Instrumental photon activation analysis of atmospheric particulate

Multielement determination in market-garden soils by instrumental photon activation analysis. Amares. Chattopadhyay and R. E. Jervis. Analytical Chemi...
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reaction product. These observations indicate that the reaction of NO with PbOz may be more complicated than that proposed by Mishmash and Meloan. Additional support for this is shown by the thermochemical calculations (19) in Table IV. The anomalous behavior of NO with PbOz is clearly evident if one considers that the net free energies and enthalpies of the reaction of COz and aqueous H N 0 3 with PbOz are somewhat more negative than that of NO. Yet NO readily reacts with electrolytically derived PbOz whereas COz and aqueous H N 0 3 do not. ( 1 9 ) "JANAF 1.hermochemical Tables," D. R . Stull, Proj. Dlr., 1960. Dow Chemical Go., Midland, Mich.

Because of its unique reactivity with NO, only electrolyticrystal forms), obtained cally derived PbOz (a and/or from the anode plates of several used lead/acid batteries and purified as described, was used for the development of sampling and analysis criteria. Further work is in progress to apply this sampling device to the measurement of HCl, C12, SO,, HF, SiF4, and CO in combustion effluents. Received for review November 13, 1972. Accepted February 20, 1973. This work was partially funded by the Environmental Protection Agency under an interagency agreement with the Air Force Rocket Propulsion Laboratory.

Instrumental Photon Activation Analysis of Atmospheric Particulate Material N. K. Aras,l W. H. Zoller, and G . E. Gordon Department of Chemistry, University of Maryland, College Park, Md. 20742

G . J. Lutz Analyticai Chemistry Division. National Bureau of Standards. Washington. D.C. 20234

Concentrations of fourteen elements in atmospheric particulate matter have been measured by irradiation of the samples with bremsstrahlung from electrons of 35 MeV from the NBS electron linac and observation of y rays from the reaction products with Ge(Li) detectors. The elements routinely observed by this nondestructive method are: Na, CI, Ca, Ti, Cr, Ni, Zn, As, Br, Zr, Sb, I , Ce, and Pb. Several other elements such as Fe, Se, Rb, and Y are marginally observable. Although, in general, instrumental photon activation analysis (IPAA) is less sensitive than instrumental neutron activation analysis (INAA), with IPAA one can measure concentrations of several elements that are difficult or impossible to measure in urban particulates with INAA, especially Ti, Ni, As, I , and Pb. Measurements of Ni, As, and Pb are quite important because of their known toxicities.

Since the development of lithium-drifted germanium [Ge(Li)] y-ray detectors, nuclear methods have been successfully applied to the measurement of concentrations of many elements in complex environmental samples, often with much greater sensitivity than had been possible with older methods ( I ) . A total of 42 elements, for example, has been observed uia instrumental neutron activation analyses (INAA) in atmospheric particulate materials collected in various areas ( 2 ) . However, because of interferP e r m a n e n t address, D e p a r t m e n t of C h e m i s t r y , M i d d l e E a s t T e c h n i c a l U n i v e r s i t v . A n k a r a . Turkev. "Nuclear Methods in Environmental Research," J . R. Vogt, T, F, Parkinson, and R . L. Carter, Ed., University of Missouri Press, GOlumbia, Mo., 1971. G. E. Gordon, in "Proceedings of the International Symposium on Identification and Measurement of Environmental Pollutants," E. Westiey, Ed., ~ a t i o n a Research i Council of Canada, Ottawa, 1971, pp 138-143.

