Anal. Chem. 1980, 52. 2218-2219
2218
Table I. Parameters for Mass Comparison by Least-Squares Linear Regression dependent variablea XRF-S IC-SO,zISE-NH;
intercept slope
(PP) 2
-22 -8
f
(9)
k
(24)
* (32)
0.96
i (0.02)b 1.04 t (0.07) 0.99 f (0.05)
a Independent variable is gravimetric mass. in parentheses show 95% confidence interval.
R2 0.999 0.993 0.995
Figures
absorbing properties, also has 6 times the thermal conductivity of air. Recently, we prepared a series of Teflon filters loaded with aerosols of (NH4)2S04.The mass of material was determined by direct weighing. In addition, S was determined by XRF, S042-by IC, and NH4+by ion selective electrode (ISE). Each of these methods was independently calibrated. Each of these individual analyses was then used to calculate the mass based on the compound stoichiometry. Comparisons of the masses based on analysis with those based on direct weighing are shown in Figure 1 along with a linear regression by leastsquares line. The regression coefficients are presented in Table I. These data indicate that excellent agreement exists among the various analytical methods and that no signficant sample loss has occurred due to X-ray fluorescence analysis. This empirical evidence certainly meets the requirements suggested by Hansen et al. for demonstrating no sulfur loss due to sample heating. In fact, our laboratory measurements of S by XRF and Sod2ions by IC in sulfate containing ambient aerosols are generally in excellent agreement. A good example is presented in a recent paper by Stevens et al. ( 4 ) . This paper is included in the references of Hansen et al.; unfortunately they do not mention these data. We cannot adduce unequivocal empirical evidence that significant loss of sulfur due to radiation-induced chemical reactions does not occur. However, such losses during operation of our apparatus seem very unlikely since only one sulfur atom in 1010is ionized by the X-ray beam during analysis. The secondary processes alluded to by Hansen et al. would require extraordinary efficiencies to result in significant loss of S in
Sir: Hansen et al. in their paper ( I ) include a "Note added in Proof' that asserts apparent losses of sulfur from ambient aerosol samples can be induced by the X-ray beam used in X-ray fluorescence (XRF) analysis. In support of this assertion, they quote a data set from a recent publication of ours (2). We feel that use of our data by Hansen et al. to support their contention is questionable for the following reasons. During the airborne measurement described in our study, the very high flow rates of air resulted in a nonuniform deposition of particulate matter onto the filters. In order to correct for this, we carried out laboratory tests in which particulate-graphite carbon was drawn through our sample system on to test filters at the same flow rate used in the field studies. The carbon particles had a size distribution similar to that of the airborne particulates sampled in the field. The distribution of carbon on the filters was determined photometrically by measuring the amounts of light absorbed by the filters as a function of the radial distance from the centers of the filters. The deposition of carbon on the filters was found
the relatively low intensity radiation environment of our apparatus. Routine comparisons of XRF sulfur analysis and IC sulfate analysis on the same ambient aerosol samples are in excellent agreement. The isolated cases where the agreement has been poor are cases when (1) the sample deposit was nonuniform rendering the XRF analysis meaningless and/or (2) the sulfate extraction procedure was demonstrated to be inefficient. These causes have been adequate to account for discrepancies without invoking any radiation-induced chemical reactions. The data and interpretive discussion presented here are specifically relevant only to the operation of the XRF analysis system used in our laboratory. However, other systems which use low power densities are likely to have similar results. The suggestion of Hansen et al. that responsible analyses include documentation of the maintenance of sample integrity during high-energy irradiation is one we fully support. Our data show clearly that our photon excited XRF method is not subject to the same errors in sulfur determinations as the particle induced methods.
LITERATURE CITED Hansen, L. D.; Ryder, J. F.; Mangelson, N. F.; Hill, M. W.; Faucett, K. J.; Eatough, D. J. Anal. Chem. 1980,52,821-824. Hegg, D. A.; Hobbs, P. V. Atmos. fnviron. 1980, 7 4 , 99. Shaw, R. W., Jr.; Willis, R. D. I n "Electron Microscopy and X-ray Application to Environmental and Occupational HeaRh Analysis"; Russell, P. A., Hutchings, A. E., Eds.; Ann Arbor Science Publishers: Ann Arbor, MI, 1978; pp 51-64. (4) Stevens. R. K.: Dzubay, T. G.; Russwwm, G.; Rickel, D. Atmos. Environ. 1978, 12, 5 5 .
