1842
Anal. Chem. 1988, 60, 1842-1845
Improved Methodology for the Determination of the Seven Elemental Tracer Long-Distance Pollution Signatures Using Thermal and Epithermal Neutron Activation Analysis S. Landsberger Department of Nuclear Engineering, University of Illinois at Urbana-Champaign, 214 Nuclear Engineering Laboratory, 103 South Goodwin Avenue, Urbana, Illinois 61801
Improved methodology In instrumental neutron activation analysls (NAA) was developed to better determine the seven-element source signature for regional air pollution. The analysis of antimony, arsenic, manganese, vanadium, and zinc was accomplished by employing thermal NAA. Special conslderatlon was given to the potential spectral interference of a secondary arsenic photopeak to the main antlmony photopeak. Selenlum was determined by uslng short-lived neutron actlvation anaiysls, whlch proved to be slgnlflcantly more reilable than the usual long-lived y-ray peaks. The analysis of lndlum was achieved by using epithermal NAA, which enhanced the preclslon and lowered the detection limit conslderably. High-efflclency detectors were employed for the short-lived analysis to achieve better statlstlcs In reasonable amounts of tkne whlk high-resolution detectors were used for the medium- and long-lived analysis to resolve any of the overlapping peaks. Reproduclbiilty tests on Mind dupikates, accuracy experiments using NBS standard reference material, overall precislon analysis, and detection llmit calculations showed the effectiveness of the new proposed scheme for these seven crucial elements in reglonal alr pollution source signatures.
Since the introduction of the elemental trace signature by Rahn and his group (1-6) there has been a great deal of interest in applying this and similar techniques to identify regional sources of air pollution in urban, rural, and remote regions (7,8). On the other hand there have been some strong exchanges between Rahn and others (9-12) on the validity of his assumptions, statistical calculations, and conclusions, but not on his analytical methods. Essentially Fiahn has proposed that seven elements, namely antimony arsenic, indium, manganese, selenium, vanadium, and zinc, can characterize regional sources of air pollution. For instance, Rahn concluded that Mn/V ratios in the Arctic aerosol are significantly different if the source is North American or Eurasian ( I ) . Another example shown is that In/Se ratios in southern Ontario, Canada, are significantly higher than ratios found in areas in the United States or Europe. Indium is a strong source signature from the smelting activities in northern Ontario. In all these investigations instrumental neutron activation analysis (NAA) was the method used to determine these seven elements. At present an intensive program to characterize both Arctic daily aerosols in Alert Bay, northern Canada (13, 14), and snowfall in the rural and remote Scottish highlands (15,16) has been under way for several years. One of our goals has been to investigate the feasibility of Rahn's seven-element signature source for our work. I t has become very apparent that due to low abundance of these elements in rural and remote regions, special care in sampling, blanks, and analytical procedures was of the utmost importance. In view that elemental ratios were the basis for signature source identification,
analytical precision and accuracy was crucial. In particular, special considerations for interferences have to be taken into account when low elemental concentrations are being determined. In fact, Rahn's papers gave little or no detailed analytical procedure to determine these elements. Also some of his results had analytical errors of 40% or higher. This was especially true for indium. The intention of this paper is to point out how these seven elements can be determined by using various improved analytical schemes in thermal and epithermal neutron activation analysis.
EXPERIMENTAL SECTION Sampling. Twelve Arctic aerosol samples used in this present study were collected on Whatman 41 filter paper on a daily basis between March 8 and 18, 1985, in Alert Bay. Five blank filters and three replicates used as blind analysis reproducibility tests were also sampled. Aerosol particulates were collected with a high-volume sampler from an air volume of approximately 2300 m3. Sample filters were cut into four equal sections, and only one section was used for NAA. The others were to be used for ion chromatography and to be archived. Irradiating and Counting. The neutron activation analyses were performed at the McMaster Nuclear Reactor in Hamilton, Canada, utilizing neutrons produced in a 2-MW swimming pool type of reactor. For short-lived nuclides ( T i e , 56Mn,and 52V) thermal neutron fluxes of 4.5 X 10l2n cm-2s-l were used. The analysis of 115mInspecifically employed epithermal neutrons having a flux of -2.5 X 10" n cm-2 s-l. For the medium-lived nuclides 76Asand "%b and the long-lived nuclides @Znand 75Se,a thermal flux of -7.5 X 10l2n cm-2 s-l was used. All samples and liquid atomic absorption solution standards were irradiated in large 7-cm3 polyethylene vials and transferred to new inert vials to minimize contamination. Since the volume of filter samples and liquid standards were identifical and counted in the same position, no corrections for differences in geometry were required. The short-lived selenium, an isotope having a half-life of 17.4 s, made it impossible for any manipulations. The counting of selenium employed a 12% efficient APTEC detector having a resolution of 2.1 keV at the 6oCo1332-keV peak. This was coupled