ences among elements, no more than about 30 elements have been measured in any one sample. Despite this success, INAA is not capable of analysis for all of the elements in atmospheric aerosols that are of considerable environmental concern. For those important elements that are apparently beyond the scope of INAA in environmental samples, alternate instrumental nuclear methods of analysis are needed. Samples of this sort generally have very complex, poorly characterized matrices. If one tries to dissolve them, great care must be exercised to prevent contamination, loss of volatile species, or loss of trace species by coprecipitation on insoluble residues or container walls. Thus, it is desirable to analyze these samples by nondestructive, instrumental methods if possible. Furthermore, to eliminate matrix effects, it is important that projectiles and radiations involved in the analysis have long ranges in the sample material. One nuclear method that meets the above criteria is instrumental photon activation analysis (1PAA)-i. e . , the irradiation of samples with bremsstrahlung produced by deceleration of high-energy electrons (215 MeV) in a stopping medium. The incoming y rays that induce nuclear reactions such as ( r , n ) , (y,p), etc. must have energies of about 7 MeV or greater, and thus have extremely long ranges in the sample material. Instrumental photon activation analysis has not been used extensively in the past, in part because sources of high-energy electrons are not widely available. Also, far less activity per unit irradiation time is produced with bremsstrahlung than with moderate-flux reactors. However, IPAA offers several advantages as a complement to INAA: (a) The reactions (y,n), (y,p), etc. of the target nuclides often lead to products other than those resulting from neutron irradiation, increasing the probability of forming products with half lives and 7-W' energies convenient for analysis. ( b ) As in the case of neutron reactions, ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973

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photonuclear reactions on low atomic number elements, which are generally the major elements present, mostly lead to products having either short half lives or decay modes with no y-ray emissions. (c) The combination of the shape of bremsstrahlung energy spectra and the excitation functions for important photonuclear reactions as a function of target atomic number 2 causes a general rise of photonuclear yield with increasing 2 ( 3 ) . This effect, coupled with point (b) offers the possibility of observing trace quantities of high-2 elements in the presence of much greater quantities of low-2 material. We have investigated the applicability of IPAA to the determination of trace-element concentrations in atmospheric aerosols. Samples were irradiated with bremsstrahlung from electrons of 35 MeV from the National Bureau of Standards (NBS) electron Linac. Following irradiations, y rays from the samples were observed with Ge(Li) detectors. We find that IPAA can be used to measure concentrations of fourteen elements routinely in urban particulate material. Concentrations of several other marginally observable elements could probably be determined with some modifications of the technique. EXPERIMENTAL Sample Collection. Samples of atmospheric particulate matter were collected by pumping air through a 20- X 25-cm Delbag polystyrene filter a t 2 m3/min using a high-volume Hurricane pump. Particulates from 2500 to 5000 m3 of air were collected on each filter and one-eighth of the filter was used in each irradiation. Samples were collected a t two sites. The first site was the roof of the six-story Chemistry Building on the College Park campus in a residential and college community about 12 km from the center of Washington, D.C., and the second site was in a residential area of Silver Spring, Md. Samples a t the second site were obtained about 2 m above the ground. One major source of atmospheric particulates near the University sampling site is automobile exhaust from the heavily travelled Highway U.S. No. 1 about 500 m to the east of the site and from several parking lots in the immediate vicinity of the Chemistry Building. Irradiations. Pilters were cut into eighths and each eighth was folded and placed in the stainless steel die of a hydraulic press and formed into a cylindrical pellet a t a pressure of 340 atm. The pellets weighed about 0.6 gram and were 1 cm in diameter and several mm thick. The samples were irradiated simultaneously with elemental monitors which contained known quantities of each element to be observed. Monitors were made from mixtures of compounds of the appropriate elements which were either sealed in polyethylene bags or placed on filter material and formed into pellets. Portions of clean Delbag filters were also pelletized for irradiation to determine blank values for the observable elements. The pelletized filters, monitors, and blanks were stacked in a cylindrical polyethylene sample carrier. Copper disks were placed between the pellets to serve as flux monitors. Samples were irradiated in the 45-degree facility of S B S electron Linac ( 4 ) .The bremsstrahlung was produced by allowing the beam of 35-MeV electrons to strike a water-cooled tungsten target just up-beam from the samples. The beam current was usually about 50 PA, and irradiations lasted several hours. Counting. Gamma rays emitted by the products in the samples were observed with a 55- or 65-cm3 Ge(Li) detector coupled with a 4096-channel analyzer. The full-width a t half-maximum of the photopeak produced by the 1332-keV line from 6oCo was 2.2 to 2.4 keV for both detectors. Spectra were first taken about one hour after irradiations. Because of the high count rate of 511-keV annihilation radiations from 20-min (mostly from the organic filter material), only the y rays from 32-min s4"'C1, 38-min 63Zn, and 67-min 204mPb could be observed well enough in short counts for accurate determinations during this time period. But after one day of cooling, essentially all of the species listed below could be observed in spectra taken for several hours. (3) G . J. Lutz. A n a / . Chem.. 43, 93 (1971) (4) P. D. LaFleur. "Activation Analysis Section: Summary of Activities. J u l y 1969 to June 1970." NBS Tech. Note. 548, 109-113 (1970). (1970).

