts are applied to 25-pm thick AI foil strips. The foil strips are exploded in 100 Torr He with a 5-kV, 282-J capacitive discharge. Analytical curves and reproducibility data are presented for Ni, Mn, Pb, and Cd, both with and without internal referencing. The relative standard deviations for these elements are 10.7, 4.1, 7.9, and 7.0%, respectively. Detection limits, usually in the low to sub ng range, are presented for 18 elements. Matrix and compound form effects are found to be relatively minor.

When a high-current source, such as a capacitor charged to several thousand volts, is switched across a thin metallic wire, rapid joule heating may result in the explosive vaporization of the wire. This exploding-wire phenomenon has been extensively studied by physicists and engineers, and a number of potential engineering applications have been documented (1-4). Preliminary studies recently were reported by Holcombe and Sacks ( 5 ) which used explodingwire excitation for the analysis of Hg, Cd, P b , and Ni electroplated onto the surface of 0.25-mm diameter Ag wires. The wires then were exploded by a 7.5-pF, 5-kV capacitive discharge. The resulting highly luminous metal-vapor plasma obtained absolute detection limits in the ng range for these elements and relative detection limits in the low ppb range for the electroplating bath. The radiative and electrical properties of Ag wires exploded under analytically useful excitation conditions also were reported recently (6). While electrodeposition a t controlled potential provides a convenient means for preconcentration and sample introduction, it is applicable only to elements which are conveniently electroplated from aqueous media a t reasonable potentials. In addition, the small surface area of the thin wires results in long deposition times and incomplete deposition. The variability in the fraction of the total amount of material deposited in a fixed deposition time contributes substantially to the overall variance of the analytical procedure. These problems can be reduced by using wires of greater surface area; however, the attendant increase in mass is deleterious in several ways. First, an excessively large energy storage facility is required to vaporize the wire while maintaining sufficient residual energy for efficient excitation in the resulting metal-vapor plasma. Second, continuum background radiation may increase with increasing Author to whom correspondence should be directed. 2074

wire mass, and, finally, explosion reproducibility is poorer with thicker wires ( 7 ) . Preliminary studies reported here explore the possible application of exploding metal foils for the excitation of trace metals deposited on their surface. While the ratio of surface area to mass of metal foils of reasonable thickness is only slightly more favorable than that obtained with wires, the presence of a flat surface does offer the possibility of the direct analysis of solutions and powders. This provides increased flexibility for exploding-conductor excitation. The preliminary results discussed here consider only the analysis of micro solution samples (10 pl). Documentation on exploding foils in the open literature is extremely limited and consists of a very few physics and engineering studies (8-14).

EXPERIMENTAL Capacitive Discharge Circuit and Associated Electronics. T h e capacitive discharge excitation source consists of a high-voltage capacitor bank using from one to four 7.5 pF General Electric Pyranol, low inductance capacitors in series with the metal foil and a three-electrode spark-gap switch. T h e capacitor is charged to 5 kV by a high-voltage neon-sign transformer through a high-voltage, half-wave rectifier stack and a 500-kil charging resistor. T h e explosion is initiated by firing the spark-gap switch via a n EG&G TR-69 high-voltage pulse transformer and a silicon control rectifier trigger circuit. T h e high-voltage excitation source, sparkgap switch, and trigger circuit are described in detail in References 5 and 6. T h e explosion current is monitored oscilloscopically using a specially constructed, four-terminal, low-inductance, triaxial current shunt similar to the design of Park (15). The current shunt uses a 2.28-mil Monel tube as a resistance element, and has a ratio of dc resistance to ac impedance greater than 0.92 a t frequencies up to 500 kHz. T h e shunt can monitor currents up to about 50 kA. Table I summarizes the source parameters and excitation conditions. Explosion Chamber. The Pyrex glass chamber is shown in Figure 1. I t is a 22-cm long, 13-cm i d . , 6-mm wall cylinder O-ring sealed to 1.8-cm thick methyl methacrylate plastic top and bottom plates. High-voltage vacuum feedthroughs and a vacuum-line connection are provided in the bottom plate. Radiation is viewed through a 4.4-cm diameter, 6-mm thick quartz window located in the chamber wall. Strips of foil are supported on 33-mm X 26-mm X 5 m m thick glass plates. While some etching of the glass plate did occur from the explosions, this appeared to have little effect on observed radiation patterns or explosion reproducibility. T h e glass support plate could be used repeatedly after cleaning in 12M HCI. T h e foil strip and glass support plate are clamped into 26-mm X 26-mm plane, parallel electrodes made from electrolytically pure copper. Copper was chosen here because it showed less erosion from the high-current discharge than other common metals. T h e distance between the electrodes is adjustable to accommodate foil strips from 20 to about 40 mm in length. However, all work reported here used 25-mm foil strips. T h e foil strip was oriented perpendicular to both the optical axis and the spectrograph slit. Foil and Sample Preparation. Foil strips, 1.6 mm wide, of Cu, AI, and Ag were cut from high-purity foil sheets (Alfa-Ventron, Beverly, Mass.). A simple cutting tool was used, which was made from two razor blades separated by a 1.6-mm thick shim. Table I1 summarizes the physical properties and dimensions of the foils. T h e strips were washed in reagent grade acetone. After positioning a foil strip and glass support plate in the electrode assembly, a 1 0 - ~ aqueous 1 sample was applied with a micro syringe. T h e sam-

ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975

Table I. Source Parameters and Excitation Conditions Residual inductance 2.8 pH Residual resistance 0.05-0.15 .Q Resistance of current shunt 2.28 mi2 Charging voltage 5 kV Ringing frequency, kHz

7.5 15 22.5 30

94 188 282 376

27.8 21.3 17.8 13.9

Table 11. Physical Properties a n d Dimensions of Foil Materials Purity, % Density, g/cm3 Melting point, K Boiling point, K Latent heat of fusion, kJ/g Latent heat of vaporization, kJ/g Mass, mg (2.5 pm thick) (12.5 pm thick) (25 pm thick) Energy to vaporize, J (2.5 pm thick) (12.5 p m thick) (25 pm thick) Resistance at 293 K , m n (2.5 pm thick) (12.5 Fm thick) (25 pm thick)

u 2"m u

Ag

CU

A1

99.99 10.5 1234 2485 0.11

99.999 8.94 1356 2868 0.204

99.9995 2.70 931.7 2600 0.398

2.35

4.79

10.50

1.07 5.36 10.71

0.91 4.55 9.10

0.27 1.38 2.75

3.2 16 32.1

5.42 27.1 54.2

4 .O 19.8 39.6

100 20.0 10.0

105 2 1 .o 10.5

167 33.4 16.7

connection

Figure 1. Explosion chamber

The glass chamber is O-ring sealed to the top and bottom plates. The electrolytically pure Cu electrodes hold the glass support plate and foil strip current shunt was displayed simultaneously with the phototube signal. In all experiments, the foil strip was located 28 cm from the spectrometer entrance slit. No focusing optics or spacial resolution was employed, and the spectrometer viewed the entire plasma volume.

RESULTS A N D DISCUSSION

No detailed spectroscopic studies of exploding foils could

ple was applied as six uniform drops equally spaced over the length of the foil strip. The samples were desolvated by evacuating the chamber. Following each experiment, the copper electrodes were polished with 600 mesh abrasive paper and rinsed with distilled water. This procedure was sufficient to reduce the contamination blank from the previous experiment to below detectability, even for the highest sample concentrations reported here. Since some metal vapor condensed on the quartz window, the window was cleaned with a three-to-one mixture of HCl and "03 after every two explosions. Sample stock solutions (1000 ppm) were prepared from reagent grade materials. Volumetric dilutions were prepared daily. Explosions were conducted in air and He a t pressures of 10, 100, and 730 Torr. The explosion chamber was evacuated to a pressure less than Torr prior to filling with support gas. Optical Monitoring and Calibration. All spectra were obtained on a 1.0-m Czerny-Turner spectrometer (Jarrell-Ash Model 78-460) using a 100-pm wide entrance slit and having a first-order linear reciprocal dispersion of 0.8 nm/mm. Time-integrated spectra were recorded on Kodak SA1 spectroscopic plates developed for 5 min in Kodak D-19 developer a t 22 "C. Optical densities were obtained on a Joyce-Loebl Mark IIIB recording microdensitometer. Plates were calibrated a t the wavelengths of interest using a two-step neutral density filter and an exploding foil as an excitation source. Several explosions under different conditions were used here to obtain calibration data from gross fog to over 2.5 optical density units. Emulsion calibration and density-to-intensity conversion were accomplished using a previously described computer procedure (16). Time-resolved spectra were obtained photoelectrically using a 100-pm spectrometer exit slit and an RCA 1P28 multiplier phototube. The tube was operated a t -900 V and used a 1-kQ load resistor. The output from the phototube was displayed on a Tektronix Type 565 dual-beam oscilloscope. The output from the calibrated

be found in the open literature. Thus, preliminary studies parallelling those reported previously for exploding wires ( 5 ) were initiated. In these studies, 25-pm Ag, 25-pm, and 2.5-pm A1 and 25-pm, 12.5-pm, and 2.5-pm thick Cu foils were exploded in air and He a t 10, 100, and 730 Torr. Explosions were conducted at 5 kV with 94 J and 282 J initially stored on the capacitor bank. Explosion reproducibility was considerably greater in the higher energy experiments. This is consistent with previously reported results (5,6) for exploding wires, which indicated that reproducible explosions were obtained only when the available energy was much greater than the energy required for thermodynamically reversible vaporization of the wire. When comparing the energy required for vaporization from Table I1 with the available energy on the capacitor bank, it is important to note that transduction of electrical energy to joule heating of the foil is very inefficient during the early stages of the explosion when the foil resistance is low. Here, much of the available energy is dissipated in the residual source resistance. In addition, the conversion of electrical energy to hydrodynamic energy of the shock wave formed during the early stage of the explosion ( I 7, 18) further reduces the overall conversion efficiency and contributes to the high initial capacitor energy required for reproducible explosions. Time-Integrated Spectra of Exploding Foils. The time-integrated radiative properties of 25-pm thick A1 foils are shown on the microdensitometer traces in Figure 2. Lines marked A and B are the 308.2-nm and 309.2-nm A1 neutral-atom resonance lines, respectively. Line marked C is the 358.7-nm line from singly-ionized Al, and D and E are the 360.1-nm and 361.2-nm lines, respectively, from doubly-ionized Al. These latter lines originate from upper states about 17.7 eV above the neutral-atom ground state. The existence of very strong lines from such high levels of excitation suggests a plasma which is spark-like in character with relatively high current density in the discharge current channel.


