Instrumental Neutron Activation Analysis of Atmospheric Pollutants Utilizing Ge(Li) 7-Ray Detectors W. H. Zoller’ and G . E. Gordon1 A . A . Noyes Nuclear Chemistry Center: Massachusetts Institute of Technology, Cambridge, Mass. 02139 Instrumental neutron activation analysis employing lithium-drifted Ge 7-ray detectors has been used to measure the concentrations of more than 20 elements (mostly metals) in atmospheric aerosols of urban areas. For most elements studied, the new technique i s more sensitive and accurate than techniques in current use. The new method is very convenient and is quite fast for several elements having short-lived irradiation products (AI, V, Mn, Na, CI, Br). The technique can also be used to measure the concentrations of many of the elements in important pollution source materials.
IN VIEW of the increasing national concern regardin,0 environmental problems and, in particular, air pollution, it is important to develop improved techniques for the measurement of concentratioris of elements and compounds in the atmosphere. The study of particulate material in the atmosphere is difticult because of many problems involving the collection and analysis of the samples and the interpretation of the results. No single analytical technique to date has been able to meet the various requirements of the problem, including sufficient speed t o deal with air pollution emergencies. In this paper, we report on a new method that is quite accurate and sensitive for the determination of about 24 elements in atmospheric aerosols. This method has great potential usefulness, particularly a s several of the elemental concentrations (Na, C1, Br, V, Mn, and, in some cases, Ca and Cu) can be determined within about a n hour after receipt of the samples. Thus, the technique could be used rapidly t o indicate pollution crises and to suggest the most serious sources contributing to the crisis. Most work on atmospheric aerosols has employed samples collected on filters or in particle impactors. Most analytical methods involve dissolution of the sample followed by chemical and instrumental analyses. A serious problem with such methods is that dissolution and subsequent wet chemical steps may introduce large errors, either by loss of material or contamination of the sample. The sample material itself, especially pollution aerosols, is largely insoluble in water, being to a great extent carbonaceous residues ( I ) . Even with use of strong acids, ion exchange of trace metals with glass and plastic containers may be quite severe (2). hlatrix effects in many of the common analytical methods (e.g. emission spectrography, atomic absorption, or microchemical techniques) may be severe if particulate material andlor interfering ions are present ( 3 4 Many of these difficulties are avoided by the method presented below in that 1 Present address, Department of Chemistry, University of Maryland, College Park, Md.
(1) C. E. Junge, “Air Chemistry 2nd Radioactivity,” Academic Press, New York, 1963. Chapters 2 and 5. (2) F. K. West, P. W. West, and F. A. Iddings, ANAL.CHEM., 38, 1566 (1966). (3) G. D. Christian, ibid., 41, 24A (1969). (4) P. W. West in “Air Pollution”, 2nd Ed., A. C. Stern, Ed.,
Academic Press, New York, 1968, Vol. 11, Chap. 19.
the filters may be irradiated with neutrons prior to any treatment. Our method involves collection of aerosols from 20 t o 50 m 3 of air on a filter, irradiation of the filter with thermal neutrons, and observation of y rays from the samples with lithium-drifted germanium [Ge(Li)] detectors. Gamma rays from irradiation products of about 24 elements can be observed and their intensities measured, allowing one to conipute the concentrations of those elements. Neutron activation analysis has been used in a wide variety of applications for many years [e.g. see Ref. (5)]. In the past, it was generally necessary chemically to separate the elements of interest t o remove the interfering activities of other elements. However, the field of instrumental neutron activation analysis (INAA), in which no chemical separations are performed, has been greatly extended by the development of Ge(Li) y-ray detectors. As Ge(Li) detectors have much better resolution than NaI(Tl), scintillation counters previously used for y-ray spectrometry, it is now possible t o resolve y rays of many radioactive nuclides from complex mixtures of activities. For example, Gordon et al. (6) demonstrated that the concentrations of 20 or more elements could be determined in a wide range of rock types by INAA with the use of Ge(Li) detectors. Activation analysis has not been very extensively used in atmospheric studies. Much of the pioneering work was done by Winchester and co-workers [see Ref. (7) and references therein] who have used radiochemical methods t o study mainly the halogens, Na, V, and Cu in the atmosphere and ocean, rain and snow. Recently, Brar et a/. (8) used NaI(T1) detectors without chemical separations to measure the concentrations of several elements in aerosols from the Chicago atmosphere. Dudey et al. (9) observed lines from irradiation products of about 23 elements in Ge(Li) y-ray spectra of marine aerosol samples after chemical removal of Na and additional ion-exchange steps. Because of the generally lower ratio of Na t o other elements in nonmarine atmosphere, it is possible t o measure concentrations of quite a number of elements in the latter type of samples, particularly from highly polluted urban areas, by high-resolution Ge(Li) y-ray spectrometry without any chemical separation. EXPERIMENTAL METHODS Sample Collection. The work reported here was done with samples collected o n the roof of the MIT Nuclear Chemistry ( 5 ) J. R. De Voe, Ed., “Modern Trends in Activation Analysis,” National Bureau of Standards, Washington, D. C., 1969. (6) G. E. Gordon, K. Randle, G. G. Goles, J. B. Corliss, M. H. Beeson, and S . S . Oxley, Geochim. Cosmochim. Acta, 32, 359
(1968). (7) J. W. Winchester, W. H. Zoller, R. A. Duce, and C. S.Benson, .4tnzos. Emiron., 1, 105 (1967). (8) S . S . Brar, D. M. Nelson, E. L. Kanabrocki, C. E. Moore, C. D. Burnham, and D. M. Hattori, in Ref. (5);Vol. I, p 43. (9) N. D. Dudey, L. E. Ross, and V. E. Noshkin, in Ref. ( 5 ) ,Vol. I, p 55.
