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Anal. Chem. 1981, 53, 1796-1798
Atomic Emission Spectrometry of Barium with a Metal Electrothermal Atomizer Masami Suzuki, * Kiyohlsa Ohta, and Tatsuya Yamaklta Department of Chemistry, Faculty of Engineering, Mie University, Kamihama-cho, Tsu, Mie-ken 5 14, Japan
The excitation of barlum In a molybdenum mlcrotube atomizer and Its use In atomlc emission spectrometry have been studied. Barlum showed a high atomic emlsslon in a molybdenum mlcrotube and the detection llmn was 0.6 pg under the optimized experlmental condltlons. Hydrogen, added to the Inert gas atmosphere, lowered the atomlzatlon temperature of barlum and Increased atomic emlssion. An Increase in the peak emisslon of barlum was shown wlth the increasing heatlng rate. No interferences from 600 ng of NaCI, 190 ng of KCI, 50 ng of Ca, 5 ng of Sr, 1000 ng of Mg, 0.1 ng of AI, and 0.1 ng of sulfate were found for measurement of atomic emlsslon of barlum. Atomic emlsslon spectrometry wlth a molybdenum mlcrotube atomizer provldes a slmple and sufllclently sensltlve technique for barlum determlnatlon. The method was applled to the determination of barium In waters.
Attention has been devoted to trace level determination of barium for study of its distribution in the environment. For this measurement, one needs a sensitive analytical technique. The electrothermal atomic absorption spectrometry with a graphite furnace has been applied for determination of barium (1). However, the determination of barium in a graphite furnace is inhibited somewhat by carbide formation. Cioni et al. (2) used the Tic-lined graphite tube for suppression of matrix effeds. Lagas (3)described the use of tubes coated with pyrolytic graphite to alleviate carbide formation problems, e.g., bad reproducibility and memory effects. Renshaw ( 4 ) found that the sensitivity for barium could be improved by a factor of 20 by lining the graphite tube with tantalum foil so that carbide formation was prevented. Goleb and Midkiff (5) applied a tantalum strip for atomization of barium in firearms discharge residues. Recently, the measurement of atomic emission from a graphite tube furnace has been shown to be useful for barium determination (6-8). The detection limits for barium determinations by carbon furnace atomic emission spectrometry are significantly better than those obtained by carbon furnace atomic absorption spectrometry (9). However, no work was done with metal-lining graphite or metal atomizers. Metal atomizers appear to be attractive for atomization of barium to prevent carbide formation. In particular, a metal microtube atomizer provides an environment with a uniform temperature and maximum atom cloud concentration in an extremely short period of time, in contrast to conventional carbon atomizers (10). The black body radiation produced by the graphite also interferes with the atomic emission measurement of elements. This work has been done to clarify the atomic emission characteristirx of barium in a molybdenum microtube atamizer for determination of this element a t extremely low levels. EXPERIMENTAL SECTION Instrumentation. The monochromator,photomultiplier,and amplifier for atomic emission and absorption measurements were the same as used previously (12). The output signal from the amplifier was fed to a microcomputer (II,12). The signals were also monitored on a memoriscope (Iwatsu MS5021).