2075

2 5 r m bl

25pm

Aq

He

Air

He

7x) Tori

2

I

0 273

&

K

L

I

o

I I

278 326

331

273

278326

T

o

r

r

33

Wavelength. nm Wavelength.

nm

Flgure 2. Microdensitometer traces obtained from 25-pm thick AI foils exploded at 5 kV and 282 J (A) AI(I) 308.2 nm;(B)Al(l) 309.2 nm: (C)AI(II) 358.7 nm; (0)AI(III) 360.1 nm; (E) Al(lI1) 361.2 nm 2.5um A I

ne

Air

730 Torr

E l

0" 0

k

-

100 Torr

10 Ton

3%

311357

362

306

Wavelength,

311 357

362

nm

Flgure 3. Microdensitometer traces obtained from 2.5-pm thick AI foils exploded at 5 kV and 282 J (A) Al(l) 308.2 nm; ( B ) Al(l) 309.2 nm; (C) AI(II) 358.7 nm: (0)Al(ll1) 360.1nm; (E) AI(III) 361.2 nm

At 730 Torr, the neutral-atom resonance lines appear as self-reversed regions in a very strong continuum. While the continuum intensity is comparable in the two gases a t 730 Torr, the self-reversals are more apparent in air. The width of these reversal regions also is noteworthy, since they are considerably greater than the 0.08-nm band pass of the spectrometer. This suggests a relatively cool but very dense region of metal vapor surrounding a hotter, radiating current channel. Similar results have been reported for exploding wires (6). The ion lines of A1 are not resolved in the 730-Torr air explosion, but they do appear as very broad emission lines in the He explosions at this pressure. A very striking similarity is observed between the 730-Torr He explosion and the 100-Torr air explosion. This is particularly apparent for the lines from both the singly- and doubly-charged ion 2076

Flgure 4. Microdensitometer traces obtained from 25-pm thick Ag foils exploded at 5 kV and 282 J ( A ) Ag(l) 328.0 nm; (B)Cu(l) 327.4 nm; (€J Ag(ll) 274.3 nm

nm;(C)Ag(I1) 276.7 nm; (0) AgQ) 275.6

species. In He at 100 Torr, the continuum intensity is greatly reduced, and the neutral-atom resonance lines appear only in emission; however, the line widths are extraordinary. The relatively large amount of metal vapor generated in a small volume early in the explosion suggests considerable collisional broadening. While self-absorption may be occurring here at certain times during the explosion, it cannot be detected in time-integrated spectra (6). The line from singly-ionized Al, while still quite broad, is considerably narrower than at 100 Torr in air or a t 730 Torr in He. The similarity between the 100-Torr He and the 10-Torr air explosions again is noteworthy. In 10-Torr He, the continuum background intensity has fallen below detection on the SA1 emulsion, and the intensity of the line radiation is greatly reduced. In addition, line widths primarily are slitwidth limited. Figure 3 shows a similar set of microdensitometer traces for explosions of 2.5-pm thick A1 foils. No self-reversals are apparent in any of these traces for the thinner foils. In air a t 730 Torr, a nearly structureless continuum is observed. In He a t this pressure, both neutral-atom and ion lines appear in emission. Again, the line widths are extraordinary. As the pressure is reduced, the same trends are observed as with the 25-pm thick foil. However, a t any given pressure, both the continuum and the line radiation intensity are lower with the thinner foils. In addition, both neutral-atom and ion lines are considerably narrower with the 2.5-pm thick foils. This is most apparent in 100-Torr He and 10Torr air. Figure 4 shows a set of microdensitometer traces obtained from explosions of 25-pm thick Ag foils under the same conditions as those used for the A1 foils in Figures 2 and 3. The line marked A is the 328.0-nm Ag neutral-atom resonance line, and the line marked B is the 327.4-nm Cu neutral-atom resonance line. Since the Ag foil used here was of very high purity, this strong Cu line probably is from sampling of the copper electrode by the high current density plasma which forms between the electrodes following vaporization of the foil. The lines marked C, D, and E are the 276.7, 275.6, and 274.3-nm lines, respectively from singlyionized Ag. Line D originates from the 6s 'D level, which is 23.1 eV above the neutral-atom ground state. Again, the presence of intense lines from highly energetic excited


