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LITERATURE CITED (1) Fed. Regist. 1977, 4 2 , 62971. (2) Annu. Book ASTM Stand. 1975, Part 31, 362 (3) "Methods for Chemical Analysis of Water and Wastes". EPA Report No 600/4-79-020, March 1979, Method 353.3. (4) Connors, J. J.; Beland, J. J.-Am. Water Works Assoc. 1976, 68, 55-56 (5) Gales, Morris E., Jr., personal communication (Feb 7, 1980, EPA Methods Development and Quality Assurance Laboratory, Cincinnati, OH) (6) Wood, E. D.; Armstrong, F. A. J.; Richards, F. A. J . Mar. B i d . Assoc. U . K . 1967. 47. 23-31. (7) Skoog, D. A.;West, D. M. "Fundamentals of Anaitical Chemistry", 3rd ed.; Holt, Rinehart and Winston: New York, 1976; p 274.
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(8) Youden. W. J.; Steiner, E. H. "Statistical Manual of the Association of Official Analytical Chemists"; Association of Official Analytical Chemists: Washington, DC, 1975; pp 33-36.
J o h n H. Margeson* Jack C. Suggs M. Rodney Midgett Environmental Monitoring System Laboratory U.S. Environmental Protection Agency Research Triangle Park, North Carolina 27711 RECEIVED for review April 28, 1980. Accepted July 15, 1980.
Determination of Lead-2 10 in Environmental Samples by Gamma Spectrometry with High-Purity Germanium Detectors
Sir: Measurement of low-level environmental 'lOPb (of the order of 0.1 dps/g) is important in many fields including health physics, geochronology, and environmental science. Presently. the standard procedures for environmental assay involve chemical separation and electrodeposition of 210Pbfollowed by a or 6 spectrometry (1-3). Although these procedures are capable of high sensitivity with gram-size samples, many processing steps are involved, as well as time delays of days and longer for ingrowth of daughter activity. Four percent of the decays of 'l0Pb are accompanied by emission of a 47-keV photon ( 4 , 5 ) , but self-absorption in the sample and low sensitivity of standard Ge(Li) detectors to this energy photon have previously prohibited practical assay by direct photon measurement. Recently, a new generation of high-purity N-type germanium detectors has become available (6). These detectors are characterized by improved sensitivity to low-energy photons (due to thin diode blocking contacts) and increased active surface area. They offer the possibility of direct assay of 210pb in moderate-sized environmental samples (of the order of 100 g) by measurement of the 47-keV photon. Good parameters for a high-purity germanium detector might be 70 cm2 active area and energy resolution of 1 keV for 47-keV photons. Due to the short range of 47-keV photons in germanium, essentially all photons striking the germanium crystal will be stopped. Consider a representative environmental surface soil sample of 0.1 dps/g and density 1.5 g/cm3. An effective thickness for absorption of 47-keV photons might typically be 2 cm. On the assumption sufficient sample is available, 210 g can be effectively utilized adjacent to the 70-cm2surface. Assuming 25 9'0 detection efficiency ( T geometry) and the 4% photon emission, one obtains 1 detector count rate = - d p s / g x 10 1 0.04 photon/decay x - count/photon x 210 g =
4
0.21 count/s (1) If no background were present, this result would imply only 7.9 min would be required to obtain 100 counts and 10% counting precision (Poisson counting statistics). The sensitivity for water samples would be improved further because 0003-2700/80/0352-1957$01 .OO/O
of larger effective sample size due to less self-absorption in the sample. In practice the above estimate is quite liberal and can only provide a bound on the sensitivity. '4ctual sensitivity will be much less due to the presence of radiation background, potentially poorer counting geometry. losses in the detector's dead layer, and ineffectual collisions in the germanium. Nevertheless, the potential feasibility of direct assay of moderate-sized environmental samples of 'lOPb without need of chemical processing or time delay is suggested. This technique has been experimentally tested with a surface soil sample and a high-purity germanium detector. The sample was surface soil from Boulder, CO, and the detector possessed a 65-cm2 active surface area. Optimum shielding for such a test was not available, so the detector was surrounded by an oversized soil sample approximately 10 cm in depth where the soil beyond the effective self-absorption layer acted as a shield from external sources. In this configuration, shielding from external radiations is adequate, but the higher energy photons in the outer layers of the soil penetrate the inner layer and add some low-energy background. Figure 1 shows the result. Figure l a is from a sealed 226Rasource which contained daughter radiations ( 4 , 5 ) including *lOPb. This source was not at equilibrium, and the ratio 214Pb/2'0Pbis enhanced over that for a source a t equilibrium. Figure 1b is the result of a 30-h count with the soil sample. The 'loPb photons are clearly present although the background is considerable. The isotope '14Pb is not visible since the sample is surface soil (top 1 cm) and the 210Pbcomes predominantly from atmospheric *l0Pb fallout (from '22Rn decay) and not decay of 226Rain the soil (7),and the emission intensity of the '14Pb 53-keV photon is less than that of the 'lOPb photon ( 5 ) . Figure ICwas taken with the soil sample removed, and the absence of the 210Pbpeak indicates that the 'loPb photon was not due to sources external to the soil. On the basis of calibration with 226Raand 241Amsources, analysis of the soil spectrum indicated an approximate concentration of 'lOPb of 0.2 dps/g. Counting error (predominantly due to the background) was *14%. Although the sensitivity is significantly worse than the bound estimate, it is still quite usable. Soil and sediment concentrations of 'lOPb typically range from 0.01 to 1.0 dps/g ( 3 , 7 ) and counting times of a C 1980 American Chemical Society
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Anal. Chem. 1980. 52, 1958-1959
1000
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Fi ure 1. Spectra with a high-purity germanium detector: (a) sealed Ra source; (b) soil sample (30 h count); (c) soil sample removed (46 h count). Low-energy electronic cutoff in all spectra corresponds to approximately 20 keV.
