appropriate number of measurements and for the 95% confidence level was used for the estimate given. By reference to Figure 3, it may be noted that the detection limit estimates obtained by this method are representative of concentrations that can be detected with reasonable certainty. It should also be noted that, although the ultrasonic system produces a higher curve slope for lead, the measurement precision for this particular element is poorer for the ultrasonic system. Thus the detection limit estimate suffers accordingly. For the other elements studied, the measurement precision estimates were found to be the same for both nebulization techniques. On the basis of the data obtained, it might be generally concluded that a significant portion of the improvement in sensitivity accrues from the increased amount of analyte reaching the flame, i.e., 34-35 % vs. 10% for pneumatic nebulization., The observed variation in the signal enhancement from element to element probably originates from differences in the various factors that tend to limit atomization in flames, e.g., tendency to form stable oxides. In general the results suggest that ultrasonic nebulization increases the analytical sensitivity by an amount sufficient to justify its application to a variety of analysis problems involving atomic absorption. The results presented in Table I are representative, however, of nearly ideal solutions. Thus it is appropriate to consider the effects of other elements at various concentrations. Effect of Total Salt Content. In order to compare the two nebulizer systems for the analysis of actual samples, the effects of varying the salt or hardness concentrations of aqueous solutions were studied. Standards for silver, zinc, and lead in 0.1F nitric acid were doped with varying amounts of NaCl and C a C 0 3 as a means of approximating a natural water system. Calibration curves were then run on these synthetic samples with the two nebulizer systems. The study showed that neither nebulizer system was affected, in terms of curve
slope, by the presence of varying amounts of NaCl or CaC03 with the ultrasonic nebulizer exhibiting the same level of enhancement over the pneumatic system observed for the distilled water solutions discussed above. Three natural water samples were also used to compare the two nebulizer systems. The total hardness of these samples ranged from 16.5 pg/ml as C a C 0 3for relatively pure water from a mountain stream to 101 pg/ml for tap water to 1050 pg/ml for water from a stagnant lake. Again the ultrasonic nebulizer produced the signal enhancement previously observed for the distilled water solutions. However, for both the natural water samples and the samples doped with NaCl or CaC03, the measurement precision was poorer for both nebulizer systems than in the distilled water solutions. For a single metal concentration, the relative standard deviations of the absorbance readings essentially doubled in value foI the solutions with high salt or hardness concentrations for both nebulizer systems. The precision estimates obtained by replicate measurements at various elemental and salt concentrations did not show a particular trend with the concentrations involved. On these bases, one would expect the detection limit estimates to become poorer than those given in Table I but the enhancement due to ultrasonic nebulization would remain essentially the same. On the basis of these experiments, it may be concluded that the ultrasonic nebulizer system is capable of conveniently providing increased analytical sensitivity for a variety of elements contained in aqueous samples. It is likely that the observations presented herein will also be generally applicable to other flame methods of analysis, e.g., flame emission and atomic fluorescence. RECEIVED for review November 4, 1971. Accepted March 29, 1972. Research supported by EPA Grant No. 16020 GDI.
Direct Isotopic Determination by Atomic Fluorescence Spectrometry Claude Veillon and John Y. Park’ Department of Chemistry, University of Houston, Houston, Texas 77004
RADIOACTIVE ISOTOPES are widely used as tracers in many applications because of the relative ease and sensitivity of detection and measurement with modern nuclear instrumentation. In many applications, such as biomedical studies, radioactive tracers have serious limitations and handling problems, making it desirable in these instances to use stable isotopes. One is then faced with the inconvenience and expense of mass spectrometric determination. Atomic spectrometry offers an alternative means of isotopic determination utilizing the hyperfine structural lines to distinguish between the various isotopes of a given element. Atomic emission, absorption, or fluorescence could be utilized, provided that the hyperfine emission or absorption lines do not overlap to a great extent. For example, Mrozowski ( I ) Present address, Department of Chemistry, California Institute of Technology, Pasadena, Calif. (1) S. Mrozowski, 2.Phys., 78,826 (1932).
