Environ. Sci. Techno/. 1083, 17, 566-567
(9) Pike, M. C.; Gordon, R. J.; Henderson, B. E.; Menck, H. R.; So0 Hoo, J. In “Persons at High Risk of Cancer”; Fraumeni, J. F., Jr., Ed.; Academic Press: New York, 1975. (10) Mantel,N.; Haenszel,W. J. Nutl. Cancer Inst. (US.) 1959, 22,719-748. Llncoln Pollssar,’ Homer Warner, Jr. Fred Hutchinson Cancer Research Center Seattle, Washington 98 104
Comment on “Synthesis and Analysis of Crystalline Silica” S I R We wish to take exception to the conclusionsand implications of the paper “Synthesis and Analysis of Crystalline Silica” ( I ) . The paper implies that the overall precision of silica analyses by X-ray diffraction (XRD) in the National Institute for Occupational Safety and Health (NIOSH) Proficiency and Analytical Testing (PAT) program should be compared to the precision attained in the author’s laboratory. Since the PAT variability is much higher, the paper concludes that the NIOSH method of analysis stands in drastic need of improvement. First, overall variability is composed of both inter- and intralaboratory variability. Moreover, the interlaboratory variability almost always exceeds intralaboratory variability. In a collaborative test (15 participating laboratories) of two analytical methods for silica, the NIOSH XRD and the Bureau of Mines (BoM) IR methods had intralaboratory relative standard deviations of 7-8%, which is nearly identical with that given by Chung for his laboratory (8%). Overall precision was another matter, ranging from 17% for the simpler samples to 30% for samples of more complex matrix. These figures are better than the PAT results quoted in the paper, probably because the collaborating laboratories conscientiously followed the methods as written, something the PAT laboratories did not necessarily do. Second, Chung is comparing the precision of a bulk powder analysis done in his laboratory to filter sample analyses done elsewhere. Filter samples are analyzed in the PAT program, and they are the focus of all the NIOSH and BoM silica methods. Filter samples may contain very small quantities of respirable silica which lie near the lower limit of detection and result in reduced precision. Last, while most methods, and certainly the current silica methods, could stand to be improved,the implication that Chung’s method would give superior results is open to question. The matrix-flushing method presented in the paper for bulk powder analyses is a very good approach to these samples. As just mentioned, however, filter samples are really the analysis problem at hand. Chung’s multiple exposure method for filter samples is relatively untried, may require modification of the sample holder on some diffractometers, and allows no correction for heavily absorbing samples. In short, Chung’s method is not a likely candidate for replacing the more general and more thoroughly tested NIOSH method and is not likely to result in improved overall precision. A good point made by Chung in this paper is the need for standard materials. Presently, a quartz standard is under development at the National Bureau of Standards, but until this is available the Min-U-Si1 5 product that Chung mentions is widely used, is available, and is of appropriate particle size. A set of 18 analytical reference 566
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minerals developed for NIOSH by IIT Research Institute includes quartz, cristobalite, and tridymite of high purity. These are available upon request. The cristobalite contains some particles larger than 10 pm and should be sieved. In conclusion we emphasize that the efforts to improve silica analyses have been continuous. The development of a quartz standard reference material and the distribution of analytical reference minerals as mentioned above are examples of these efforts. The PAT program also continues; a paper to be presented at the 1983 American Industrial Hygiene Conference will show that interlaboratory precision for PAT silica analyses has improved over the last few years. Another recent effort has been the collaborative test already mentioned, the planning for which began in 1977. The work was cosponsored by NIOSH and BoM and performed by SRI International. A report of the work, which was done in three phases and ultimately involved 30 laboratories, will be available in mid-1983. Registry No. SiOz, 7631-86-9.
Literature Cited (1) Chung, F. H. Environ. Sci. Technol. 1982, 16, 796. Martin Abell* NIOSH Cincinnati. Ohio 45226
Ciarine Anderson
SRI International Menlo Park, California 94025
SIR: Before getting to the specific points commented on by Abell and Anderson, I’d like to make a point that the data of the round robins and the dispute over St. Helens’ ashes were cited in my paper (I) to demonstrate a fact: inadequate interlaboratory precision of silica analysis for either bulk powder or filter samples. My intention was to share our experiences in dealing with silica analysis. (1)The soundness of the NIOSH/XRD method was never questioned. However, its round robin data are disappointing. In my opinion, the NIOSH method is valid, but its sample preparation procedure is demanding, which hurts its precision and defeats the simplicity feature of the XRD technique. The lack of certified filter standards deteriorates its precision further. (2) For bulk powder analysis, the precision of the XRD technique was found to be 8% or better. For filter samples, a precision of 7-8% was obtained in their 15-laboratory collaborative test of the NIOSH/XRD and the BOM/IR methods. It is quite clear that the precisions of bulk powder analysis and filter sample analysis are comparable indeed. I believe that by using certified reference standards distributed by a single source, the overall precision of silica analysis could be greatly improved regardless of which XRD method was chosen or what sample type was involved. (3) Consensus has been obtained on the merjt of the matrix-flushing method for bulk powder analysis which is half of the essence of my paper. The other half is synthesis of standards. As for filter samples, I have never claimed that the multiple-exposuremethod (2) is superior.
0013-936X/83/0917-0566$01.50/0
0 1983 American Chemical Society