**B
day or longer are practical with germanium detector systems. The above results demonstrate the feasibility of assaying concentrations of 210Pbof the order of 0.1 dps/g in moderate-sized environmental samples with high-purity germanium detectors. They were substantiated by additional measurements. For environmental samples, identification of the 47keV photon from zloPb in spectra should be relatively unambiguous. The nearest commonly observed y emissions due to natural radioactivity (53 keV, 214Pb;42 keV, 231Th;51 keV, are easily resolvable with a germanium detector (8). Due to the few steps involved, the procedure should be convenient and reliable. Significant improvements in sensitivity should be possible with shielding and detectors optimized for such measurement. A major focus of design for future systems optimized for z*oPbmeasurement should be reduction in the low-energy background. For example, for the experimental measurements described above the sensitivity for the same counting time and error would be increased by over a factor of 10 if the background were not present. Several points are important about reduction of low-energy background in 'lOPb measurements.
The background is largely due to the Compton edge of higher energy photopeaks. These higher energy photons come from both external sources and the sample itself. Hence the sample size should be no larger than that which gives a thickness where self-absorption of the 47-keV photons becomes dominant. Larger samples add background from more penetrating high-energy photons without adding zloPbcounts. Environmental samples with large amounts of higher energy y emitters will be more difficult to count than those with small amounts. Shielding from external sources of 'loPb should be relatively easy due to the short range of 47-keV photons. The greater challenge is to reduce the higher energy photons. Compton suppression systems (9) would be one effective, but expensive, way to reduce background from both sample and external sources. Standard methods must still remain the choice when maximum sensitivity is required. Emission of a and radiation follows all the decays of 210Pband its daughters (not just 4% as is the case for the 47-keV photon) and there is no fundamental limit to the size sample that can be processed. However, many environmental samples involve 'lOPb concentrations and sample sizes suitable for photon measurement, and in these cases high-purity germanium detectors offer a technique that is significantly more time saving and convenient.
ACKNOWLEDGMENT The assistance of the staff of the Nuclear Physics Laboratory of the University of Colorado in providing facilities and carrying out these experimental measurements is gratefully acknowledged.
LITERATURE CITED Rama; Koide, M.; Goidberg, E. D. Science (Washington, D.C.) 1981, 734, 98-99. Flynn, W. W. Anal. Chim. Acta 1988, 43, 221-227. Krishnaswamy, S.;Lai, D.: Martin, J. M.; Meybeck, M. Earth Pbnet. Sci. Lett. 1971, 1 1 , 407-414. Lederer, C. M.; Shirley, V. S. "Table of Isotopes", 7th ed.;Wiley: New York, 1978. "Radiological Health Handbook"; U.S. Department of Health, Education, and Welfare: Rockville, MD, 1970. Pehl, R. H. Phys. Today 1977, 30, 11, 50-61. Moore, H. E.; Poet, S. E. J. Geophys. Res. 1978, 87, 1056-1058. Perkins, R . W.: Thomas, C. W. "Workshop on Methods for Measuring Radiation in and around Mills"; Atomic Industrial Forum: Washington, D.C., 1977; pp 255-274. Camp, D. C.; Gatrousis, C.; Maynard, L. A. Nucl. Instrum. Methods 1973, 117, 189-211.
Stephen D. Schery Geophysical Research Center New Mexico Institute of Mining and Technology Socorro, New Mexico 87801 RECEIVED for review March 25, 1980. Accepted July 25, 1980. This research was supported in part by U S . Department of Energy Contract No. DE-AS04-78ETO5364.
Comments on Experimental Studies on Spatial Distributions of Atoms Surrounding an Individual Solute Particle Vaporizing in an Analytical Flame Sir: On page 1903 of the paper by Boss and Hieftje ( I ) , the authors make the following statement: "West, Fassel, and Kniseley (16) determined that Ca atoms released in N20/CzH2 0003-2700/80/0352-1958$01 .OO/O
flames from solutions containing phosphate migrate toward the edges more slowly than do those from phosphate-free solutions". We can find no statements in the two papers (2, 0 1980 American Chemical Society