showed that mercury vapor excited by a given group of hyperfine structure components fluoresced only those components contained in the exciting source. The observed isotopic shifts in spectral lines of an element are due to the mass effect and to the nuclear volume effect (nonzero volume of nuclear charge) (2). The mass effect results in decreasing hyperfine component separation with increasing atomic mass, while the nuclear volume effect results in increasing separation with increasing nuclear size, so the two effects act in opposite directions. Consequently, one finds relatively large isotopic line separation only for very light and very heavy elements, with mid-range elements having very small separation. In his original paper on atomic absorption spectrometry, Walsh (3) suggested the possibility of isotopic determination (2) H. Kopfermann, “Nuclear Moments,” Academic Press, New York, N. Y., 1958, p 161. (3) A. Walsh, Spectrochim.Acta, 7,108 (1955). ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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1coolant
Table I. Hyperfine Structure of Mercury Component and Contributing isotopes, displacement, A amu -0.025 -0.010 0.0 $0.012 $0.022
Figure 1. Source lamp cooling arrangement
by that technique. Since that time, isotopic determinations for uranium (4-6), lithium (7-ZO), and lead (11) by atomic absorption have appeared in the literature. Isotopic determination by atomic fluorescence spectrometry has not been reported, but we chose to concentrate on that technique in this study because of its simpler instrumental requirements, such as low dispersion monochromator and the fact that the source is not viewed directly, and because of the ease of preparation of electrodeless discharge sources with very small quantities of the isotopically pure elements. For most elements of moderate mass, where the separations due to mass and nuclear volume effects are both small, one can expect a large degree of overlap of the hyperfine components unless line broadening in the source and sample atomization cell are minimized. Little quantitative information is available on the actual absorption and source emission linewidths for the atomic lines, much less for the individual hyperfine components. Therefore, the applicability of this technique is determined to a great extent by the shape and width of these line profiles. EXPERIMENTAL
Apparatus. A 0.5-m,f/8.6, 16 A/mm Ebert monochromator with an 1180 grooves/mm, 52 X 52 mm ruled area grating blazed for 3000 A was used. Curved entrance and exit slits of 0.4-mm width and 20-mm height were used, resulting in a spectral bandwidth of 6.4 A. An HTV Type R106 photomultiplier detector was used, with a conventional dynode resistor arrangement and overall voltages between - 350 and -900 V dc, as required. A mechanical chopper modulated the source radiation at 330 Hz and the resulting ac fluorescence signal was amplified by a phase-sensitive, synchronous amplifier tuned to that frequency (Model 120, Princeton Applied Research Corp., Princeton, N.J.). A time constant of 3 seconds was used, and the output was displayed on a 25-cm strip-chart recorder. The reference signal for the synchronous detector was generated by the circuit described previously (12). (4) J. A. Goleb, ANAL.CHEM., 35,1978 (1963). (5) G. Rossi and M. Mol, Spectrochim. Acta, 24B,389 (1969). (6) G . Rossi, ibid., 26B,271 (1971). (7) A. M. Zaidel and E. P. Korrenoi, Opt. Spectrosc., 10, 299 (1961). (8) D. C. Manning and W. Slavin, A t . Absorption Newslett., No. 8 ( 1962). (9) J. A. Goleb and Y. Yokoyama, Anal. Chim. Acta, 30, 213 (1964). (10) .I A. . Wheat, Appl. Spectrosc., 25,328 (1971). (11) W. H. Brimhall, ANAL.CHEM., 41,1349 (1969). (12) C. Veillon and M. Margoshes, Specrrochim. Acta, WB,503 (1968). 1474
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
199, 201 198, 201 200 202 199, 201, 204
Unmagnified images of the source on the flame, and the flame on the entrance slit, were formed with 25-mm diameter X 100-mm focal length fused silica lenses. The electrodeless discharge lamp sources were powered by a 2450-MHz, 0-100 W generator (Raytheon Co., Lexington, Mass.) using a parabolic reflector antenna. A light trap, matched to the aperture of the system, was placed on the monochromator optical axis behind the flame cell to prevent detection of unwanted radiation. To minimize Doppler broadening of the hyperfine absorption lines using a flame for sample atomization, one must use a flame with as low a temperature as is compatible with the required sensitivity. The elements investigated in this study are easily atomized and a low temperature (13) flame was used. The burner consisted of two concentric quartz tubes of 3-mm and 8-mm 0.d. and mounted in a water-jacketed quartz sheath of 23-mm i.d., closed at the bottom and having a 10-mm opening at the top. Sample aerosol in the Ar/Oz mixture is conducted into the inner tube and Hz into the outer tube. The fuel-rich flame burns within the water-cooled quartz sheath and a secondary flame forms above the sheath opening, where the fluorescence measurements are made. Gas flow rates were monitored with calibrated rotameters (14). Flow rates used were: 0.85 l./min HP, 