The molybdenum microtube atomizer (20 mm long and 2 mm bore) and absorption chamber (300 mL) have been described (12). A microtube was fabricated from 0.05 or 0.1 mm thick molybdenum sheet. The argon used as purge gas in the absorption chamber was mixed with hydrogen. Two light apertures (0.5mm diameter) were positioned in front of the monochromator entrance slit so that the light beam from the hollow cathode lamp passed through the center of the microtube and through the apertures. The apertures greatly reduced the amount of tube-wall radiation reaching the monochromator. A barium hollow-cathode lamp (HamamatsuTV Co.) was used for atomic absorption measurement at 553.55 nm and for ionic measurement at 455.40 nm. Atomic and ionic emission measurements were made at the same wavelengths as in absorption. For atomic emission measurements, the wavelength of the monochromator was adjusted to the desired lines using a hollow-cathode lamp which was then disconnected. The tube temperature was measured as described previously (12). The signals from the photodiode were fed to the microcomputer and memoriscope and recorded simultaneously with the emission or absorption signal. All sample solutions were injected into the microtube by use of a 1-pL glass micropipet. Reagents. Stock solutions, 1mg/mL, of barium were prepared by dissolving ita carbonate (dried at 110 "C for 2 h) in hydrochloric or nitric acid and diluting with demineralized water. Dilute working solutions were prepared immediately before use by diluting appropriate volumes of stock solutions. These solutions were 0.001 M in acid. Demineralized water gave no signal for alkaline earth elements. All reagents were of analytical reagent grade and checked for barium as an impurity. Procedure. Samples of 1p L of barium were injected into the microtube and dried at 100 "C for 30 s, and then atomized by heating to a find temperature of 2350 "C. All of the atomization signals were stored in a microcomputer. Signals were subject to background (noise) subtraction, compensation of base line, and smoothingas described for atomic absorption (11,12).Then, the resulting signals were displayed on a cathode ray tube (CRT). The programming was also made to indicate the well-defined appearancetemperaturesof atomizationprofiles. The atomization signals were traced on a memoriscope separately to observe the atomization profiles. RESULTS AND DISCUSSION Atomic Emission Profile of Barium. The atomic emission profile in a molybdenum microtube atomizer is presented together with the atomic absorption profile in Figure 1. These profiles are shown for 100 pg of barium, measurements being made with similar photomultiplier voltage and sensitivity of the memoriscope for both atomic emission and absorption. The atomic emission and absorption profiles processed by the microcomputer are also shown. Atomic emission for barium was comparatively high, probably becawe of its lower excitation energy (2.24 V for the 553.6-nm line). The peak at about 2100 "C on the atomic absorption profile cannot be regarded as maximum absorption. The measurement of atomic absorption signal of barium was difficult due to the atomic emission because an unmodulated light source was employed in the present measuring system for fast tracing of the atomization signals. Maximum emission signal of strontium was shown at the same temperature at which atomic absorption signal was maximum (13). From the result for
0003-2700/81/0353-1796$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
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Time, scc
Time , sec
display for effect of heating rate on atomic emission of barium: (a) 4.1 deg/ms, (b) 2.9 deg/ms, (c) 2.6 deglms, (d) 2.0 deg/ms, (e),(f), (g), and (h) temperature Increase for (a), (b), (c),rand (d), respectively. Figure 3. CRT
Time, sec
Atomic emission and absorption profiles for barium: (1) not computer processed; (2) computer processed; (a) and (c) atomic emisslon; (b) and (d) atomic absorption; (a') and (b') background; (e) temperature increase. Photomultlpiler voltage was 750 V, with 200 mL/min H2 and 300 mL/min Ar. Flgure 1.
Time, sec
CRT display for atomic emission of different amounts of barium: (a) 50 pg, (b) 100 pg, (c) 200 pg, (d) temperature increase. Flgure 4.
Time,
SQC
Figure 2. CRT dlsplay for effect of hydrogen flow rate on atomic emisslon of barium: (a)500 mL/min H2,(b) 100 mL/min H2 and 400 mL/min Ar, (c) 200 mL/mln H2 and 300 mL/min Ar, (d) 300 mL/min H2 and 200 mL/min Ar, (e) 50 mL/min H2 and 450 mL/min Ar, (f) 20 mL/min H, and 480 mL/min Ar, (9) 500 mL/mln Ar, (h) temperature Increase.
