1153
Anal. Chern. 1980, 52, 1153-1154
origin can be found in texts on statistics and is not new. I was unable to consult the two references cited by him, but found the derivation in Bennett and Franklin (6). I do apologize for this oversight; I have always considered inadequate survey of the literature to be inexcusable. T o the best of my knowledge, my originality consisted of the application of b = Sxiyi/Zxi2
(3) "Statistical Analysis in Chemistry and the Chemical Industry", Carl A. Bennett and Norman L. Franklin, John Wiley 8 Sons, New York, 1954, p 28. (4) L. Endrenyi, Department of Pharmacology, Universty of Toronto, Toronto M5S 1AB. Canada, private communication. (5) K. Buijs and M. J. Maurice, Anal. Chim. Acta, 47, 469-74 (1969). (6) Ref. 3, p 232.
Frederick C, Strong I11 Faculdade de Engenharia de Alimentos e Agricola Universidade Estadual de Campinas Caixa Postal No. 1170 13100 Campinas, S.P., Brasil
for the first time to a series of spectrophotometric measurements.
LITERATURE CITED (1) Frederick C. Strong 111, Anal. Chem., 51, 298-9 (1979). (2) "Introduction to Statistical Methods", D. L. Harnett, Addison-Wesley Publishing Co., Reading, Mass., 1970.
RECEIVED for review October 1, 1979. Accepted February 4, 1980.
Exchange of Comments on Neutron Activation Analyses for Simultaneous Determination of Trace Elements in Ambient Air Collected on Glass-Fiber Filters Sir: Lambert and Wilshire ( 1 ) are to be commended for attempting the difficult task of determining trace element concentrations in ambient air using what appear to be careful experimental techniques and methods of data reduction. However, the reliability and significance of trace element analysis of air samples collected on glass-fiber filters are questionable. For example, only 3 elements out of the 26 reported by Lambert and Wilshire were above their discrimination limit in 80% or more of the measurements. Dams et al. ( 2 ) have stated that glass-filters must not be used for nondestructive neutron activation analysis (NAA) because they contain high concentrations of trace metals. Our work on cotton dusts supports the contention of Dams et al. We sampled air in a cotton processing area on vinyl chloride filter media. The sample and filter were irradiated in the nuclear reactor at North Carolina State University (the same facility employed by Lambert and Wilshire). We were concerned with elements of both short and long half-lives. As expected, chlorine dominated the background, which showed considerable variation. Similar to the results of Lambert and Wilshire. our data were obscured by the background. There was no consistent pattern even for elements of long half-lives which were expected to be present in high quantities, and we were unable to differentiate between actual differences in dusts and sampling errors. Similar background problems were encountered using X-ray fluorescence (XRF) spectrometry when filter media containing high trace element concentrations were employed. However. there is little problem with back-
ground interference using cellulose acetate filter media ( 3 ) . Teflon filter media may be a better choice as it I S less sensitive to water when gravimetric measurements are desired. LVhile Lambert and Wilshire have developed a procedure that is adequate for a few elements with long half-lives and adaptable to the EPA requirement ihat fiber-glass filters be employed, we conclude that the NAA method is unreliable for samples collected on filters with a high trace element content. We suggest that, in general, dusts on glass-fiber filters cannot be properly analyzed by NAA or by any other purely instrumental multielement analytiral method (e.g., XRF).
Sir, Fornes and Gilbert have apparently had an unfortunate experience. However. it is not clear how that can lead reliably to their rather sweeping nonquantitative generalization. First, Fornes and Gilbert seem to deplore the use of glass-fiber filters. Yet nowhere do they mention their experiences with such filters, ambient particulate or the application of a discrimination limit. We did not intend to recommend glass-fiber filters and would relish a low background filter as would any analyst. However, all legitimate aspects of any situation need consideration and tradeoffs which produce less than ideal conditions are possible. We encountered a non-ideal situation and had, we believe. some success. It was necessary to move
beyond refusal to do the analyses or making unshaded assertions of unsuitability. The alternative was the implementation of a practical procedure which yielded a quantitative estimate of the smallest quantity of any element in the sample which could he distinguished from the filter contribution, i.e., the discrimination limit. This quantity could be determined before sampling is begun and the approach is applicable to other filter media and analytical techniques. Next, Fornes and Gilbert question ihe reliability of our data. In a n article by Walling et al. ( I ) , arsenic data acquired by us using this technique on ambient samples from glass-fiber filters was compared to those from the same fxlters using an acid extraction and flameless atomic absorption spectrometry.
This article not subject to U.S. Copyright
LITERATURE CITED (1) Lambert, J. P. E.: Wilshire. F W. Anal. Chem. 1979. 57. 1346-1350. (2) Dams. R.; Robbins, J. A . ; Rahn. K . A , ; Winchester, J. W. Anal. Chem., 1970, 4 2 , 861 (3) Fornes, R. E.;Gilbert, R. D.: Hersh, S. P.; Dzubay, T. G. Textile Res. J . , in press.
R. D. Gilbert
K. E. Fornes* School of Textiles North Carolina State University Box 5006 Raleigh, North Carolina 27650 RECEIVED for review September 16. 1979. Accepted March 20. 1980.
Published 1980 by the American Chemical Society