1.17 l./min Ar, and 0.65 l./min 02. The Ar and O2were mixed and used to nebulize the sample solutions in the heated chamber-condenser sample introduction system described previously (15). Excitation Sources. The electrodeless discharge lamps used as excitation sources were prepared according to the general procedures described by Mansfield et al. (16). Small quantities of the stable isotopes of Hg, In, Zn, and Cu were obtained as oxides from Oak Ridge National Laboratory, and a few milligrams were placed in the manifold system and reduced with hydrogen. Except for mercury, which was used as the metal, iodine was then introduced and the system heated to form the metal iodide, which was then distilled into the lamp blank. Argon was introduced and the lamp sealed off at a pressure of 2 Torr. Other gases (Ne, He, Hz) and pressures were also tried, but the most satisfactory performance in terms of intensity, lifetime, and broadening was obtained under the conditions mentioned above. To minimize overlap of the hyperfine source emission lines, the electrodeless lamps were operated at low temperatures and R F power levels. The arrangement used for cooling the source lamps with water is illustrated in Figure 1, with the coolant flow adjusted so that only a thin stream of water flowed over the lamp surface. This was necessary in order to minimize absorption of the microwav: energy by the coolant. Observation of the mercury 2537 A hyperfine structure in the 3rd order spectrumoof a 1.5-m Wadsworth spectrograph, and the copper 3z47 A doublet using a Fabry-Perot interferometer of 0.013 A instrumental width, indicated that slight broadening occurred when going from water cooling, forced air cooling, and ambient air cooling of the source lamps. Therefore, to minimize contributions by source line (13) R. M. Dagnall et ul., A d y s t , 93,72 (1968). 42,684 (1970). (14) C. Veillon and J. Y. Park, ANAL.CHEM., (15) C. Veillon and M. Margoshes, Specrrochim. Acta, 23B, 553 (1968). (16) J. M. Mansfield et ai., ibid., p 389.
broadening to the observed overlap in the atomic fluorescence measurements, the source lamps were operated with water cooling.
Table 11. Overlap Observed with Various Source-Isotope Combinations Source used
Solution RESULTS AND DISCUSSION
Mercury. The hyperfine system of this element results from contributions by six isotopes and is somewhat complex, as illustrated in Table I. The displacements given in Table I represent the distance in wavelength units of each component from the line center (Hg-200 component), and the isotopes contributing to each component are also indicated (17). Considering the component spacing, the probable component absorption linewidths in the flame, and the contribution to some components by more than one isotope, one can expect that complete separation of the hyperfine absorption components cannot be obtained. Looking at the Hg-200 and Hg-202 components, and using the Hg-200 source with a Hg-202 solution (and vice-uersa), correcting for the difference in source intensities using a solution of the same isotope present in each lamp, one obtains fluorescence signals which represent the extent of overlap in the hyperfine absorption lines of each isotope. Comparing these data with those for the isotopic sources used with a solution of the same isotope, an overlap of 56 was observed in both cases. Without knowing the half-widths and actual shapes of the spectral lines involved, one can not predict either, but based on the relatively high degree of overlap one can conclude that the component lines are considerably wider than 0.012 A or that the wings of the line contribute significantly. It is assumed here that the overlap is due to the hyperfine absorption lines and not to the source emission lines, based on the source temperature effects mentioned earlier. Similar data were obtained for the other combinations and are summarized in Table 11. Measurement of the fluorescence of a mercury sample with each isotopic source (correcting for source intensity differences) yields six fluorescence intensity values, each of which is the sum of the six isotope concentrations, each multiplied by the appropriate factor from Table 11. Thus, one obtains six equations with six unknowns which must be solved simultaneously, a task easily performed by a computer. Using a sample of natural mercury and the factors from Table 11, we obtain the results shown in Table 111. Also shown in Table I11 are the results of a similar analysis performed on a synthetic sample containing 50 pg/ml of each isotope. The errors in these isotopic analyses are due primarily to inaccuracies in the overlap measurements shown in Table I1 and the measurement of relative source intensities in each case. Indium and Zinc. Indium has two stable isotopes of 113 and 115 amu with abundances of 4.16 and 95.84%, respectively. Zinc has essentially four stable isotopes of 64,66,67, and 68 amu with abundances of 48.87,27.62,4.12, and 18.71%, respectively. For both of these elements, very little isotopic separation could be observed, with the overlap being above 85% in all cases. This is probably the result of the very close spacing of the hyperfine structure components (18) in these elements.