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ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973

Irradiation of Specific Elements and Standards. After most radioactive products were identified, a number of pure elements or simple compounds of them were irradiated to determine the major production modes for each product and to see if the same activity could be produced by reactions on neighboring elements. Also, as a check on the absolute accuracy of the IPAA method for some representative elements, a sample of the NBS Orchard Leaves Standard Reference Material (1571) was irradiated along with known quantities of CaC03, PbO, and As203 monitors The observed concentrations for Ca, Pb, and As were in excellent agreement with the values previously established by a variety of other analytical methods.

RESULTS Gamma-Ray Spectra. Examples of y-ray spectra are shown in Figures 1 to 3. Activities responsible for the y rays were determined on the basis of y-ray energies (determined to h0.5 keV or better) and the intensity patterns of the several y rays emitted by most products. Furthermore, in the case of each product with a half life of from one hour to thirty days, decay curves for the major y rays were plotted to check for interferences. The spectrum of Figure 1, taken one hour after irradiation, is so strongly dominated by 511-keV annihilation radiations from positron emitters that no y rays below that energy are observable. Many lines observed a t higher energies result from (n,y) products of secondary neutron reactions, e.g. 24Na, 56Mn, SOBr, and 82Br. The products of major interest a t this time period are 34mCl, 63Zn, and 204mPb. At 24 hours after irradiations (Figure 2), the products of major interest are 22-hr 43K, 4.5-day 47Ca, 44-hr 48Sc, 36-hr 57Ni, 62-hr 67Cu, 56-hr 77Br, 67-hr IzzSb, and 52-hr 203Pb. Several y rays in the spectra of Figures 2 and 3 have not been identified. These lines are listed in Table I along with the apparent half-lives. The y-ray energies and intensities of some of the neutron-deficient species encountered are not as well characterized as for the neutronexcess species generally observed in INAA. The spectrum observed 11 days after irradiation is shown in Figure 3. Some of the major peaks arise from 3.35-day 47Sc, 52-hr 203Pb, and 53-day 7Be, the latter produced mainly by 12C(y,cun). The 137Cs peak appearing in the spectra served as an internal standard. Elements Measurable by IPAA. Reactions that can serve as bases of concentration measurements of atmospheric aerosols are listed in Table I1 along with properties of the product nuclides and possible interfering reactions. In the second part of Table I1 are listed other observed activities and their major production modes. Some of these species could be used for concentration measurements, although for the reasons listed below we have not used them. In Table 11, we have not given detailed y-ray lists for observed species that were not used for elemental determinations; however, energies of their prominent lines are given in the list of y rays in order of energy in Table I. More details on decay of the photonuclear products and the neutron-capture species are given in References 5 and 6, respectively. Table I lists, in order of increasing energy, all of the y rays typically observed in the photon-irradiated atmospheric particulate samples. This listing includes some y rays of species given in Table I1 that were deemed too weak for inclusion there. Also listed are y rays emitted by species in the second part of Table 11, which were not used as the basis of elemental determinations. Comments on Individual Elements. In the following paragraphs, we have noted any problems encountered in (5) V . Galatanu and M . Grecescu, J. Radioanai. Chem.. 10, 325 (1972).

(6) I . M . H . Pagden, G . J . Pearson, and J. M. Bewers, J. Radioanai.