states suggests a high-current density channel following vaporization. In general, the trends observed in Figures 2 and 3 for A1 foils also are observed for Ag foils. The noteworthy differences are in the narrower ion lines a t reduced pressure, the lower intensity of the background continuum, particularly a t 730 Torr in He and 100 Torr in air, and the lack of strong-reversals of the neutral-atom resonance lines at high pressure in the Ag spectra. It is of interest to note that the width of the Cu neutral-atom resonance line is much less than that of the Ag neutral-atom resonance line and that the decrease in intensity of the Cu line with decreasing pressure is less obvious than that of the Ag line. This further suggests that the Cu is introduced by electrode sampling rather than by vaporization of the foil. Explosions of 2.5, 12.5, and 25-pm thick Cu foils produced spectra showing the same qualitative trends as those obtained from Ag and Al. Since all three foil materials produced similar results, the decision to use A1 foils for the development of analytical procedures was based primarily on the fewer spectral lines and, hence, fewer potential line interferences observed in the A1 spectra. The lower continuum emission and the narrower lines observed with the 2.5-pm A1 foil relative to the 25-pm thick A1 foil suggests the use of the thinner foil for analytical studies. However, difficulty in cutting and handling this extremely fragile material decreased the explosion reproducibility to an unacceptable value, Hence, all analytical studies reported here used 25-pm thick A1 foil as an explosion substrate. Helium a t 100 Torr was chosen as a support gas since continuous background would just be detected under these conditions using SA1 emulsion. While the background continuum intensity is similar in air a t 10 Torr, the greater difficulty in controlling the pressure in this range adversely affects explosion reproducibility. Discharge Current and Background Radiation Waveforms from A1 Foils. Figure 5 shows current waveforms and multiplier phototube response waveforms for 25-pm thick A1 foils exploded under the six conditions of Figure 2. The upper waveform of each pair is the discharge current with zero current indicated by the dashed line. The lower waveform of each pair is the multiplier phototube response in a region of continuous background near 319.9 nm. The intensity scale here is in arbitrary units but is consistent throughout the figure. In air at 730 Torr, the smooth, damped sinusoidal current waveform is interrupted by a kink about 8 psec after the start of the trace. The first detected radiation occurs simultaneously with this kink. A number of investigators (19, 20) have suggested that similar kinks occurring in the current waveforms from exploding wires indicate the onset of vaporization. Bennett (21) has suggested the formation of a vaporization wave which propagates through the wire (or foil) at the local speed of sound. This vaporization wave represents the movement of the phase boundary through the liquid metal. As the metal vaporizes, it is converted from a conducting liquid to a gaseous dielectric. This results in a rapid decrease in conductance of the metal with a corresponding increase in voltage drop across the conduction path. However, before the current has fallen very far, the increased voltage drop results in dielectric breakdown of the metal vapor or the surrounding gas (6). Following this restrike, the sinusoidal current resumes with current conduction through the metal-vapor plasma. A similar current waveform is obtained in He a t 730 Torr. However, here the kink a t about 8 psec is less pronounced. In air a t 100 Torr, the kink appears more as a slight discontinuity in the current waveform. In He a t 100 Torr and in both air and He at 10 Torr, no discontinuity is

2oL

20L

:0

20

40

$3 T i m e , psec

Figure 5. Discharge current and radiation waveforms obtained from 25-pm thick AI foils exploded at 5 kV and 282 J The radiation was measured in a region of continuum background near 319.9 nm

detected, and a smooth, damped sinusoidal waveform is obtained. However, the first detected radiation a t about 8 psec suggests that vaporization begins a t about the same time. What is not readily apparent in Figure 5 , but is of considerable significance, is the steady increase in the peak current of the first current halfcycle as the vaporization kink becomes less apparent. Sacks and Holcombe (6) have observed similar trends for Ag wires exploded in air and He. They suggested that reduced vaporization efficiency at lower pressure and in He relative to air, results in less ohmic energy dissipation in the metal prior to plasma restrike, thus resulting in a smaller current fall during vaporization and increased residual capacitor voltage following vaporization. This reduced vaporization efficiency results from earlier restrike or plasma formation because of the easier dielectric breakdown of either the metal vapor or the surrounding gas a t reduced pressure and in He relative to air. The peak radiation intensity occurs just after vaporization in both air and He a t 730 and 100 Torr. I t is significant to note that the background intensity decreases rapidly after the initial peak about one quarter of the way through the first current halfcycle. This decrease in background intensity with increasing discharge current suggests that this initial background peak is associated with the vaporization processes and is not controlled by the discharge current following restrike. This is confirmed in the second and subsequent current halfcycles where this background radiation does follow the current amplitude. This is most readily apparent in the He explosions. It is also significant to note that the amplitude of this initial background peak decreases not only in an absolute sense with decreasing pressure and in He relative to air, but also relative to the background intensity in the second and subsequent current halfcycles. At 10 Torr in both gases, this initial peak loses its sharp leading edge, more closely follows the current waveform in the first current halfcycle, and no longer dominates the overall background waveform. If this initial background peak is associated with the vaporization processes, perhaps as blackbody radiation, then the decreased energy deposited in the foil prior to plasma


2077

Air

He

I

730 Torr

,-:.