--
used
198
199
200
201
202
204
198 199 200 20 1 202 204
100 59 66 52 37 29
61 100 35 80 38 42
68 33 100 46 56 38
52 83 44 100 62 78
40 33 56 60 100 68
29 42 40 75 70 100
Table 111. Analysis of Natural Mercury and Synthetic Mixture
Isotope 198 199 200 201 202 204
Natural Hg sample, Z Abundance Found 10.02 16.92 23.10 13.22 29.72 6.84
9.70 18.4 23.8 12.6 30.0 6.71
Synthetic Hg sample, I.Lg/ml Concentration Found
50 50 50 50 50 50
51.5 52.0 47.5 51 .O 48.0 53.0
z
(17) A. C. G. Mitchell and M. W. Zemansky, “Resonance Radiation and Excited Atoms,” Cambridge University Press, New York, N. Y., 1961, p 38. (18) G. Crawfordetal., C a x J . Res., A28,138(1950).
Copper. This element consists of two stable isotopes of 63 and 65 amu with abundances of 69.09 and 30.91%, respectiyely. The separation cf the doublet components in the 3247 A Cu resonance line is 0.039 A (19), although these components are not due to the isotopes, the isotopic splitting being much smaller. Utilizing the isotopic source lamps, no isotopic separation could be observed. The emission line profile was obtained with a scanning Fabry-Perot interferometer having an instrumental bandwidth of 0.01 3 A (the details of this interferometer system will be given in a later publication). The observed profile of the 3247 A source emission showed the two components of the doublet to be well separated with less than 50z overlap, oeach component having an observed halfwidth of about 0.028 A, but no isotopic separation could be observed. This indicates that the hyperfine components are sufficiently broadened so that complete overlap occurs. CONCLUSIONS
Direct isotopic determination by atomic fluorescence spectrometry employing flame atomization cells is limited to elements having relatively large isotopic displacements, because of Doppler broadening of the isotopic absorption lines in the flame cell. Two alternatives suggest themselves in overcoming this limitation. If the sample atoms must be maintained in a high temperature environment, one could employ Zeeman splitting of the hyperfine structure components to increase the isotopic separation, by placing the atomization cell in a magnetic field. A second, and probably more practical, alternative would be to utilize a nonflame atomization cell, generating the sample atoms in an inert atmosphere like argon and observing these a t o m in a low temperature region of the atmosphere, thus minimizing Doppler broadening of the sample absorption lines. For (19) P. Brix and W. Humback, 2.Phys., 128,506 (1950). ANALYTICAL CHEMISTRY, VOL. 44, NO. 8,
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example, in the case of Hg, one could employ the Hatch and Ott method (20), generating the sample atoms at room temperature. For other elements, one might employ a
(20) W. R. Hatch and W. L. Ott, ANAL.CHEM., 40,2085 (1968).
region downstream from the atomization cell. Both of these possibilities will be investigated in the near future.
Work supported in part by the Robert A. Welch Foundation and in part by the National Science Foundation.