Chem.. 8, 127 (1971).

6

I

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I

ATMOSPHERIC PARTICULAK

I

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1

MATERIAL

CHANNEL NUMBER

,

300

PART 250

I

I

I

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1

I

I

I

I

I

II

24

'

Na

-

38

CHANNEL

NUMBER

Figure 1. Gamma-ray spectrum taken with a 55-cm3 Ge(Li) detector starting one hour after a 4-hr irradiation of a Delbag filter sample with bremsstrahlung. S.E. and D.E. designate single and double annihilation photon escape peaks

150

CHANNEL NUMBER

-

I PART

I

I

1

I

I

I

I

I

I

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I

I

IL

,25

- 124Hr Cooling1

00

-

24

NO

-

75t 75

I

F2

1044 2 4

50

77

62

166 I k O

25-

48

,

48

s c ~ '

~

,

2m CHANNEL NUMBER

Figure 2. Gamma-ray spectrum taken with a 55-cm3 Ge(Li) detector starting one day after a 4-hr irradiation of a Delbag filter sample with bremsstrahlung

8

I

I

I

I

I

I

I

I

1

-

203

Pb

7-

1

74 As 596

279

47

6-

ATMOSPHERIC PARTICULATE MATERIAL GelLi) Y-ray Detector

1

-

W

z z a TI Kg

CHANNEL NUMBER

i W

z z a 0 I 12: a W

rz 3

8

1400

1600

1800

2200

2000 CHANNEL

2400

2600

NUMBER

Gamma-ray spectrum taken with a 55-cm3 Ge(Li) detector starting eleven days after irradiation of a Delbag filter sample with bremsstrahlung Figure 3.

the analyses for each of the elements. Note that it is desirable to avoid the use of (n,y) reactions because of uncertainties of the flux and energy spectrum of neutrons a t various sample positions. The flux monitor reaction of Cu depends on the bremsstrahlung flux and is not necessarily representative of the neutron flux. Sodium and Magnesium. Although some of the observed 24h'a results from the 23Na(n,y) reaction of secondary neutrons, a major amount results from the 25Mg(y,p) and 26Mg(y,pn) reactions. In fact, the Z4Na activity has been used for the determination of Mg concentrations in terrestrial and lunar rocks ( 7 , 8). Perhaps by using a lower beam energy ( e . g . , 23 us. 35 MeV) and minimizing the amount of thermalizing materials near the sample area, one could use 24Na as the basis for Mg determinations as Schmitt et al. have done in the case of geological samples (7, 8 ) . (7) R. A. Schmitt, T. A. Linn, Jr., and H. Wakita, Radiochim. Acta. 13, 200 (1970). (8) H. Wakita, R . A. Schmitt, and P. Rey, in "Proceedings of the Apollo-1.1 Lunar Science Conference," Pergamon Press, New York, N.Y., 1970, Vol. 2, pp 1685-1717.

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ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1973

The 23Na(y,n)Z2Na reaction can be used conveniently for Na determinations. Possible interfering reactions are Z4Mg(y,pn) and 27Al(y,an), but for the same weights of elements, the 22Na production ratios are about l O 3 / l O / l for Na/Mg/Al, so only about 1 to 2% of the 22Na comes from the interfering reactions. Chlorine. The only problem with C1 determinations is the high C1 blank for Delbag filter material which contributed from one-half to one-third of the total 34mCl activity. Interference from the 39K(y,an) reaction is less than 1%. Calcium. Either reaction, 44Ca(y,p)43K or 48Ca(y,n)47Ca, can be used for Ca measurements, although the former generally gives higher precision. The 373-keV line of 43K is the best to use, as the 617-keV line is subject t o interference from 82Br. Interference from the 374-keV line of 67-min 204mPb can be avoided by using spectra taken a day or more after irradiation. Titanium. The 4 8 T i ( y , ~ ) ~ ~reaction Sc produces the greatest activity from Ti, but is subject to interference from the p- decay of 47Ca resulting from the 48Ca(y,n) reaction and, perhaps, from the 51v(Y,a)47sC reaction. The 49Ti(y,p)48Sc reaction is not subject to serious inter-