200 p9 NI 0

Figure 7. AI vapor condensation patterns obtained from 25-gm thick AI foils exploded at 5 kV and 282 J

'

0

-

0 0

.

o 20

s 40

L

,b,

QOO,J&J

60 Time,

0

20

40

60

prcc

Figure 6. Discharge current and radiation waveforms obtained from 2.5-pm thick AI foils exploded at 5 kV and 282 J The radiation was measured in a region of continuum background near 319.9 nm

restrike a t reduced pressure and in He relative to air is consistent with the decreased prominence of this initial background peak. The absence of any kink in the current waveforms a t 10 Torr, coupled with the shape of the background radiation waveform, suggests very inefficient foil vaporization and a time-integrated background intensity controlled more by processes in the restrike plasma than by the initial vaporization. These conclusions are consistent with the microdensitometer traces shown in Figure 2. Figure 6 shows current waveforms and multiplier phototube response waveforms for 2.5-pm thick A1 foils corresponding to the six microdensitometer traces in Figure 3. No kink or discontinuity is observed in any of these current waveforms, and the first radiation is detected just after the start of the current waveform. These observations are consistent with the very low energy required for reversible vaporization of these thinner foils and the more efficient energy conversion in the higher initial foil resistance. This results in the very early vaporization and subsequent current transfer to the metal-vapor plasma. The background radiation patterns observed for the 2.5pm thick foils are considerably different than those observed for the 25-pm foils in Figure 5 . In all cases for the 2.5-pm thick foils, the peak in the background intensity nearly coincides with the current peak during the first as well as during subsequent current halfcycles. Only in air a t 730 and 100 Torr does the sharp leading edge of the background waveform a t the start of the discharge indicate significant background contribution from the vaporization processes. The intensity during the second and subsequent radiation peaks relative to the first peak also indicates that, with thinner foils, the background intensity is controlled by the plasma current following restrike. When comparing the background radiation waveforms for the two foil thicknesses in the 730-Torr air explosions, it is interesting to note the shoulder on the falling edge of the first radiation peak for the 25-pm foil. The location and amplitude of this shoulder suggest that the radiation waveform is the composite of the plasma radiation background similar to that obtained with the 2.5-pm foil and an initial radiation peak from the foil vaporization. The nearly similar background intensity obtained for both foil sizes at 10 Torr in both air and He again suggests that little contin2078

* ANALYTICAL CHEMISTRY, VOL.

uum radiation is produced during the foil vaporization. The origin of the background peak at about 47 psec in the 10-Torr He explosion is not clear. This peak, however, is a reproducible feature of explosions conducted under these conditions. These results again suggest the use of very thin foils exploded in He at reduced pressure to obtain more favorable time-integrated line-to-background intensity ratios. Condensation Patterns. The room-temperature glass plate used to support the foil provides a condensation SUIface for A1 vapor immediately after vaporization. Conn (22) and Mattox et al. (23) used the condensation of metal vapor from exploding wires to obtain mirror-like thin film deposits. The nature of the condensation patterns shown in Figure 7 is strongly dependent on the ambient gas and pressure as well as on the sample concentration. As the pressure is reduced, the longer mean free paths for the vapor particles results in less condensation on the glass plate. In He, a bright metallic deposit is obtained at 730 and 100 Torr. At 10 Torr in He, only a few streaks of metal are observed perpendicular to the foil strip axis. This appears to result from splattering rather than vaporization of the Al. This again suggests incomplete vaporization of the foil. I t is also interesting to note that the air explosions produce a gray to black deposit indicating some reaction with the air. The presence of a sample on the foil produces no apparent change in the condensation patterns until the sample size exceeds about 50 pg. As seen in Figure 7 , the presence of 200 pg of Ni results in a highly striated condensation pattern, where the voids in the condensation film correspond to the location of the six sample drops. This suggests that the sample retards vaporization of the foil. This is not surprising at such high sample loadings, since the sample must vaporize before the A1 itself can undergo normal vaporization. Since the heating mechanism for the solid sample residue probably is controlled by relatively slow thermal conduction across the foil-sample interface, the dielectric breakdown of the gas or metal vapor may interrupt the current in the foil prior to complete vaporization in local regions of high sample loading. While incomplete vaporization of the foil and possibly the sample may significantly affect radiation patterns, this poses no real limitation since the 50 pg or more of sample required to observe this effect corresponds to at least a 5000-ppm solution for the 10-pl samples used for analytical studies. Analytical Curves. Analytical curves were prepared using 10-111 aliquots of aqueous solutions of the metal salts. All explosions were conducted a t 5 kV and 282 J initially stored on the capacitor bank. Analytical curves were prepared for Mn, Pb, Ni, and Cd. These elements were chosen because of their wide range of thermodynamic properties. Each point on these curves rep-

47, NO. 13, NOVEMBER 1975

Table 111. Compounds a n d Analysis Lines Used for Analytical Curves Sample

Sample line, n m

Mn as MnSOI.H,O P b as Pb(NO,), N i as NiC1,*6H,O Cd as CdC1,

(11) 280.5

(I) 283.3 (I) 356.6 (11) 274.8

Reference

1 pg C r as C r ( N 0 , ) , * 9 H 2 0 1 yg N i as NiC1,.6H20 1 pg C r as C r ( N 0 3 ) , . 9 H 2 0 1 0 pg Ni as NiC1,*6H,O

Reference line, n m

A1 l i n e , nrn

(11) 286.2 (I) 352.4

(11) 288.1 (I) 283.7

(I) 357.8 (11) 286.4

(I) 348.2 (11) 288.1

Table IV. Detection l i m i t s for Exploding-Foil Excitation Elen ent

Compound

‘itavelength,

nm

328.1 278.0 267.6 306.8 346.6 34 5.4 358.8 253.7 323.3 Li 285.2 MR 279.8 Mn 352.4 Ni 283.3 Pb 283 .O Pt 252.9 Sb 284.0 Sn SnC1,*2H,Cl 377.6 TI TIC1 Zn Z nO 328.2 a Rased on 10.~1 aqueous ,samples.

