Anal. Chem. 1980, 52, 125-129
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Atomic Emission Spectrometry of Solid Samples with Laser Vaporization-Microwave Induced Plasma System Toshio Ishizuka * and Yoshinori Uwamino Government Industrial Research Institute, Nagoya, 1- I, tfirate-machi, Kita-ku, Nagoya 462, Japan
A laser vaporization-microwave Induced plasma system was used for the determination of various elements in solid samples such as brass, steel, and aluminum alloy. The emission signals of AI, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Ti, and Zn were traced with an oscllloscope, and those elements were determined with high-speed peak-holder and integrator. Precision (relative standard deviation) was 1.2% to 13.8% for the peak-height method, and 2.3% to 12.1% for the peak-area method. The detection limits in the solid samples ranged from 0.9 ppm (0.7 pg) for Zn in aluminum alloy to 22 ppm (20 pg) for Mo in steel.
Low-power, microwave-induced plasmas have been used as excitation sources in atomic emission spectrometry. Most reports have dealt with the analyses of trace elements in solution samples by using the microwave-induced plasmas (1-13). T o maintain a stable plasma, the aqueous phase must be largely removed prior to the plasma (1). For example, solution sample has been deposited onto a filament t o evaporate solvent. Residue on the filament has been vaporized by heating of the filament and introduced into the plasma for excitation of the atomic spectra ( 2 , 10-12). Therefore, the sample has been introduced into the plasma from the solid phase rather than from the aqueous phase. No reports for microwave plasma emission spectrometry (MPES) have dealt with the direct vaporization of solid samples and the introduction into the plasma. In the case of solid samples, they are converted into solution samples, and the analyses are performed by the methods mentioned above. If a system for analyzing solid samples directly by MPES were developed, the system would be useful in the field of analytical chemistry. As the introduction of large amounts of material may cause significant changes in the plasma impedance, a microsample introduction system must be used to maintain the plasma stability. It is said that the excessive introduction rate (greater than 1 mg/s) of material extinguishes the plasma (1).
T h e authors have used a Q-switched ruby laser beam as the atomizer for the atomic absorption of solid samples ( 1 4 ) . Amounts of about 1 p g have been sputtered by one laser shot and introduced into an absorption cell in the order of milliseconds. The rate of sample introduction into the absorption cell has been below 1 mg/s. Even if the laser shot system is used in MPES of solid samples, this rate does not disturb the stability of plasma and/or extinguish the plasma. A system consisting of the laser vaporization and a microwave-induced argon plasma was developed for the analysis of solid samples. There are various kinds of solid samples, such as metals, biological materials, ceramics, minerals, semiconductor materials, etc. As the system takes advantage of sputtering phenomena of solid samples by the laser shot, both conductive and nonconductive solid materials can be analyzed with the system. Because of the excellent focusing characteristics of the laser beam and the practicality of microsampling on a sample surface, the system would be useful for the microprobe analysis of solid samples. In conventional laser microprobe analysis, a laser plume produced on a sample surface by a laser shot has been used as an emission source. T h e emission 0003-270018010352-0 125$01 ,0010
spectra of the laser plume have the following disadvantageous characteristics: intense background, spectral broadening, remarkable self-absorption, etc. (15,16). On the other hand, the emission spectra of microwave-induced plasma as the emission source in the system do not have the above char acteristics. First, the usefulness of the system for the analysis of solid samples must be confirmed. Therefore, metal samples were used as solid samples, and MPES of solid samples was. studied in view of macroanalysis.
