Ultrasonic Nebulization in Atomic Absorption Spectrophotometry T. R. Copeland, K. W. Olson, and R. K. Skogerboe Department of Chemistry, Colorado State University, Fort Collins, Colo. 80521
ALTHOUGH THERE HAS BEEN a recent emphasis on developing nonflame atomization systems for atomic absorption (I-@, it is quite likely that flames will continue to be generally considered as the preferred atomization media. One criticism of flame systems originates from the lack of adequate analytical sensitivity for some elements and/or analysis problems. This deficiency is at least partially due to the relative innefficiency of the nebulization process which is commonly accomplished with pneumatic systems. Willis (7), for example, has demonstrated that approximately 90 of the sample solution never reaches the atomizing flame. The sample aerosol delivered to the flame consists of a range of droplet sizes with an average diameter of 8 microns (7). Any one aerosol droplet has a very short residence time in the flame and must undergo several processes during that time (8), so the ability to measure an atom population must be kinetically limited. If a more uniform dispersion of smaller droplets could be produced, it is intuitive that the analytical sensitivity might be improved via at least two routes. A decrease in droplet size should increase the delivery efficiency though a decrease in condensation probability and the atomization efficiencies should also increase because of the anticipated reduction in the time required for evaporation of the droplet. An increased uniformity of size distribution suggests that the flame zone of maximum atom population should be much more localized with a concomitant increase in sensitivity. These potential improvements coupled with the advantages listed by Stupar and Dawson (9) suggested the need for further investigation of ultrasonic nebulization for atomic absorption. In the earlier reports on the use of ultrasonic nebulization for flame methods, focusing nebulizers were used. In these systems, a relatively large volume of liquid was required and constant, reproducible nebulization was difficult to obtain (]&I#). More recently, Kirsten and Bertilsson (15) and Stupar and Dawson (9) reported on the use of ultrasonic nebulizers which could be continuously fed sample solution from a source external to the nebulizer. Although both of these reports are rather brief, they indicated that atomic absorption sensitivities might be enhanced by factors of 2-5 over those (1) R. Woodriff and G. Ramelow, Spectrochim. Acta, 24B, 864 (1968). (2) H. Massman,Z. A n d . Chem., 225,203 (1967). (3) H. L. Kahn, AmericanLaborarory, 1970,41, Aug. (4) R. H. Wendt and V. A. Fassel, ANAL.CHEM., 38,337 (1966). (5) G. H. Morrison and Y. Talmi, ibid., 42,811 (1970). (6) B. G. Gandrud and R. K. Skogerboe, Appl. Spectrosc., 25, 243 ( 197 1). (7) J. B. Willis, Spectrochim. Acta, 23A, 81 1 (1967). (8) R. Mavrodineanu and H. Boiteux, “Flame Spectroscopy,” Wiley and Sons, New York, N.Y., 1965. 9) J. Stupar and J. B. Dawson, At. Absorption Newslett., 8, 38, ( 1969). (10) H. Dunken, G. Pforr, and W. Mikkeleit, Z. Chem., 4, 237 (1964). ( 1 1 ) H. Dunken, G. Pforr, W. Mikkeleit, and K. Geller, ibid., 3, 196 (1963). 36,412 (1964). (12) C . D. West and D. N. Hume, ANAL.CHEM., (13) J. Stupar and J. B. Dawson, Appl. Opt., 7,1351 (1968). (14) J. Spitz and G. Uny, ibid., p 1345. (15) W. S. Kirsten and G. 0. B. Bertilsson, ANAL.CHEM.,38, 648 (1966).
69
H SAMPLE IN
EPOXY TEFLON PYREX ULTRASONIC CRYSTAL
TO BURNER
Figure 1. Schematic of system for coupling the ultrasonic nebulizer to a burner
obtainable with pneumatic nebulizers. The present report presents a further comparison of pneumatic us. ultrasonic nebulization for a representative group of elements. The results indicate that ultrasonic nebulization can be conveniently used to increase the sensitivity of atomic absorption analyses. The analysis sensitivity is generally enhanced by factors of 2 to 4 for the five elements studied depending on the element and the measurement conditions. The measurement precision is also generally improved particularly at the lower concentrations. Consequently, ultrasonic nebulization offers desirable sensitivity improvements that should find application in atomic absorption. EXPERIMENTAL
All measurements were made with a Techtron Model AA5 atomic absorption unit using an air-acetylene flame. A Tomorrow Enterprises ultrasonic nebulizer was coupled directly to the nebulizer chamber of the burner as indicated in Figure 1. Flame conditions were optimized for each element studied and for both the pneumatic and ultrasonic nebulizer systems. Instrument conditions such as slit width, lamp current, and read-out time constant were held constant for any one element for both nebulizer systems. Thus, the results presented below are representative of optimum conditions with regard to the nebulization and atomization systems at constant measurement conditions and offer a reliable means for comparing the two nebulization systems. RESULTS AND DISCUSSION
The ultrasonic nebulization chamber and the means of coupling this to the nebulization chamber of the existing Techtron burner are illustrated in Figure 1. The system is designed to minimize condensation of the aerosol while maintaining nebulizer interchange simplicity. In this system more than 95% of the condensation drainage occurs at the overflow of the ultrasonic unit. Experiments indicate that the memory of the system is reduced to approximately 15 seconds when the rapid flushing feature of the solution pumping system on the ultrasonic generator is used and, consequently, compares favorably with the pneumatic nebulizer. Results of the optimization study indicate that the ultrasonic frequency should be controlled to =tl%or better at 1.4 MHz in order to maintain maximum aerosol production ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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Table I.