Table I. List of Gamma Rays Observed in Photon-Irradiated Air Filter Samples Energy, keV

82.5 84.9 87.8 92.2 93.3 101.0 121.1 122.0 127.1 135.9 136.5 139.2 145.4 146.5 159.4 161.9 165.8 175.1 184.5 186.1 196.7 200.4 219.2 221.5 238.6 238.9 241.9 249,7 264.6 270.6 273.5 279.2 279.5 281.6 291.2 295.2 297.2 300.2 303.8 320.1 Q

Nuclide

TI Ksi

Energy, keV

332

TI K 3 2

77Br 82Br

349

"CU

351

8'Br

75Se 57Co

351.9 362

57Ni

75Se 57Co 77Br

141Ce 34mCI

47sc 77Br

l39Ce 45sc 67Cu

226Ra(Bkg) 120rnSb 77Br 43K

82Br 228Th (Bkg) 77Br 226Ra(Bkg) 77Br

75Se 7?Br 82Br

203Pb

'5Se 77Br

43K

226Ra(Bkg) 77Br 67Cu

77Br 51Cr

373.0 374.7 385.1 388.6 397.8 400.6 401.3 438.7 439.7 477.6 484.8 488.9 491.3 51 1 .O 521 .O 548.2 554.3 559.1 564.1 568.6 575.1 579.4 583.1 585.9 592.5 595.7 596.6 606.8 608.5 609.3 61 7.0

Nuclide

unknown f l I 2 I 1 day unknown f1/2 45 hr unknown tl l 2 45 hr n26Ra(Bkg) unknown tl 2 60 hr

43K

-

no4mp)J

77Br 1261

43K

75Se 203Pb GgmZn

77Br 7Be 77Br

47Ca 1261

Annihilation 77Br S2Zn

82Br

76As 122Sb 77Br 77Br 77Br 228Th (Bkg) 77Br

43K

74As 6'Zn 82Br

74As 226Ra(Bkg) 80Br

Energy, keV

61 7.2 619.1 634.8 640.4 657.0 657.6 665.4 665.7 666.4 667.7 669.6 680.7 692.8 698.3 704.3 744.1 754.0 756.0 776.5 807.9 810.8 818.5 827.8 834.8 846.8 879.7 882.9 884.5 889.3 898.2 899.1 908.0 909.1 91 1.7 935.5 937.3 952.4 961.9 983.4 1005.7

Nuclide

43K 82Br

74As 5oBr 76As 1 1 OmAg

226Ra(Bkg) 80Br 1261

'32Cs

Energy, keV

1368.5 1377.6 1395 1434.3 1460.0 1474.8 1524.7 1596.2 1604

122Sb 82Br 80Br 52Mn 1261

77Br 82Br

47Ca 58c0

77Br 82 Br 54Mn 56Mn 1261

84Rb 11OrnA

9

46Sc 88Y 204rnPb

2 2 8 A ~(Bkg) 89.3

204rnPb 52Mn 11OmA

9 82Br 632n

45sc 77Br

57Ni

unknown t 1 / 2 long 52Mn 40K (Bkg) 82Br

42K

140La

1642.0 1650.3 1731.9 1757.6 1764.5 1779.5 1810.9 1836.2 1919.5 2112.7 2128.5 2243.1 2753.9 1009.7 1022.8 1037.4 1044.0 11 15.4 1120.3 1120.5 1140.5 1171.0 1177.5 1256.6 1256.7 1274.6 1296.8 1311.7 1317.4

- i o days

unknown

tlf2

63zn

203Pb

Nuclide

24Na

38c1

82Br

24Na(DE)= 57Ni

226Ra(Bkg) 82Br 56Mn BEY

57Ni 56Mn

s4mCI 24Na( S E ) b z4Na 82Br

120rnSb 48Sc 82Br 65zn

226Ra(Bkg) 4%

lZ2Sb

120rnSb 34mci

lZ2Sb 80Br

22Na 47Ca 45sc 82Br

DE = double escape of 51 1-keV photons. b SE = single escape peak.