EXPERIMENTAL Apparatus. Table I shows the instrumentation and experimental conditions used in this work. Figure 1 shows a block diagram of the apparatus set up with the instruments listed in Table I. Figure 2 shows the schematic diagram of the laser vaporization-microwave induced plasma cell system. The vapor produced from a solid sample by a laser shot interacts with the plasma for the time of millisecond order. The very rapid emission signal of the vapor ww traced with the oscilloscope and measured with the high-speed peak-holder and integrator constructed in this laboratory. Figure 3 shows the measuring system. In Figure 3, the emission signal from the photomultiplier follows into the peak-holder and integrator through the current-to-voltage converter and the FET switches. The FET switches are turned ON by the gate pulses generated from the oscilloscope. The oscilloscope is triggered by the pulse radiation from the xenon flash lamp used to pump the laser rod. The peak-holder was used for measuring the peak height of the emission signal profile. The integrator was used for measuring the peak area of the profile. The signals stored in the peak-holder and integratnr were rewrded with the two-pen strip chart recorder. Samples. Metal samples were used as solid samples in this work. The metals were as follows: brass (NBS, C1103-CI105): plain carbon steel (BAS, SS431-SS435); and aluminum alloy (ALCOA, KA-213-B, KA-356-E, KB-356-B, KC-356-E, SS-24242. SS-A332-D, and SS-F332-D). The flat and smooth surfaces of the samples were obtained by polishing with a 400-mesh silicon carbide paper. The samples with the polished surfaces were washed with distilled water and ethyl alcohol, and dried with hot air. Procedure. A sample was mounted in the laser vaporization cell with an O-ring seal by suction of the vacuum pump. Argon gas was streamed into the cell and adjusted at an appropriate flow rate with the needle valve. The argon plasma was initiated with the Tesla coil. The laser beam was shot after stabilizing the plasma for about 1 min. The laser beam was directed downward with a 45O-quartz prism and focused on the sample surface with a 10-cm focal length lens. The diameter of the laser beam was about 1 mm on the sample surface. A plume was produced from the sample by the laser beam. The vapor in the laser plume was introduced into the plasma by the flow of argon gas, and excited to atomic emission by the plasma. The emission signal from the plasma was traced with the oscilloscope and measured by means of the measuring system shown in Figure 3. The laser beam was shot on the sample at 30-s intervals. Data were treated in the same manner as employed in the preceding paper (14). That is to say, the same point on the sample surface was shot five times with the laser beam. The average of five values obtained by each laser shot was treated as the value at the point. The following elements in each sample were determined: iron, lead, and nickel in brass; aluminum, copper, molybdenum, and nickel in steel; and chromium, copper, iron, manganese. nickel, titanium, and zinc in aluminum alloy. Table I1 shows the an8 lytical lines for each element studied in this work. C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980 Q-swi tched ruby laser
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Figure 1. Block diagram of apparatus Laser beam
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Ar
Figure 2. Schematic diagram of laser vaporization-microwave induced plasma cell system. The scheme is represented in inside dimensions
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RESULTS AND DISCUSSION Oscillograms of Emission Signals. The emission signals for each element were traced with the oscilloscope. The profiles of emission signals were affected by (1)the argon flow rate (AFR, i.e., pressure in the cell system), (2) the laser energy (i.e., capacitor voltage, CV, for the power supply to the xenon flash lamp used to pump the laser rod), and (3) the microwave power (MWP) supplied to the microwave coupling cavity. In
this section, the results obtained under the experimental conditions shown in Table I are discussed. Figure 4 shows the oscillograms of emission signals for nickel (0.16%) in brass, aluminum (0.24%) in steel, and copper (0.055%) in aluminum alloy. The emission signals for these elements were observed after about 1.2 ms from the oscilloscope triggering. The emission profiles for nickel and copper were similar in pattern. The emission peaks appeared after 1.7 to 1.8ms from the triggering. The half-widths of the peaks for nickel and copper were about 0.9 and 0.8 ms, respectively. The emission peak for aluminum appeared after 1.3 ms from the triggering, and the half-width was about 0.3 ms. The reproducibilities of the delay times of the signal and peak appearances from the oscilloscope triggering were good for each element, respectively. The good reproducibilities would facilitate the development of electronic signal processing. The emission profiles for chromium, iron, manganese, lead, and zinc were similar to those for nickel and copper. The profiles for molybdenum and titanium were similar to that for aluminum. The narrow peak widths for aluminum, molybdenum, and titanium may be due to the shorter lifetimes of those atoms in the plasma, that is, the immediate combination of those atoms with oxygen as an impurity in the plasma gas, because those elements have a strong affinity for oxygen. In this work, the argon gas in a cylinder was used directly without further purification, since the elements studied were not anticipated to be contained as impurities in the argon gas. Effect of Argon Flow Rate. The effect of AFR on the emission intensities for iron in brass, molybdenum in steel, and copper in aluminum alloy was studied by varying AFR from 20 to 500 mL/min. Other experimental conditions were identical to those shown in Table I. Figure 5 shows the effect of AF'R on the emission intensity for copper in aluminum alloy. In Figure 5 , each plot is the average of values obtained a t two points on the sample, and the two ends of the vertical line on each plot represent the values obtained a t the two points. Both the peak height and peak area increased with an increase in AFR. In the present system, AFR is coupled directly to plasma pressure. The pressure which excites copper atoms most efficiently was not found a t AFR up to 500 mL/min. The amounts of scatter of values in Figure 5 were larger a t AFR over 200 mL/min (about 2.5 Torr) than at AFR of 100 mL/min (about 1.5 Torr) or below. Similar results were obtained for iron in brass and molybdenum in steel.