strontium it can therefore be presumed that maximum absorption of barium coincides in temperature with maximum emission of this element. As shown in Figure 1the computer processing of the atomic emission signal facilitates the higher sensitive measurement of barium. This comes from the extreme reduction of background (noise). Similar atomic emission profiles were shown for both chloride and nitrate of barium. The dimension of the microtube was important for measuring atomic emission of barium. Smaller diameter of the microtube was not favorable because of tube wall radiation reaching the monochromator. The use of larger diameter tube resulted in poor sensitivity of barium emission. A microtube of 2 mm diameter was recommended in order to minimize the impact of tube wall radiation without the pronounced impairment of sensitivity. Effect of Hydrogen. Figure 2 demonstrates the dependence of atomic emission profiles for barium on flow rates of argon and hydrogen as purge gases. The peak emission of barium initially increased with increasing hydrogen flow up to 200 mLJmin and thereafter decreased with further increase in hydrogen flow. The increased atomic emission can be attributed to the increased atom formation of barium in the reducing chemical environment created by hydrogen. The optimum flow rate of hydrogen is 200 mL/min for the present
atomization device. Under this condition the ionization of barium was negligible. Poorer atomic emission was shown in pure argon atmosphere. The appearance temperatures were the same at higher flow rates of hydrogen, while at lower flow rates of hydrogen they shifted to higher regions. The effects of hydrogen flow on atomic absorption of barium were analogous to those on atomic emission. Effect of Heating Rate. The influence of heating rat8es of the microtube atomizer on atomic emission profiles of barium was studied. Figure 3 presents the change of atomic emission profiles as u function of the microtube heating rate. As the heating rate of the microtube is increased, the atomic emission profiles are characterized by sharper and narrower curves showing higher peak emission. The appearance ternperatures appear to be independent of the heating rates of the microtube. However, the peak temperatures were found to be very dependent on the heating rates. Although tin increase in peak emission of barium was shown with the increasing heating rate, the measurement was limited by the short lifetime of the microtube when atomizing at higher temperature. The measurement at temperatures lower thtm 2350 "C was recommended for the present atomizer. Atonnization characteristics of barium with the 0.1 mm thick miicrotube were similar to those with the 0.05 mm thick micrlotube, although the heating rate of the microtube was decreased when the same power was applied for heating. Detection Limit and Reproducibility. The atomic emission signals obtained with increasing amounts of barium are presented in Figure 4. In the atomization of barium tat the 200-pg level the decay of emission signal to base line was not shown even after 1 s. However, a log-log plot of barium amount and emission intensity was linear. The integration of emission profiles over a period of 1 s also gave a linear calibration curve. The detection limit. was calculated from peak height in the atomization profile and defined as that the weight of analyl;e giving an emission signal equal to twice the standard deviation of the background signal. In the metal microtube atomizer, the detection limit is restricted by the small sample volume
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Table I. Determination of Barium in Some Water Samples amt of barium. d m L sample added found tap water 0,019,0.021, 0.019 0.010 0.032,0.035, 0.028 river water 0.017,0.017,0.018 0,010 0.028,0.025,0.030 pond water 0.026,0.026 0.010 0.035,0.036 (1pL). Therefore, the detection limit is given in absolute terms (picograms). As little as 0.6 pg of barium could be detected under the optimized conditions. The coefficient of variation was 4.8% (10 determinations) for atomic emission of 100 pg of barium. Interference Studies. The interferences were investigated by analyzing computer-processed atomic emission profiles of barium in the absence and presence of concomitants. Entire overlap of two profiles was regarded as no interference from concomitants. No interferences from 600 ng of sodium chloride and 190 ng of potassium chloride were observed on atomic emission of 100 pg of barium. Larger amounts of alkali chlorides led to depression of barium emission. In the presence of 1 pg of sodium chloride, the depression of the atomic emission signal of barium was by 5% in peak height and the profile was somewhat broadened. The limiting amounts were 600 and 190 ng for sodium chloride and potassium chloride, respectively. Calcium is known to cause serious interference in the determination of barium. The permissible amount of calcium was 50 ng. The emission signal from calcium (probably CaOH) was observed in lower temperature region prior to barium emission signal if large amount of calcium was present. Although two emission peaks were differentiated, atomic emission of barium was difficult to be measured even if correction of calcium emission is made. This is due to the depression of atomic emission of barium in the presence of large amounh of calcium. The depression of barium emission in the presence of 100 ng of calcium was 52%. The effects of calcium on barium emission were identical for both chloride and nitrate. Cioni et al. (2) described the depressing effect of calcium on barium absorption in hydrochloric acid solutions and enhancing effect in nitric acid solutions for atomization in carbon furnace atomizer. The effects of magnesium, strontium, aluminum, and sulfate on atomic emission of barium were also tested. The permissible amounts were 100, 5,O.l and 0.1 ng for magnesium, strontium, aluminum, and sulfate, respectively. Large amounts of these elements depressed barium emission. Mineral acids such as hydrochloric and phosphoric acids had no effect on barium emission at the 0.01 M level. Cioni et al. (2) showed that phosphoric acid interfered in the atomic absorption measurement of barium in a carbon furnace, even when present in low concentrations.