Curve slope, absorbance/pg/ml PN us
Element
0.41 1.44 2.17 0.23 1.74
Ag
Cd Mga Pb
Zn a
Comparison of Curve Slopes and Detection Limits for Ultrasonic and Pneumatic Nebulization
Slope ratio, USjPN
1.19 2.86 3.23 0.52 5.23
Detection limit, (rglml)
2.9 1.9 1.5 2.3 3.1
PN
us
0.02 0.008 0.002 0.02 0.006
0.005 0.003 0.001 0.03 0.002
Enhancement factor, US/PN 4.0 2.7 2.0 0.7 3.0
Mg comparison made at X 3 scale expansion, all others were made at X 6 scale expansion.
I
3 5 7 DISTANCE ABOVE BURNER SLOT, mm
J
Figure 2. Vertical profile of an atomic population in the flame 0 Ultrasonic nebulizer 0
Pneumatic nebulizer
and to prolong the life of the ultrasonic crystal. The tuning system associated with the unit readily allows this type of control. The sample feed rate, which is controlled by the pump on the nebulizer unit, is easily maintained at the optimum level of 3.0 i= 0.1 ml/minute. The absorption signal is maximized when 35% of the total oxidant (air) flow is introduced into the ultrasonic nebulization chamber. The remainder of the oxidant is introduced at the auxiliary air input of the burner. Measurements indicate that 34-35z of the analyte delivered to the nebulizer reaches the flame under the conditions specified above. Thus, the nebulization-delivery efficiency is increased by approximately a factor of 3.5 over the efficiencies observed by Willis (7) with the pneumatic nebulizer. An indication of the relative droplet size distribution for the two nebulizers can be obtained from the vertical flame population profiles given in Figure 2. When such profiles are measured with the standard optical arrangement, no difference can be observed for the two nebulizers. Thus, the measurements presented in Figure 2 were made using a vignetting system arranged so the image of the light source was approximately 2-3 mm in diameter at the source end of the burner. Under these conditions, the maximum population of atoms is observed very close to the burner slot and the decrease on moving upward in the flame may be attributed to a dilution effect. The maximum atom population for the pneumatic nebulizer occurs higher in the flame implying, but not conclusively proving, that the sizes of the aerosol droplets are larger and therefore have a greater kinetic limitation on the atom formation. Since these measurements were made at constant lead concentration, the curves are also indicative 1472
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0.04
Figure 3.
0.12 0.20 0.2 8 METAL CONCENTRATION, ug/ml
0.36
Comparative analytical curves for lead and zinc US = Ultrasonic nebulizer PN = Pneumatic nebulizer
of the enhanced sensitivity that can be realized through the use of the ultrasonic nebulizer. A further indication of the increase in sensitivity realized through the use of ultrasonic nebulization can be obtained from the example analytical curves presented in Figure 3. These curves demonstrate the general observation for the five elements studied, Le., ultrasonic nebulization produces a curve of considerably greater slope than can be realized with pneumatic nebulization. An additional indication of the advantage realized may be obtained from the data in Table I. The ratios of the ultrasonic-to-pneumatic curve slopes reflect the general sensitivity differences between the two techniques for the elements studied. While the curve slopes are indicative of sensitivity, the ability to determine a small change in concentration is ultimately limited also by the measurement precision (16, 17). Consequently the detection limits given in Table I have been defined as outlined by Skogerboe et al. (16, 17), i.e., the concentration required to produce a signal t times the standard deviation of measurement. For the limits given in Table I, the measurement precision was estimated by pooling the standard deviations of replicate measurements for each elemental concentration used in establishing the analytical curves (18). The t statistic for the (16) R. K. Skogerboe and C. L. Grant, Spectrosc. Left., 3, 215 (1970). (17) R. K. Skogerboe, Ann T. Heybey, and G . H. Morrison, ANAL. CHEM., 38,1821 (1965). (18) W. J. Youden, in “Handbook of Analytical Chemistry” L. Meites, Ed., McGraw-Hill, New York, N.Y., 1963, Sect. 14.
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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