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Figure 5. Effect of argon flow rate on the emission intensity for copper (0.04%) in aluminum alloy
Flgure 4. Oscillograms of emission signals for several elements. (A) Ni (0.16%) in brass; (B) AI (0.24%) in steel; (C) Cu (0.055%) in aluminum alloy. Signal, (A) 0.1 V/div; (B) 0.02 V/div; (C) 0.5 V/div. Time scale. 0.5 ms/div
The profiles of emission signals for the above elements were also obtained a t various AFR’s with the oscilloscope. The profiles obtained a t AFR of 20 to 100 mL/min showed a simple peak pattern. However, those obtained at AFR of 200 and 500 mL/min showed complex emission patterns after f i s t main peaks occurred. The complex profiles were not constant in pattern and intensity for each laser shot, and the reproducibilities were not good as can be seen from Figure 5. The complex emission profiles a t AFR over 200 mL/min may be due to the turbulence of argon plasma. From the results mentioned above, the conditions of AFR over 200 mL/min are undesirable for quantitative analysis. Therefore, emission intensity was sacrificed t o obtain data with a high precision, and AFR of 100 mL/min was selected as optimum conditions. Effect of Laser Energy. The effect of laser energy on the emission intensities for the same elements as discussed in the preceding section was studied by varying CV from 3.8 to 4.4 kV (Le., about 0.4 to 1.3 J as laser energy). Figure 6 shows the effect of CV (laser energy) on the emission intensity for iron in brass. Each plot in Figure 6 is the average of values obtained a t two points on the sample. Both the peak height and peak area were low a t CV of 3.8 kV (0.4 J) near the threshold voltage for the Q-switched ruby laser used. Those
3.0 (0.4)
4.0 (1 0)
4.2 (1.2)
4.4 (1.3)
Capacitor Voltage, k V (Laser Energy, J 1
Figure 6. Effect of laser energy (capacitor voltage) on the emission intensity for iron (0.26%) in brass
highly increased with an increase in CV from 3.8 to 4.2 kV (about 1.2 J). The increase of the intensities was slight with an increase in CV over 4.2 kV. Similar results were obtained for molybdenum in steel and copper in aluminum alloy. It has been shown that for a given material, the amounts sputtered by a laser shot are linear functions of laser energy over limited energy ranges (17). In Figure 6, the increased intensities with an increase in CV must be due to the increase of amounts sputtered from the sample by the laser shot. From the results mentioned above, 4.2 kV was selected as the suitable CV for all the elements studied. The amounts of brass, steel, and aluminum alloy sputtered by the laser shot were about 1.0, 0.9, and 0.8 p g under conditions of CV of 4.2 kV, respectively. It should be noted that not all of the materials sputtered from the samples by the laser shot were introduced into the plasma tube. A portion of the sputtered materials must have deposited on the wall of the vaporization cell. This was confirmed by the presence of thin metallic films plated out on the wall by the repeated laser shots.
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Table I. Instrumentation and Experimental Conditions excitation system Q-switched ruby laser JEOL, model JLR-02A; max. peak power, 7 0 MW; max. laser energy, about 1.5 J; pulse-width, less than 20 ns; laser energy, usually operated at about 1.2 J (capacitor voltage, CV, 4.2 kV). about 1.0, 0.9, and 0.8 p g for sputtered mass brass, steel, and Al alloy, respectively, were sputtered by one laser shot under conditions of cv of 4.2 kV. microwave generator Ito Chotanpa, model MR-1s; 200 W max.; usually operated at 60 W. Ito Chotanpa, CTCR-S15/25; microwave coupling Evenson type, 1 / 4 wave. cavity stainless steel wall and Pyrex glass laser vaporization window; see Figure 2. cell quartz tube (7-mm i.d.); see Figure plasma tube 2. Ar 99.995%; argon flow rate plasma gas (AFR), usually 100 mL/min. Edwards LBlB needle valve plasma gas control Ueshima Brooks R-2-15-D flow meter AFR of 100 mL/min corresponds pressure to the pressure of 1.5 Torr; AFR of 20 to 500 mL/min corresponds to the pressure of 0.5 to 7 Torr. pressure measurement Daia Shinku, model PT-S2OO Pirani gauge rotary vacuum pump Hitachi 4VP-C Tesla coil; Tokyo Koshuha plasma ignition Denkiro. optical system monochromator Nippon Jarrell-Ash, model 82-000; 0.5-m Evert mounting; 1180 g/mm-grating brazed at 300 nm; dispersion, 1.6 nm/mm; slit widths (entrance and exit), 50 pm for brass and Al alloy and 10 pm for steel. quartz lens with 10-cm focal optics length and 25-mm diam. ; plasma image was focused on the entrance slit. analytical lines for each element wavelength setting were set with the hollow cathode lamps (Westinghouse and Hamamatsu TV). measuring system detector Hamamatsu TV R-106 photomultiplier John Fluke, model 415B detector power supply oscilloscoDe National. model VP-5410A trigger signal Hamamatsu TV R-106 detector photomultiplier with HewlettPackard, model 651 5A power supply. peak-holder and constructed from operational integrator amplifiers (Teledyne Philbrick 1026) and FET switches (Teledyne 2110BE) in this laboratory; see Figure 3. recorder National, model VP-654A two-pen strip chart recorder