ference. Although 46Sc can have interference from the 45Sc(n,?) reaction of secondary neutrons, good agreement of the Ti concentration between 4Y3c and 4 % ~determinations was obtained. Chromium. The 52Cr(y,n)5lCr reaction is subject to interference from the 56Fe(?.on) reaction. For equal weights of Fe and Cr, the 51Cr production ratio is about l o 3 for Cr/Fe; however, the atmospheric concentration of Fe is about 100 times that of Cr, so a correction of about 10% must be made for the iron contribution to the 51Cr activity. Because of this correction, one can make more accurate Cr measurements by INAA. Nickel. There are no difficulties in the measurement of Ni concentrations by the 58Ni(y,n) 57Ni reaction. Zinc. Any of the three reactions listed in Table I1 can be used for Zn determinations, but the greatest accuracy is obtainable with the 68Zn(y,p) 67Cu reaction. Arsenic. The 74As line a t 597 keV has some interference from a line originating from 9.1-hr 62Zn produced by 64Zn(y,2n); however, the 17.9-day half-life of 74As allows one to wait for complete decay of 62Zn. Arsenic-74 is also produced by the 79Br(y,on) reaction. For equal weights of As and Br, the amount of i4As originating from As is

about 104 times t h a t from Br, but since Br concentrations may be 100-fold greater than those of As a t some sites, a 1 to 2% correction for the Br contribution may be necessary. Bromine. Either reaction listed for Br can be used without interference and good agreement was obtained between the two methods. However, because of the lack of neutron flux monitors, the results from the 79Br(y,2n) reaction are more reliable. Zirconium. Determinations were made of Zr concentrations by the gOZr(y,n)*gZr reaction which is free from interference by reactions of Nb and Mo a t 35 MeV. Antimony. Antimony concentrations can be determined via the 123Sb(y,n)122Sb reaction. Although (y,p) and (y,pn) reactions of Te isotopes could interfere, yields from them would be lower than those of the S b reactions by factors of lo2 to 103, whereas atmospheric Te concentrations (9) are comparable with those of Sb (10). Iodine. The iodine concentration can be determined by the 127I(y,n)126I reaction. Either lz6I y ray, 389 or 666 keV, can be used, but about ten days cooling should be (9) J. A. Reednick, M . S. Thesis, Universityof Maryland, 1972. (10) W. H. Zoller and G. E. Gordon, A n a / . Chem., 42, 257 (1970)

ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1973

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Table II. Production and Properties of Nuclides Observable with Ge(Li) Detectors in Bremsstrahlung-Irradiated Atmospheric Aerosols A. Activities Useful for Analysis

Element

Na

Nuclear reactions

23Na(y,n)

Target isotopic abund., %

100

Best photopeaks for determination, keVa

Product nuclide

Best time after irrad. for countingb

2.6-yr "Na

1275

D

32-rnin 34mCI 22-hr 43K 4.5-day 47Ca 44-hr 48SC 84-day 46Sc 27.8-day 51Cr 36-hr 5 7 N i 38-rnin 63Zn 243-day 65Zn 62-hr 67Cu 17.9-day 7 4 A s 56-hr 77Br 35.3-hr 82Br 78.4-hr 89Zr 67-hr 12%b 12.8-day l Z 6 1 138-day 67-min loarnPb

1 1 77, 2128

A

373, 398, 617 489,808, 1297 983. 1037. 1312 889, 1120 320 127, 1378, 1919 670, 962 1115 93, 185 596, 635 239, 521 554. 776, 1044 909 564, 693 389. 491, 666 166 899. 912

52-hr 203Pb

279, 401

Possible interfering reactionsc

27Al(y,cun).24Mg(y,pn)

M g ( y ,p2n )