Ag As Au Bi Cd co Cr Hg

Relative detection l i m i t , ppma

Absolute detrcnon limit, ng

0.17 8 .O 2.5 0.5 4 .O

1.7 80 25 5 .O 40

1.2

12 1.7

0.17 1.5 2 .o 0.04 0.04 0.2 1.6 10 4.5 3.0 0.7 1.6

15 20 0.4 0.4

2 .o 16 100 45 30 7 .O 16

resents the average of four or five determinations a t the indicated concentration. A.ll spectra for a given element and concentration were recorded on a single photographic plate to minimize photographic processing error. Since exploding foil excitation is based on a transient, nonrepetitive event, conventional noise reduction devices such as lock-in amplifiers cannot be used. Thus, internal referencing may be necessary to compensate for certain problems, such as positional instability of the plasma. Each Mn and Ni sample contained 1.0 p g of Cr as an internal reference. For the P b and Cd analyses, 1.0 pg and 10 pg of Ni, respectively was added to each sample as an internal reference. These internal :reference elements were chosen strictly on the basis of convenience and the availability of lines of suitable intensity in the appropriate wavelength regions. There was no attempt to match either thermodynamic properties or excitation energies of the reference and sample. Holcombe and Sacks ( 5 ) suggested’ using a convenient line from the Ag-wire matrix as an internal reference for the analysis of metals electroplated onto the surface of the Ag wire using explo1din;g wire excitation. This procedure also was evaluated here using a convenient A1 line from the foil matrix. Table I11 summarizes the compound forms and analysis lines used in these studies. Figure 8 shows a composit of three analytical curves for Mn. The reduction in point scatter obtained by ratioing the intensity of the Mn line to that of the Cr internal reference is significant. However, no significant improvement is observed using A1 as an internal reference relative to the Mn analytical curve obtained without line intensity ratioing. All three curves are linear with nearly the same slope for Mn samples less than about 2 pg. The negative deviation observed for larger samples suggests either incomplete sample vaporization or self-absorption. Since spectral line

Figure 8.

Analytical curves for Mn

Each point represents the average of four or five determinations at the indicated concentration

shapes may change drastically during the course of an explosion ( 6 ) , line profile studies on time-integrated spectra are not a reliable indicator of self-absorption, and photographically recorded time-resolved spectra with a narrow bandpass spectrometer will be required. The other elements evaluated gave similar results except that the most satisfactory P b analytical curve was obtained without internal referencing. Figure 9 shows these curves. The Mn curve has been repeated for comparison with the other elements. In general, the curves have a linear dynamic range of a t least two to three decades of concentration. The Ni curve as well as the Mn curve shows a negative deviation for large samples. Again, this is attributed to either incomplete vaporization or self-absorption. The need for more definitive studies is indicated here. Detection Limits. Detection limits were determined for the 18 elements listed in Table IV. These detection limits are defined as the minimum amount of the element required t o produce a line intensity equal t o three times the intensity equivalent of the root-mean-square noise on the microdensitometer trace in a nearby wavelength region of continuum background. T o obtain reliable detection limits, the SA1 photographic plates were prefogged with a Kodak Wratten series OA filter to obtain an optical density of 0.25 on the otherwise unexposed emulsion. This ensured operation on the linear region of the emulsion. The relative detection limits in ppm included in Table IV are based on 10-pl samples. While increasing sample volume will reduce these relative detection limits, 50 p1 is about the maximum sample size consistent with the dimensions of the foil strips used here. Larger samples can be used only through a time-consuming series of application and desolvation steps. Thinner, larger surface area foils should make the use of larger samples quite straightforward if the cutting and handling problems can be over-

ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975 * 2079

Ic o r

20r

0

2

-

b

NI (Cd)

mi

K

0.01

1.0

0.1 ,U9

100

IO

Sample

Figure 10. Intensities of the internal reference lines as functions of the analyte concentrations For each curve, the internal reference was present at fixed concentration: while the analyte, which is indicated parenthetically, varied in concentration over the indicated range 0.1

pg

1.0 Sample

10

100

Figure 9. Analytical curves for Mn, Ni, Pb, and Cd Each point represents the average of four or five determinations at the indicated concentration