The permissible amounts of concomitants were independent of amount of barium under the experimental conditions applied. Although the alleviation of interference from concomitants is expected in higher atomization temperature, the heating of the atomizer is limited by the melting point of molybdenum. Determination of Barium in Water Samples. Atomic emission spectrometry with the molybdenum microtube atomizer was applied to the determination of barium in water. The water samples were acidified with hydrochloricacid, and diluted 2-fold with demineralized water. Some of the results are shown in Table I. The calibration curve was prepared from standard solutions without any treatment. The data presented in Table I indicate that the added barium was recovered quantitatively. Evaluation of the accuracy of the method was difficult because no certified samples of low-level barium content were available. Although this method was applied to the samples which contain extremely higher concentrations of sodium chloride and calcium, the results obtained were unsatisfactory. Some separation procedures may be necessitated. Therefore, the application of this method to other environmental samples is a future problem. The results show that atomic emission spectrometry in the metal microtube atomizer offers a simple and sufficiently sensitive technique for determination of barium. The ionization of barium in the molybdenum microtube is much less and there is no fear of unfavorable carbide formation. Tube wall emission in this atomization device is lower than that in carbon atomizers, and the effect of tube wall emission can be reduced by the use of the optimum diameter of the microtube. In spite of the low temperatures compared to other emission sources, the observed detection limit for barium suggests that this technique may be of analytical value. LITERATURE CITED Berggren, Per-Olof Anal. Chim. Acta 1980, 119, 161-166. Cionl, R.; Mazzucoteiii, A.; Ottoneiio, 0. Anal. Chlm. Acta 1978, 82, 415-420. Lagas, P. Anal. Chim. Acta 1978, 98, 261-267. Renshaw, G. D. At. Absorpt. News/. 1973, 12, 158-160. W e b , J. A.; Miikiff. C. R., Jr. Appl. Spectrosc. 1975, 29, 44-48. Epstein, M. S.; Rains, T. C.; O’havor, T. C. Appl. Spectrosc. 1976, 30, 324-329. Hutton, R. C.; Ottaway, J. M.; Rains, T. C.; Epstein, M. S. Anawst (London) 1977, 102, 429-435. Epstein, M. S.; Rains, T. C.; Brady, T. J.; Moody, J. R.; Barnes, I. L. Anal. Chem. 1978, 50, 874-880. Ebdan, L.; Hutton, R. C.; Ottaway, J. M. Anal. Chlm. Acta 1978, 96, 63-67. Ohta, K.; Suzuki, M. Talanta 1975, 22, 465-469. Suzuki, M.; Ohta, K. Anal. Chim. Acta 1981, 133, 209-213. Suzuki, M.; Ohta, K.; Yamakita, T. Anal. Chem. 1981, 53, 9-13. Suzuki. M.; Ohta, K. Talanta 1981, 28, 177-181.
RECEIVED for review March 23,1981. Accepted June 26,1981. This work was supported by the Ministry of Education, Science and Culture of Japan through a Grant-in-Aid for Special Project Research.
