Test of X-Ray Fluorescence Spectrometry as a Method for Analysis of the Elemental Composition of Atmospheric Aerosols R. H. Hammerle and I?.H. Marsh Scientific Research Staff. Ford Motor Company. Dearborn. Mich. 48127
K. Rengan Department of Chemistry. €astern Michigan University. Ypsilanti. Mich. 48 797
R. D. Giauque and J. M. Jaklevic University of California. Lawrence Berkeley Laboratory. Berkeley. Calif. 94 720
Recent papers (1, 2) demonstrate that an analysis of the elemental composition of particulate matter can be very useful in determining the sources of pollution. The elemental analysis is usually done by neutron activation, which requires access to a nuclear reactor and which presently requires weeks to complete. A field instrument which can analyze the elemental composition in a matter of hours would greatly facilitate such studies. An X-ray fluorescence spectrometer using a semiconductor detector appears to be such an instrument (3, 4). To determine if X-ray fluorescence analysis with a semiconductor detector is sufficiently sensitive and accurate to analyze atmospheric particulate matter, a comparison was made of the results obtained by X-ray fluorescence and the results obtained from the same samples by neutron activation and, for Pb, by atomic absorption. For the comparison, ten elements (Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Br, and Pb) that are present in typical air pollution samples and that are most easily detected by the X-ray fluorescence spectrometer using a single excitation radiation (Mo K) were chosen. Neutron activation was used to determine all of these elements except Pb, which was determined by atomic absorption.
EXPERIMENTAL Samples. A number of samples were taken in Dearborn, Mich., during January and February, 1971. The length of time air was pumped through the filters, the volume of air passing through them, and the particulate mass collected on the filter are shown in Table I. Gelman type GA membrane filters with 0.2-k pores and 47-mm diameters were employed because of their excellent particle-trapping efficiency and because of their low mass (2.0 mg/cm2), which had the effect of minimizing the scattered X-ray background and reducing the absorption of the lower energy Xrays by the filter material. Since these filters have appreciable concentrations of some of the elements of interest, appropriate blank corrections were made. X-Ray Fluorescence Analysis. The samples were sent to Lawrence Berkeley Laboratory, where the X-ray fluorescence analysis was done using a low-background guard-ring detector with pulsed light feedback electronics. The exciting X-rays were produced by a molybdenum transmission tube ( 4 , 5 ) . The intensity of the primary radiation was monitored by measuring the current which struck the molybdenum anode during the analysis. K . R a h n , R.
Darns. J A Robbins. and J .
W.
Winchester. Atmos
Environ.. 5. 413 (1971).
P R . Harrison, K . A R a h n . R. Darns. J A Robbins. J. W Winchesfer. S. S. Brar, and D. M Nelson, J. Air Poiiut. Cont. A s s . , 21. 563
(1971)
Dittrich and C . R Cothern. J A i r Poiiuf Cont. A s s 21, 716 (1971). F S. Goulding and J . M Jaklevic. 'Trace Element Analysis by X-Ray Fluorescence." Lawrence Radiation Laboratory Report No. U C R L 20625.May. 1971 J M . Jaklevic and F . S. Goulding. Proc. IEEE. NS-19.384 (1972). T R.
The method of standardization was based on calculations using theoretical fluorescence yield and cross section data. A single element thin-film standard was employed to calibrate for X-ray geometry (6). Absorption of the X-rays by the air path and the detector window, and detector efficiency (both a function of X-ray energy), were also taken into account. Filter absorption effects were negligible for the elements of interest. A 2-cm2 portion from the center of each filter and one blank filter were analyzed for 1000 sec with the X-ray tube operated at 42 kV and 250 FA. The Ka X-rays were used for analysis, except for Pb, where the Lp X-rays were used. Neutron Activation A n a l y s k T h e neutron activation analysis was carried out using a procedure similar to that outlined by Dams et al. (7). A 30-cm3 Ge(Li) detector coupled to a Nuclear Data Model 2200, 4096-channel analyzer was used. For the first irradiation, standards (prepared by evaporating a known volume of a standard solution drop by drop uniformly over the surface of a membrane filter), blank filters, and the samples were heatsealed individually in very pure polyethylene bags. Each of the filters was irradiated for 5 min, employing a piece of Ti foil as a flux monitor, in the pneumatic tube system of the Ford Nuclear Reactor, Phoenix Memorial Laboratory, The University of Michigan. The flux was approximately 2 X 1012 neutrons/cm2/sec. After a 105-sec cooling period, the filters were counted in the polyethylene bags for 400 sec live time. Then the flux monitors were counted. For the second irradiation, the samples, standards, and blanks which had been removed from the polyethylene bags were individually heat-sealed in very pure polyethylene vials. Five of these vials were irradiated together for 2 hr in the reactor pool; the flux was approximately 1.5 X 1013neutrons/cm2/sec. Attached to the outside of each of the vials was a piece of Fe foil that was used as a flux monitor. After about a 24-hr cooling period, the filters, still in the vials, were counted for 2000 sec live time. The flux monitors were then counted. After approximately a 20-day cooling period, the samples, one standard and one blank filter, were counted for 24 hr clock time. The Cu determination was performed using the 511 keV y-rays. Since the major source of high-energy y-rays in the samples after 24 hr cooling was Na, a Na spectrum was obtained using the same procedure described above. Thus, the contribution of Na to the 511keV peak in the spectra could be subtracted. The Ni determination was done using the y-rays from 58C0, which was produced by the reaction 58Ni (n,p) 58C0. The remaining elements were determined using the X-rays and isotopes listed by Dams et al. (7).