come. I t should be noted here that these detection limits are based on the use of a rather slow photographic emulsion. I t is suggested that some improvement may be obtained through photoelectric detection. Reproducibility of Exploding-Foil Excitation. The reproducibility of the foil cutting and sample introduction steps was evaluated along with the reproducibility of the overall procedure. Eight strips of foil were cut using the previously described cutting tool. Each strip was cut 30 mm X 1.6 mm. The strips were weighed on a Perkin-Elmer Electro Autobalance and obtained a % relative standard deviation of 1.0%. The reproducibility of the sample introduction technique was evaluated spectrophotometrically using a green food dye solution. Ten-pl aliquots of the concentrated food dye solution were introduced onto five foil strips. Each strip then was immersed in 25.0 ml of distilled water and the absorbance measured a t 625 nm with a Beckman Model DB spectrophotometer. A second group of five foils was similarly treated except that the solvent was removed under vacuum a t room temperature using a procedure identical to that used for the preparation of analytical samples. The dye residue on each foil strip was then dissolved in 25.0 ml of distilled water and the absorbance measured. The results indicated an average sample recovery of 97% with a % relative standard deviation of 1.4%. The overall method reproducibility was determined for the replicate determinations used in obtaining the data for the analytical curves in Figure 9. The % relative standard deviations were computed for one concentration of each element with no internal referencing, using A1 from the foil matrix as an internal reference, and using an added internal reference. The results are listed in Table V. For Mn, Ni, and Cd, the results using the added internal reference were far superior to those obtained from the other procedures. For Pb, the poorest results were obtained with the added internal reference and the most satisfactory results with no internal reference. There is no obvious explanation for this anomaly, which is observed a t all P b concentrations. I t is significant that with Ni and Mn, poorer results are obtained using A1 as an internal reference than with no internal reference. Previously reported work ( 5 ) with exploding Ag wires indicated that the Ag wire matrix could be used effectively as an internal reference. The difference here is that using electrodeposition onto wires for sample introduction results in a conductive, metallic sample coating, which can be heated and vaporized directly by joule heating. The solution samples used with exploding-foil excitation form a dielectric coating on the foil surface which 2080

L_

Table V. Percent Relative Standard Deviations for Exploding-Foil Excitation \In(lOw)

No internal

reference A1 internal reference Added internal reference

Yi(1Oug)

Cd(1Oug)

16

17

29

21

20

22

4.1

10.7

7.0

i’d(O.5~~)

7.9

16

33

is heated indirectly by conduction across the sample-foil interface. As shown in the condensation patterns of Figure 7, the presence of the dielectric sample can affect the normal vaporization of the foil. Thus, shot-to-shot variation in the distribution of sample on the foil will affect the amount of foil vaporized and thus the intensity of the A1 line radiation. This is supported by a significant increase in the relative standard deviation of A1 line radiation intensity as the sample size is increased. This is observed for all four elements tested. Compound F o r m a n d M a t r i x Effects. The thermodynamic properties of the sample compound should be of only minor significance with exploding-foil excitation. The high effective current-channel temperature suggested by the high intensity of lines from doubly-ionized A1 species indicates that compound formation in the metal vapor plasma following vaporization should not be significant. In addition, the rapid vaporization and the total-sample-consumption nature of the sample vaporization suggests that compound form effects, such as fractional distillation, should not be important during the sampling or vaporization step. Table VI shows the results of a preliminary study of compound form for Cd and Ni. A series of foils was exploded, each containing 2.0 fig of Cd as the nitrate, chloride, or fluoride and 1.0 fig of Ni as internal reference. Each value in Table VI represents the average of five determinations. No statistically significant compound form effect was observed. A similar study for the determination of Ni as the chloride and nitrate with each sample containing 2.0 fig of Ni and 1.0 fig of Cr as internal reference produced similar results. Only with very large samples, where incomplete vaporization may occur, should compound form be significant. A considerably more detailed study, however, will be required to confirm these preliminary observations. Matrix effects can be evaluated by observing the behavior of the internal reference elements as the sample concentration is changed. Here, the sample represents a matrix with its concentration changing over two or more orders of magnitude, while the internal reference is present a t a fixed concentration. Figure 10 shows the intensities of the Ni and Cr internal reference lines as functions of the sample concentrations.


Table VI. Effect of Compound Form in Exploding-Foil Excitation Amount lourd, pg Cd(11 326.1 nm

Element

Cc,mpound

Cd Cd Cd

Ccb(NO, ) 4 H,0 CC.Cl, CciF2

342 841 1373

Bai!ing point, li

a d d e d , uil

Cd(1) 326.1 n n i

N i ( 1 ) 313.4 nrn

405 1233 2031

2 .o 2 .o 2 .o

2.1 2 .o 2.2

2.2 2 .o 1.9

An?OWt

A i i I ) 300.2 n ' ~ N i ( 1 ) 305.5 nm

Ni Ni

NiC1,-6H20 Ni(N0,),.6HZ0

*..

...