Arsenic Determination at Sub-Microgram Levels by Arsine Evolution and Flameless Atomic Absorption Spectrophotometric Technique Richard C. Chu, George P. Barron, and Paul A. W. Baumgarner Pesticide Research Laboratory and Graduate Study Center, The Pennsylvania State Unicersity, University Park, Pa. 16802 THEDETERMINATION of arsenic by Atomic Absorption Spectrophotometry (AAS) using the conventional air/acetylene flame presents some difficulties because of strong flame absorption at the far ultraviolet region where the most sensitive resonance lines of arsenic lie. The introduction of an argon/hydrogenentrained air flame considerably reduced the flame absorption in this region of the spectrum ( I ) . However, because of the low temperature of the argon/hydrogen flame compared to the air/acetylene flame, interferences due to molecular absorption and incomplete salt dissociation were inevitable. The use of a nitrogen/hydrogen-entrained air flame and a nitrogenseparated airlacetylene flame have been reported (2, 3) with the advantage of minimizing the interferences. Recently, the chemical conversion of arsenic to arsine and its subsequent introduction into an argonlhydrogen flame has been developed (4-6). This technique eliminated the interference resulting from the matrix effect and improved the detection limits. This paper describes a flameless AAS method for arsenic determination which also involves the chemical conversion of arsenic to arsine. The arsine evolved is swept into an electrically-heated absorption tube by means of an argon carrier gas. Since no flame is employed in this technique, the background absorption is less than that of the argonlhydrogen-entrained air flame method, and a considerable increase of sensitivity is obtained. EXPERIMENTAL
Apparatus. All measurements were made with a PerkinElmer Model 403 Atomic Absorption Spectrophotometer equipped with an arsenic hollow cathode lamp and a digital readout and printer. The apparatus used for the generation (1) H. L. Kahn and J. E. Schallis, A t . Absorption Newslett., 7 , 5 (1968). ( 2 ) A. Ando, M. Suzuki, K. Fuwa, and B. L. Vallee, ANAL.CHEM., 41,1974 (1969). (3) G. F. Kirbright, M. Sargent, and T. S . West, At. Absorption Newslett., 8,34 (1969). (4) W. Holak, ANAL.CHEM., 41,1712 (1969). ( 5 ) E. F. Dalton and A. J. Malonoski, Ar. Absorption Newslett., 10,92 (1971). (6) F. J. Fernandez and D. C. Manning, ibid.,p 36. 1476
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8,JULY 1972
of arsine and the electrically-heated absorption tube is shown in Figure 1. The absorption tube (part No. 7, Figure 1) was constructed from a 2.5-cm i.d. X 15-cm piece of Vycor glass tubing with both ends left open and with a gas inlet inserted in the tubing at approximately the mid-point or 7.5 cm from each end. The absorption tube was placed above the burner head of the instrument and aligned to obtain minimum background absorption. Heat was applied to the absorption tube through asbestos-covered wire (part No. 8, Figure 1) that was coiled around the middle section and connected to a variable transformer (The Superior Electric Co., Bristol, Conn.). The temperature in the tube was measured with a thermocouple (Therm0 Electric Mfg. Co., Dubuque, Iowa). Instrument settings used in the atomic absorption were hollpw-cathode arsenic lamp Furrent, 18 mA; wavelength, 1937 A ; spectral band width, 7 A; and argon flow, 6 SCFH. Reagents. Analytical grade reagents were used. The arsenic standard was a stock solution of 1.O mg of arsenic per ml prepared by dissolving 0.1465 gram of NasHAsO,, 7 H 2 0 in 5 % H2S04-20% HC1 solution in a 100-ml volumetric flask. Also used were: stannous chloride solution (40% SnC12. 2 H 2 0 in concentration HCl) ; potassium iodide solution (15% KI in glass-distilled water); and zinc granules (20 mesh). The diluent for stock solution was 5% H2s04-20% HC1 solution, prepared by adding 50 ml of concentrated HzS04 and 200 ml of concentrated HC1 to 60 ml of glass-distilled water in a liter volumetric flask. Dilute arsenic standards were prepared immediately before use by dilution of the stock solution. All glassware was washed with concentrated HzS04 and rinsed several times with glass-distilled water. Procedures. An amount of arsenic ranging from 0.05 to 1.0 pg was added to a 250-ml beaker containing 25 ml of 5 % Hzs04-20% HCI solution. To this solution, 2 ml of 15 % KI and 1 ml of 20 % SnClz.2 H 2 0were added and mixed well. The solution was heated to 85 "C for 5 minutes and cooled to room temperature to ensure complete reduction of arsenic from the pentavalent to the trivalent state. After the solution was transferred to the reaction flask (part No. 1, Figure l), the flask was connected to the apparatus with the argon flow by-passing it, One gram of 20-mesh zinc granules was added through the side neck of the flask and an adapter with a balloon tied at one end was immediately inserted into the