RESULTS The elemental compositions of the collected particulate matter plus the filter as determined by the two methods are shown in Table 11. The contribution of the particulate matter can be determined by subtracting the contribution of the filter, which is given in Table 111. The agreement between the two methods, except for Br, is generally with(6) R D Giauque F S Goulding J M Jaklevic and R H Pehl Ana/ Chem 45,671 (1973) (7) R Darns J A Robbins K A Rahn a n d J W Winchester A n a / Chem 42,861 (1970)
A N A L Y T I C A L CHEMISTRY, VOL. 45, N O . 11, SEPTEMBER 1973
1939
Table I. Collection of !he Samples Sample
Time, hr
1 2 3
24 24 10
Air,
v01,O
m3
Particulate mass, mg
20 22 9
2.0 2.2 0.6
a Flow rate throuah the filter is 0.016 m3 min-’ cm-’.
Table II. Comparison of X-Ray Fluorescence and Neutron Activation Analysisa X-ray fluorescence
Element
Neutron activation
Sample 1
Ca
48
&I4 Not seen 1.2 0.3 3.44 f 0.08 55.7 f 4.3 0.40f 0.24 1.22 f 0.78 5.50f 0.18 2.56f 0.15 26.7 f 1.4b
45 f2 2.0 f 0.2 0.92 I 0.08 3.62f 0.14 52.5 f 0.3 0.47f 0.03 1.59f 0.06 5.50 f 0.20 5.04 f 0.20 28.5 I1.l
Ti Cr Mn
Fe Ni
cu Zn
Br Pb
*
Sample 2
Ca
54
45 f2 1.9 f 0.2 0.92I 0.08 3.53f 0.14 37.1 f 1.2 0.43 f 0.03 1.36f 0.05 5.40f 0.22 5.04f 0.20 29.6 I 1.2
Ti Cr Mn
Fe Ni
cu Zn
Br Pb
f15
Not seen Not analyzed 3.8 I 0.1 Not analyzed
Not analyzed Not analyzed Not analyzed 2.96 f 0.15 29.7 f 1.5b
in experimental errors. In the case of Br, the cause of the discrepancy is not understood. The errors listed are principally statistical errors. By testing a number of identical standards with the same concentrations of elements as samples 1 and 2, we found that the neutron activation analyses were reproducible to within the statistical errors (except for Cr and Cu, whose concentrations vary widely in the blanks or vials). The errors obtained by X-ray fluorescence analysis were smaller than those obtained by neutron activation analysis for most of the ten elements studied. This may be due in part to the fact that these elements were chosen because they are easy to determine by X-ray fluorescence analysis. However, in comparing the two methods for Mn, which is very sensitive by neutron activation, one finds the two methods are similar in accuracy. In addition to the direct comparison of the two methods shown here, the X-ray fluorescence analysis has been compared with the neutron activation analysis of particulate samples which were collected for only 1.5 hr ( I ) . In that work, a filter with a larger pore size was used, giving a flow rate 7.5 times greater than that used in this work. Thus, the 10-hr sample of this study has the same volume of air drawn through the filter per cm2 as the 1.5-hr samples. The compositions of the particulate matter in both cases were measured with about the same accuracy. In conclusion, the two methods of analysis generally agree t o within 20%. Considering the small quantities of the elements on the filters and considering that the accuracy can be improved if necessary by increasing the flow rate through the filter or by increasing the collection time, we feel that X-ray fluorescence analysis with a semiconductor detector is accurate and sensitive enough for the analysis of atmospheric particulate matter.
Sample 3
Ca
36
30 f1 0.5 f 0.1