75.5 2.06 0.145 5.51 7.28 83.76 67.88 48.89 27.81 15.57

CI

Ca Ti CR Ni

Zn As

100

50.54 49.46 51.46 42.75 100 88.48

Br

Zr

Sb I

Ce

23.6 1.48 1.48

Pb

I

B . Other Activities Producedd Active species

53-day 7Be 15-hr 24Na 37-rnin 38Cl 12.4-hr 4 2 K 3.35-day 47Sc 5.7-day 5 2 M n 303-day 5 4 M n 2.6-hr 56Mn 270-day 57C0 71 -day 58C0 9.1-hr "Zn 26.4-hr 7 6 A s 120-day 75Se 4.4-hr 80mBr 18-rnin 80Br 33-d ay 8 4 R b 107-day 253-day l o m A g 5.8-day lnomSb 6.5-day 13%s 40.2-hr laoLa

-

Production modes

B

C,D D B A D B,C C,D B,C B,C C C, B C,D D

50T~(y,2~n) 51v(y,2pn),50v(y,2n) 45Sc(n,y) 56Fe(y.an),54Fe(y.2pn)

79Br(y.a n )

A

B,C Comments

"C(y,an) 23Na(n,y),2 5 M g ( y , ~ ) 37Cl(n,y) 4 3 C a ( y , ~41K(n,y) ), 47Cap- decay; 48Ti(y,p) 54Fe(y,~n) 5 6 F e ( y , ~ n55Mn(y,n) ), 55Mn(n,y), 57Fe(y,~) 5 7 N i p' decay, 58Ni(y,p) 55Co(y,n), 6oNi(y,np), 63Cu(y,an) 64Zn(y,2n) 75As(n,y), 77Se(y,~) 74Se(n,y), '%e(y,n) B'Br(y,n),"Br(n,y) 85Rb(y,n),86Sr(y,np) 69Y ( y , n )

B C,B

39K(y.an)

Could be used for C detn. U n d e r proper conditions could b e used as basis for Mg detn.

Possible basis for Fe d e t n .

Possible for Se detn Possible basis for R b detn. Possible basis for Y detn.

O9Ag(n,y) lZ1Sb(y,n) l33Cs (y,n ) 35La(n,y)

a ?-Rays used for determination in italic type. Time periods: A , 1 to 2 hours: 6 , 1 to 3 days: C , 10 to 12 days: D, greater than 3 weeks. of about 7 % or greater in i t a l c type. Mafor 7 rays of these species listed in Table 1

used to eliminate SOB, and 132Cs interference with the 666-keV line. Cerium. The only possible interference is from l4IPr(y,pn) which in this region of atomic numbers should be quite negligible a t the irradiation energy used. Lead. Either reaction listed in Table I1 can be used without interference, with the zo4Pb(y,n)203Pbbeing the more sensitive. Reproducibility and Accuracy of Analyses. In order to check on the reproducibility of the analyses, a 20- X 25-cm filter with collected particles was cut into eighths and three of the eighths were irradiated and analyzed. Results of the experiment are listed in Table 111. For most elements, the standard deviation of the three values is between 5 and 10% of the mean value. For most elements, the values obtained from Sample 1 are significantly lower 1486

ANALYTICAL CHEMISTRY, VOL. 45. NO. 8, JULY 1973

Interferences

than those from the other two samples, suggesting that the particulate material may not have been deposited uniformly over the filter. Considering possible variations in filter material, perturbations of the air stream near the filter and errors in the analysis. the observed 5 to 10% overall variations are about what one would expect. Taking into account statistical fluctuations of photopeak areas and underlying backgrounds and uncertainties in the make-up of the elemental monitors, we estimate that the accuracy of the analysis itself (of the filter plus collected particles) is within &5% for most elements. (The absolute accuracy is being investigated in more detail in an inter-laboratory comparative analysis of pollutionsource and fly ash standards organized by the Environmental Protection Agency and the Yational Bureau of Standards.) When additional uncertainties, such as that

Table I l l . Analyses of Three Equal-Area Samples of Filter from Run No. 4 Collected at the University of Maryland