330

410

The element indicated parenthetically by each plot is the matrix whose concentration is shown on the abscissa. Relative intensities along the ordinate are plotted logarithmically so that the four plots could be shifted for display clarity while preserving relative changes in intensity. While there is some indication of a decrease in intensity of Cr radiation for M.n and Ni samples greater than 1.0 gg, the relative standard deviations for these nonratioed measurements are in the 20 to 30% range. From the 530% error bar shown on the plot of Cr line intensity vs. pg of Ni, it is apparent that no statistically significant matrix effects can be observed a t this level of precision. I t is clear, however, that matrix effects are not severe over the range of sample size used here. A more detailed study is suggested using independent matrix materials as well as an added internal reference. Figure 11 shows the intensities of the A1 lines used as internal references as functions of the corresponding sample concentration. Only i n the case of Cd present a t 100 pg is there significant indication of a change in the A1 line radiation intensity. However, this is not surprising since a 100pg sample represents ;about 3.6% of the foil mass. Thus, the Cd may be present in sufficient quantity to affect the restrike plasma as well as .the foil vaporization. The relative invariance of the A1 line radiation intensity suggests that for samples under abl3ut 10 pg (1000 ppm in a IO-pl sample), the vaporization of the A1 foil is not significantly affected by the presence of the sample. CONCLUSIONS These preliminary :studies suggest the potential application of exploding-foil excitation for multielement analyses of micro solution samples. Relative standard deviations in the 20 to 30% range sihodd be attainable without internal referencing and in the 5 to 10% range with an internal reference. For this range of precision, it should not be necessary to match the reference t o the sample with respect to either thermodynamic properties or excitation energies. Matrix and compound. form effects do not seem outrageous, and a single working curve for each element may be applicable to a wide range of sample compositions and concentrations. However, experimental verification of this clearly is required. The relative freedom from matrix and compound form effects i:s consistent with the vaporization of the sample in a time short relative to the radiative observation time, the total sample consumption, and the high apparent temperature of the metal-vapor plasma. The choice of foil material, thickness, and excitation conditions needs considerable attention. A detailed study of the factors which control linear dynamic range, precision, and detection limits is under way. The use of thinner foils is strongly suggested. This should reduce line interference problems from the foil material as well as reducing the sample blank for the foil impurity elements. By reducing the background radialion associated with the initial vaporization, thinner foils should obtain lower detection limits

-

i.1, 0

2-

U

1 1 0.01

2 .o 2 .o

Ci(1) 3 0 1 . 7 n m

2 .o 2 .o

1.8 2 .o

,

, T

0.1

1.0

10

,,i 100

,ug S a m p l e

Figure 11. Intensities of the AI internal reference lines listed in Table Ill as functions of the analyte concentrations

by increasing sample line-to-background ratios. Very little is known about the mechanisms responsible for the radiation patterns observed with exploding foils. However, the importance of the surrounding gas in controlling the plasma restrike is obvious. Whether dielectric breakdown occurs through the metal vapor or the surrounding gas may be determined using time-resolved spectrometry. The importance of this distinction has been demonstrated for exploding-wire excitation (6). The simplicity, the relatively short analysis time (about 10 minutes exclusive of photographic processing), and the low operating cost (about 7 cents per analysis for the highpurity foil and the He support gas) as well as a number of attractive analytical features of the exploding-foil system should provide the impetus for further basic studies of this novel excitation source and the development of multielement analytical procedures. LITERATURE C I T E D (1) "Exploding Wires", Vol. 1, W. G.Chace and H. K. Moore, Ed., Plenum, New York, 1959. (2) /bid., Vol. 2, 1962. (3) /bid., Vol. 3, 1964. (4) /bid., Vol. 4, 1968. (5)J. A. Holcombe and R. D. Sacks, Spectrochim. Acta, Part B, 28, 451 (1973). (6)R. D. Sacks and J. A. Holcombe, Appl. Spectrosc., 28, 518 (1974). (7)J. A. Holcombe, Ph.D. Thesis, Department of Chemistry, University of Michigan, 1974. (8) D. Schiff, Ref. 1, p 283. (9)D. V. Keller and J. R. Penning, Jr., Ref. 2,p 263. (10)A. H. Guenther, D. C. Wunsch, and T. D. Soapes, Ref. 2,p 279. (1 1) G.Schenk and J. G. Linhart, Ref. 3,p 223. (12)E. C.Cnare, J. Appl. Phys., 23, 1275 (1961). (13) P. B. Higgins, Rev. Sci. Instrum., 45, 602 (1974). (14)J. Bealing and P. G. Carpenter, J . Phys. E.: Sci. Instru., 5, 889 (1972). (15)J. H. Park, J. Res. Natl. Bur. Std. (U.S.), 39, 191 (1947). (16)J. A . Holcombe, D. W. Brinkman, and R. D. Sacks, Anal. Chem., 47, 441 (1975). (17)F. D. Bennett, Phys. Fluids, I , 515 (1958). (18)F. D. Bennett, Ref. 1, p 211. (19)W. G.Chace, Ref. 1, p 7. (20)E. David, 2.Phys., 150, 162 (1958). (21)F. D. Bennett, Prog. High Temp. Phys. Chem., 2, l(1968). (22)W. M. Conn. Phys. Rev., 79, 213 (1950). (23)D. M. Mattox, A. W. Mullendore, and F. N. Regarchik, J. Vac. Sci. Techno/., 4, 123 (1967).

RECEIVEDfor review May 27, 1975. Accepted July 28, 1975. This work was supported by National Science Foundation Grant GP-37026X.


2081

Atomic emission determination of selected trace metals in micro

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