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Figure 1. Schematic diagram of p r a y spectrometry equipment used Building in Cambridge, Mass. Samples were collected by pumping 20 to 50 m 3 of air through 57-mm diam Millipore EH (0.5-p pore diam) membrane filters or Delbag polystyrene filters with a n electric vacuum pump. This required about 12 hours with our present systems. Both filter materials have high collection efficiencies for particulate material down to the 0.1-p range (IO). Irradiations. The filters were cut into halves and each half was sealed in a precleaned polyethylene vial. All polyethylene which came into contact with the samples was washed with hot concd HNO, followed by rinses with distilled-demineralized water and acetone. The two halves of each filter were irradiated separately for different lengths of time in the MIT reactor at a flux of 2 X 1013 n/cm2 sec. In each irradiation, one or more sample vials were placed in a sample carrier (“rabbit”) along with a vial containing monitor solution. The monitor solution contained known concentrations of all the elements that were expected to be observed in the irradiated filters. Following the short irradiations (-5 min), the rabbit was delivered to the laboratory within a few seconds aia a pneumatic-tube system (11) for observation of species with half lives from a few minutes to several days. After long irradiations (up to 9 hours), the samples were allowed to “cool” for several days before observation of species with half lives from several hours up to many years. Treatment of Samples and Monitors. Following irradiations, each filter was transferred to a clean glass vial. The irradiation container was washed with 1 ml of concd HnS04 (and a few drops of water) which was then added to the filter sample, dissolving much of the filter and sample, leaving -1 ml of solution and suspended residue in the bottom of the vial. Tests with the volatile elements C1 and Br showed that these elements are not appreciably lost in the acid dissolution step. This step was performed to collect any material that flaked off the filter in the irradiation container and to reduce the volume of the filter and particulate material. In comparing the activity of a given species in a sample with that in the monitor, it is important that the two sources of activity have about the same volume and shape. Otherwise, when they are counted close to the Ge(Li) detectors, the counting efficiencies for the different sources may be considerably different. An aliquant of monitor solution was placed in another glass vial and its volume made up to about 1 ml by addition of HzO. Gamma-Ray Spectrometry. Gamma-ray spectra of each sample were taken at several times after each irradiation. A (10) E. R. Henrickson in “Air Pollution,” (see Ref. 4), Vol. 11, Chap. 16. (1 1) C. E. Bemis, Jr., and J. W. Irvine, Jr., NucL Znstrum. Methods, 34, 57 (1965). 258
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Figure 2. Gamma-ray spectra taken with the 26-cm3 Ge(Li) detector at three different times following a 5-min irradiation of a Millipore filter sample S.E. and D.E. designate single and double annihilation-photon escape peaks. Bk indicates peaks occurring in background block diagram of the counting equipment is shown in Figure 1. Two different Ge(Li) detectors were used: a large, co-axial detector of effective volume 26 cm3 (Canberra) and a wafertype detector of 0.5 cm3 volume (Ortec). Because of its greater efficiency, the large detector was particularly useful for oberving high-energy y-rays. The full-width a t halfmaximum (FWHM) of the photopeak produced by 1332-keV y rays from 6oCowas about 2.3 keV. The small detector (which we designate the Ge-LEPS, low-energy photon spectrometer) is designed for very high resolution (FWHM s 0.4 keV at 14.4 keV) of low-energy X- and y-rays. The high resolution is achieved by keeping the detector quite small transistor (giving low capacity) and by placing the field-effect transistor of the pre-amplifier in the detector cryostat (12). Although the Ge-LEPS is most effective for photons from about 10 to 150 keV, its efficiency drops off slowly enough with energy that it can be used up to about 500 keV. Pulses from the detectors were passed through preamplifiers and linear amplifiers which included a base-line restorer to (12) H. R. Bowman, E. K. Hyde, S. G. Thompson, and R. C . Jared, Science, 151, 562 (1966).
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