Table V. Concentrations of Elements in the Delbag Filter Blanks

Observed atmospheric concentration ( p g / m 3 ) Element

Sample 1

Sample 2

Sample 3

Na

0.120 1.31 1.32 0.167 0.0073 0.132 0.0045 0.29 0.0084 0.0015 0.0018 0.0033 0.86

0.1 47 0.94 1.56 0.199 0.0050 0.1 19 0.0053 0.34 0.0087 0.0018 0.0021 0.0042 1.03

0.136 0.91 1.62 0.180 0.0043 0.135 0.0056 0.35 0.0085 0.0020 0.0019 0.0044 1.11

CI

Ca Ti Cr Zn As Br Zr

Sb I

Ce Pb

Average

Blank concentration, ng/crn2 This work

Hoffmana

Na

70 26,000 125 39 110 13 1,200 1 4 12 I 1 11 10.5 13 72

64 29,000 19 ... 4 11 618

90 27,000 300 70 2 I25 51 5

-. . .

1,000

CI

Ca Ti Cr Ni Zn

As Br Zr

Sb I

Ce Pb a

Table I V . Approximate Limits of Detectability by Several Methods for Some Elements in Aerosols from Urban Atmospheres Limit of detectability, ng/m3 Element

I PAAa

IN A A ~

Na

2 0.4 30 0.9 4.5 0.05 3 0.20 0.20 30 0.3 0.17 0.4 12

0.002 0.05 2.0 ... 4 x 10-5 0.25 4 x 10-5

CI

Ca Ti Cr

Ni Zn

As Zr Br

Sb I

Ce Pb

, . .

... 0.005 2 x 10-5 ... 2 x 10-5

...

Flame methodsC

0.002 ...

0.05 4 0.15 0.15 0.0025 5 100 ... 2.5

... ... 1

a Assumes a particulate sampie collected from 1000 m3 of urban air, 4-hr irradiation with bremsstrahlung from a 50-pA beam of 35-MeV electrons; y-ray spectra taken with a 55-cm3 Ge(Li) detector with samples 5 c m from detector; for short-lived activities, spectra taken for about one hour, for long-lived species, about ten hours. Same conditions as in a , except irradiation with neutrons at flux of 6 X n/cm2-sec. Very shortlived activities observed for about one half life as close to detector as possible with analyzer dead-time kept below 20%. Based on Zoller and Gordon ( 7 0 ) , modified for improvements in available instruments in 1973. Combination of various flame methods such as atomic absorption, atomic fluorescence, and flame emission. Assumes same particulate sample as in a and b, dissolved and diluted to 50 ml. with aliquots taken for analysis for each element. Based on Refs. ( 1 7 ) and (72). updated to 1973 capabilities.

of the air flow rate through the filter, are considered, the overall accuracy of the atmospheric concentrations of most elements is probably no better than about 10 to 15%. Sensitivity of Analyses for Various Elements. The limits of detectability for elements observable by IPAA are given in Table IV along with limits for INAA and various flame methods. In each case it is assumed that particulates are collected from 1000 m3 of air. One should use some care in interpreting the limits of detectability for complex samples such as atmospheric particulates, as the limit for a specific element depends strongly upon the amounts of interfering elements present. Limits for IPAA were computed by estimating how much the appropriate peaks in the spectra from the samples could be reduced while still allowing a reasonably accurate peak area to be determined. Thus, values given in column 2 of Table IV (11) H L Kahn Advan Chem Ser 73, 183 (1968) (12) J D Winefordner and T .J Vickers, Ana/ Chem 42, 206R (1970)

Dams et a/.b

Element

0.13 f 0.01 1.05 f 0.15 1.53 f 0.12 0.18 f 0.01 0.0055 f 0.001 1 0.13 i 0.01 0.0051 f 0.0004 0.33 f 0.02 0.0085 f 0.0001 0.0018 f 0.0002 0.0019 f 0.0001 0.0040 31 0.0004 1 .oo f 0.09

... 42

... ... 1

10.2

... ...

-

24

,..