Effect of Microwave Power. The MWP supplied to the microwave coupling cavity is an important factor affecting spectral emission from the plasma. The effect of MWP on the emission intensities for copper in aluminum alloy and molybdenum in steel was studied by varying MWP from 40
100
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to 100 W. Figures 7 and 8 show the results obtained for copper and molybdenum, respectively. In Figure 7 , the emission intensity for copper decreased little by little with an increase in MWP. In Figure 8, that for molybdenum increased with an increase in MWP. The effect of MWP on the emission intensities was not the same for both elements. T h e effect of M W P on the emission intensity for copper was similar to that for a plasma at atmospheric pressure reported previously by Runnels and Gibson (2). They have reported that an increase in MWP lengthens the plasma considerably and copper emission is localized almost a t the leading edge of the plasma. As a portion of copper metals has been plated out on the wall of the plasma tube a t the leading edge, it has not been introduced further into the plasma center. In this work, the spectral emission of the argon plasma was measured a t the cavity center. Therefore, the effect of M W P on the emission intensity for copper would be explained as stated above. For a refractory element such as molybdenum, a higher MWP is considered necessary to increase the emission intensity. In subsequent studies, all data were obtained by using M W P of 60 W, since this power was sufficient to maintain a stable plasma and could be utilized for all the elements without major loss in emission intensity. Precision Data. Table I1 shows the precision data for the peak-height and peak-area values obtained for various elements. The precision (relative standard deviation) in Table
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Table 11. Precision Data, Detection Limits, and Concentration Ranges in Working Curves for Various Elements precisiona detection limit rel. std. dev., %
analytical line, n m Al 396.15 Cr 357.87 Cu 324.75 Al alloy Cu 324.75 Fe brass 371.99 Fe Al alloy 371.99 hln Al alloy 403.08 Mo steel 386.41 Ni brass 352.45 Ni steel 341.48 Ni Al alloy 352.45 Pb brass 283.31 Al alloy 283.31 Pb Al alloy 365.35 Ti Zn AJ alloy 213.86 a The precision was evaluated from beam. element
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slopes of about unity, indicating a linear relat.ionship between emission intensity and concentration.
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12 9.6 21 17 0.15 8.4 12.1 22 18 15 1.2 0.035 7.3 6.3 0.9 0.7 1.2 1.0 0.035-0.20 the data obtained a t the five points which were shot every five times with the laser
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peak height concn., ppm wt, pg 9.3 8.4 13 10 2.4 2.2 2.1 1.7 2.7 2.7 12 9.6 5.4 4.3 22 20 3.8 3.8 12 11
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Detection Limits. T h e samples containing the lowest concentrations of each element were used for the determination of detection limits. T h e detection limit was defined as the concentration or weight of element which gives a signal equal to twice the standard deviation of background level. The background levels were measured a t wavelengths near the analytical lines for each element. The weight of the detection limit was calculated from the concentration of detection limit and the weight of sample sputtered by the laser shot. Table I1 shows the detection limits obtained. The detection limits ranged from 0.9 ppm (0.7 pg) for zinc in aluminum alloy t o 22 ppm (20 pg) for molybdenum in steel with the peakheight method. Those ranged from 1.2 ppm (1.0 pg) for zinc in aluminum alloy to 25 ppm (23 pg) for molybdenum in steel with the peak-area method. T h e detection limits for copper, iron, and nickel were determined by using a few kinds of samples The detection limits for iron and nickel in brass were lower than those in steel or aluminum alloy. T h e detection limits for copper and nickel did not indicate significant differences between steel and aluminum alloy. LITERATURE CITED
Metal Concn, %
Figure 9. Working curves for iron and nickel
in aluminum alloy
I1 was evaluated from the data obtained a t the five points on a sample. T h e precision was 1.2% to 13.8% for the peakheight method. T h a t was 2.3% to 12.1% for the peak-area method. The results shown in Table I1 presented no difference in precision for the peak-height and peak-area methods. Working Curves. The working curves for several elements were constructed over one or two orders of magnitude in the concentration ranges shown in Table 11. Figure 9 shows the working curves for iron and nickel in aluminum alloy. Each plot in Figure represents the average Of the Obtained a t the five points. T h e curves for iron have slopes of about unity, indicating a linear relationship between emission intensity and Concentration. However, the curves for nickel have slopes less than unity, indicating a nonlinear relationship between emission intensity and concentration* relationships between emission intensitv and concentration were also obtained for copper (0.04% t o 1.01%) and zinc (0.035% to 0.20%) in aluminum alloy. The nonlinearities may be due to self-absorption. T h e other working curves have
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RECEIVED for review April 10, 1979. Accepted September 27, 1979.