Robert W. Shaw* Robert K. Stevens Environmental Sciences Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, North Carolina 27711 William J. Courtney Northrop Services, Inc. Research Triangle Park, North Carolina 27711 RECEIVED for review May 30, 1980. Accepted August 6,1980.
to be nonuniform. Since the X-ray beam employed in the XRF analysis was relatively narrow compared to the diameters of the filters, we employed a correction factor in interpreting the XRF results which was derived from the measured distribution function of the carbon on the test filters. The uncertainty in the correction factor, which is included in the variances in XRF concentrations shown in Table VI1 of ref 2, was derived from analysis of the deposition of carbon on the various test filters. However, the uncertainty derived in this way does not include any systematic errors that might have existed due to differences between the distribution function measured in the laboratory and the actual distribution function on the filters used in the field. While the correction factor we derived is probably reasonably accurate, we believe that its use precludes the employment of our data for the type of precise comparison between XRF and ion-exchange chromatography (IEC) results discussed by Hansen et al. Indeed, we feel that any statement beyond the qualitative one that we made in our paper (namely,
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Anal. Chem. 1980, 52, 2219
that there was “reasonable” agreement between the XRF and IEC results) is unwarranted. LITERATURE C I T E D (1) Hansen, L. D.; Ryder, J. F.;Mangelson, N. F.; Hill, M. W.; Faucett, K. J.; Eatough. D. J Anal. Chem. 1980, 52, 821-824. (2) Hegg, D. A.; Hobbs, P. V. Atmos. Environ. 1980, 14, 99-116.
Dean A. Hegg P e t e r V. Hobbs*
Sir: Shaw et al. have convinced us that the XRF data for sulfur in Hegg and Hobbs article (I) do have much larger uncertainties than those given in the article. We thus agree that these data cannot be used to show that sulfur is lost during X-ray irradiation. Measurements of total sulfur by PIXE and of sulfate ion by ion chromatography (IC) done in our laboratory are also generally in agreement; i.e., see Table I1 in our paper. However, as documented in Table I of our paper, there is serious disagreement on some samples, including coal fly ash. The purpose of our study was to determine the reasons for the disagreement between the methods in those cases where disagreement was found. A discussion of those cases in which the methods are in agreement is not germane. Likewise, proving that the XRF method gives accurate results for pure ammonium sulfate does not prove that accurate results will be obtained for the compounds of sulfate in coal fly ash. (Which most likely are not ammonium sulfate.) We suggested that a series of standards including at least ammonium sulfate, sulfuric acid, and ferrous sulfate need to be run to evaluate an X-ray method. We do not know if even these standards will detect all of the potential errors. The results reported by Shaw et al. on ammonium sulfate do establish an upper limit for the temperatures reached b y their samples in the X-ray beam. We have recently analyzed some samples collected from the plume of a large coal-fired power plant in the western U.S. These samples were collected by aircraft and on Ghia Teflon filters in a manner very similar to the samples described in the article by Hegg and Hobbs except that the particulate deposition was uniform. The filters were divided in half, half analyzed by IC after exposure to an X-ray beam (for 5 min in a Philips Model PW1410 X-ray spectrometer with a Cr tube operated a t 40 kV and 50 mA) and half analyzed by IC with no exposure to the beam. The results, summarized in Table I, indicate a consistent increase in F- and either no change or a decrease in sulfate ion after exposure to the X-ray beam. Deviations of the Na+ and NH4+ratios from 1may be a result of changes in the extractability of Na+ caused by irradiation or may be due to experimental errors in dividing and analyzing the filter. We propose that the increase in F- arises from decomposition of the Teflon filter and the decrease in sulfate ion from radiation-induced chemistry similar to that observed
0003-2700/80/0352-2219$01 .OO/O
2219
Cloud and Aerosol Research Group Atmospheric Sciences Department University of Washington Seattle, Washington 98195
RECEIVED for review June 16.1980. Accepted August 6,1980. Contribution No. 547, Atmospheric Sciences Department, University of Washington, Seattle.
Table I. Comparison of IC Determinations of Water-Extractable Anions and Cations from Ghia Teflon Filters with Particulates from a Coal-Fired Power Plant Plume, before and after Exposure to X-rays dist
from
stack, km
5 80 5 80
ratio of IC after to bePore XRF exposure Fso:Na’ NH,‘
1.73
0.53 0.7 8
1.24 1.37 1.15
4.04
0.56
1.51
1.72 2.09
1.02
1.18 0.92 0.95 1.25
for fly ash samples in a PIXE beam. It should be emphasized that the comparison given here is an analysis of XRF exposed samples vs. unexposed samples and not sequential analysis. It would appear that a detailed evaluation should be made of possible losses of S during XRF analysis. Such losses will clearly be a function of the chemical state of the S in the sample being analyzed. Shaw et al. point out that only one sulfur atom in 10” is affected by the X-ray irradiation in their system. This number (1/10lo) refers only to the primary excitation event while it is the much more probable, low-energy valence shell events, i.e., charge-transfer events induced by ejected electrons and secondary photons, which probably lead to the formation of volatile sulfur species. T o reiterate one of the conclusions in our paper: the results of species specific analysis of samples which have been exposed to high-energy radiation will always be subject to question. LITERATURE C I T E D (1) Hegg, D. A,; Hobbs, P. V. Atmos. Environ. 1980, 74, 99-116.
L. D. Hansen* N. F. Mangelson D. J. Eatough Department of Chemistry and Thermochemical Institute Brigham Young University Provo, Utah 84602 RECEIVED for review June 30, 1980. .Accepted August 7,1980.
0 1980 American Chemical Society