to a computer-controlledautomated rabbit system capable of receiving the samples within 5 s after irradiation. The accuracy of the timing system is &0.5 s, thus contributing to the overall error to less than 1% for a 60-s irradiation. Filter samples and selenium standards were irradiated and counted all under identical conditions. An irradiation time of 60 s, decay time of 5 s, and counting time of 30 s (almost two half-lives of 77mSe)was employed. Concentrations for liquid standards were prepared so that they closely match the dead times for samples. A simple arithmetic dead time correction from live time to true time was done for each individual sample. This technique has been proven to be very reliable in previous 77mSedeterminations (17). All samples for short-lived and long-lived irradiations were done sequentially and together, respectively. Manganese (E7= 1810.7 keV) and vanadium were determined by using a 5-min irradiation time, a 5-min delay time, and 10-min counting time. To augment the precision of manganese, a different photopeak (El= 846.7 keV) was employed with a delay time of 60 min and counting time of 30 min. A high-efficiency 28% Aptec detector possessing a resolution of 1.9 keV at the 6oCo1332-keV
0003-2700/88/0360-1842$01.50/0Q 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60,
NO. 18, SEPTEMBER 15, 1988
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Table I. Irradiation, Decay and Counting Conditions, and Typical Detection Limits type of neutron element
isotope
antimony arsenic indium
76As
lZ2Sb 116mIn
half-life 2.7 days 26.2 h 54.1 min
flux
thermal thermal thermal thermal epithermal
manganese
S6Mn
2.58 h
thermal
selenium
77mSe 75Se
17.4 s 118.5 days
thermal
52v
3.76 min 244.1 days
thermal thermal
vanadium zinc
65Zn
tc
ti
td
10 h 10 h 5 min 5 min 5 min 5 min 10 min 10 min 5 min 5 min 60 s 10 h 10 h 5 min 10 h
3-4 days 3-4 days 5 min 5 min 45 min 45 min 20 min 20 min 5 min l h 5s 3 weeks 3 weeks 5 min
3 weeks
y-ray, keV
typical detection limit,” ng
564.0 559.5 416.9 1097.3 416.9 1097.3 416.9 1097.3 1810.7 846.7 161.7 136.0 264.6 1434.4 1115.4
9 20 1 0.9 0.9 0.8 0.3 0.2 80 70 18 36 25 10 120
3h 3h 10 min 10 min
30 min 30 min l h l h 10 min 0.5 h 30 s
3h 3h 10 min 3h
ODetection limit based on Currie’s criterion (ref 18). peak was used. The analysis of indium using both thermal and epithermal activation analysis was attempted. Thermal NAA gave poor precision primarily due to the high continuum arising from the 24Naand 37Clisotopes. An epithermal irradiation time of 10 min followed by a delay time of 20 min and counting time of 45-60 min improved sensitivities considerably. Arsenic and antimony determinations were carried out by using a 10-h irradiation followed by a 3-4-day delay time and 3-h counting time. Zinc and long-lived 75Sewere subsequently analyzed by using a 3-week delay time and a 3-h counting time. In these determinations for the medium- and long-lived nuclides, a 12% efficient ORTEC detector having a 1.7-keV resolution was used. This was essential to completely resolve the 554.3-, 559.5, and 564.0-keV y-rays belonging to 82Br,16As,and 122Sb,respectively, and the 1115.5- and 1120.5-keV photopeaks characteristic of &Zn and 9%.All spectra were collected with a Canberra Series 90 multichannel analyzer having both pile-up rejection and dead-time correction units.
RESULTS AND DISCUSSION For the elements in question in this study the resultant radioactive isotope with its half-life and main y-ray used in the analysis is presented in Table I. The type of neutrons as well as irradiation, decay, and counting conditions and typical detection limits (given in nanograms) are shown in Table I as well to better understand the scheme for the activation procedures. The detection limits were calculated according to Currie’s criterion (18). Finally the results for three blind replicate samples and the average and standard deviation for five blank values for the Whatman 41 filter paper are given in Table 11. Calculated Errors. The errors on a given peak resulting from calculating the expression d ( N A UV,) where NA is the number of counts in the photopeak and NB is the average background on either side of the photopeak. The number of channels for peak and background were identical. All prepared standard solutions were determined in triplicate and used for calibration. A weighted standard deviation for the calibrating solution was then combined in quadrature with the statistical error of the photopeak to derive a final error. The contribution of the error of the prepared solutions never contributed more than 0.5%-2.0% to the overall error. A discussion of the salient features in the determination of each element now follows. Antimony. Good resolution of the detector system is imperative to resolve the peaks at 554.3, 559.5 and 564.0 keV belonging to %r, I6As,and ‘%b. The precision of the analysis is between 6% and 10% with typical detection limits of 20 ng. There is however a small but significant spectral interference arising from the 563.5-keV y-ray peak belonging to
+
Table 11. Results for Blind Duplicates for Three Pairs of Samplesn element
antimony arsenic
indium manganese
selenium vanadium zinc a
set 1 concn, ng
concn, ng
set 2
set 3 concn, ng
57 f 3 61 f 3 218 f 10 227 f 10 0.62 f 0.07 0.65 f 0.07 720 f 40 690 i 40 50 f 7 57 f I 182 f 9 185 f 9 14600 f 400 12700 f 400
23 f 2 20 f 2 106 f 7 97 f 7 0.46 k 0.05 0.39 k 0.05 341 f 20 316 f 20 37 f 5 35 f 5 91 & 5 87 f 5 1990 f 80 1610 f 80
26 f 3 28 i 3 141 f 14 131 f 14 0.59 f 0.09 0.62 f 0.09 323 f 20 317 f 20 54 f 6 41 f 6 134 f 7 134 f 7 1950 f 90 1860 f 80
blank, ng
