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
