cineration of 1 kg of trichlorobiphenyl (0&12H,C13 1.1).
T., Shimokawa, T., Shinozaki, Y., Bull. Chem. SOC.J p n . , 47 (a),
ratio:
Acknowledgment The authors are greatly indebted to N. Kase and K. Seki, Nihon Sanso Co., Tokyo, for their help in the construction of the incineration plant and to H. Matsui for his help with the computer calculations. Literature Cited (1) Nishiwaki, T., Ninomiya, J., Anda, K., Yamanaka, S., Nippon Kagaku Kaishi, 12,2225 (1972). (2) Swai, T., Shinozaki, Y., Chem. Lett., 1 (lo), 865 (1972); Sawai,
1889 (1974). (3) Kawamura, Y., Toba, Y., Oghisu, Y., Tanaka, Y., Kogai, 10 (2), 15 (1975). (4) Ackerman, D., Clausen, J., Johnson, R., Tobias, R., Zee, C., Adams, J., Harris, J., Levins, P., Stauffer, J., Thrun, K., Woodland, L., EPA Contract 68-01-2966, Facility Rep. No. 6 (PB-270 8971, 1977. (5) Morisaki, S., Komamiya, K., A n t e n Kogaku 15,8 (1976). (6) Akita, K., Ogiso, C., Kitagawa, T., ibid., 10,276 (1971). (7) Zelezink, F. J., Gordon, S., NASA T N D-1454, 1960. (8) Gaydon, A. G., Wolfhard, H. G., “Flames, Their Structure, Radiation, and Temperature”, p 289, Chapman and Hall, London, England, 1970. Receiued for reuiew October 17,1977. Accepted M a y 31, 1978.
NOTES
Imaging and Analysis of Airborne Dust for Silica Frank H. Chung Sherwin-Williams Research Center, Chicago, 111. 60628
Chronic exposure to high concentrations of silica would cause lung damage (silicosis). Therefore, the amount of airborne silica in factories is regulated and monitored by OSHA. Among the six techniques available for the analysis of crystalline silica in airborne dust collected on membrane filters ( I ), chemical separation (colorimetric) and x-ray diffraction are most widely used. The advantages of the x-ray method over the chemical .method are simplicity and rapidity. However, these advantages are lost in NIOSH P&CAM Method No. 109 (x-ray diffraction) where an internal standard and a silver membrane are utilized to increase precision ( 2 , 3 ) .An in situ x-ray technique is more attractive when a large number of samples are to be screened and particularly when many of the samples contain little or no free silica. A crucial factor causing the spread of results of an in situ x-ray method is the distribution of dust particles on the filter. However, the dust distribution is generally invisible and hence overlooked.
by the developer, forming a transparent thin film. The dust particles are insoluble and thus are imbedded into the PVC film leaving a sharp image. The thin film dries in about a minute, giving a permanent sample or standard. Other than optimum solubility and fast drying, the developer system is chosen so that the filter retains its shape and the dust particles are not disturbed in the process of image development. Some typical examples are shown in Figure 1. A study of these developed images indicates that the distribution of dust particles on filters tends to be uneven when the dust level in a factory fluctuates, or when a short sampling period is necessary due to high dust levels. Uneven distributions are also likely to be found in standards made by using a wet suspension/filtration procedure. Generally speaking, neither aerosol nor suspensoid collected on a filter would produce a statistically uniform distribution of particles over the filter. Improperly collected, overloaded, or locally compacted samples may cause larger errors in subsequent analysis. The imaging technique can reveal the presence of these problems and warrant precautions or extra care. Besides image development, another application of this technique is the fixation and preservation of standards or samples in their undisturbed condition for shipping, stock, rescanning, or reference. By the same principle, for filters made of mixed esters of cellulose such as MF-Millipore, the image of dust particles can be developed with straight ethyl acetate or acetone in a similar manner.
Imaging the Dust Particles Membrane filters used for airborne dust sampling are made of either polyvinyl chloride (PVC) or mixed esters of cellulose (nitrocellulose/cellulose acetate). PVC filters are recommended for free silica monitoring, while cellulose filters are routinely used for asbestos and heavy metals. A simple technique has been found to develop the image of dust particles collected on these membrane filters as described below. A developer is made of one part methylene chloride and five parts chloroform by volume. A few drops of this developer are spread on an aluminum disk about 45 mm in diameter (or square). Surface tension prevents the developer from spilling over the edge of the disk. The PVC filter is then carefully placed on the developer. The.filter is dissolved immediately
I n Situ X-ray Diffraction Analysis for Silica The quickest way to determine the quantity of free silica in airborne dust collected on a membrane filter is to measure the x-ray intensity of the characteristic silica peak from the filter as received without tedious sample preparation. The silica concentration is then read from a calibration curve previously prepared by use of external standards. Two conditions must be met to make this simplest scheme feasible: First, the matrix effect must be negligible; second, the sample composition must be uniform. In the case of a few milligrams of dust collected on a PVC filter, the matrix effect is indeed negligible ( 4 , 5 ) , but the dust particle distribution is far from uniform. Note that even though the dust distribution is uniform, the silica distribution might still not be uniform, espe-
A technique to develop the image of dust collected on membrane filters is reported. The developed images reveal that the dust particles from either aerosol or suspensoid are not uniformly distributed over the filter. This uneven distribution affects the precision of in situ x-ray diffraction analysis. To cope with this situation, a multiple-exposure x-ray diffraction method is developed and applied to free silica analysis. This method achieves substantial timesaving (15 min instead of 3 h/sample) and delivers reliable data.
1208 Environmental Science 8. Technology
0013-936X/78/0912-1208$01.00/0 @ 1978 American Chemical Society
E 100-
-
- .
u
c (
=
Y
.
(
Y
.
L
Y
50
50 -
-
0
40
1
2,3,3+
:: *+
h m m * m m m t . m t . t . m ~ m
Flgure 1. image of dust on PVC filters Except lor blank and three PAT samples. ail samples were taken from paint factories where various resins, pigments. and extenders were loaded into milis and processed. (1) AS received, result of improper sampling. (2) As received. dust laden, dust particles invi8ible. (3) After imaging. clean blank filter.(4) After imaging, PAT Sample S41-B, no free silica. (5)After imaging. PAT Sample 541-1, 0.105 mg SiO2Ifilter. (6)After imaging, PAT Sample S41-2, 0.077 mg SiOI/lilter. (7) After imaging. overloaded sample. (8)After imaging. even distribution of dust particles. (9) After imaging. uneven distribution of dust particles
cially when the type and level of dust in air fluctuate. Therefore, some sort of remedy is needed to obtain a meaningful average x-ray intensity, Naturally, spinning the sample during exposure to the x-ray beam would probably be the first choice (6). However, the x-ray intensity obtained from a spinning sample grants too much weight to dust particles deposited near the center of the filter as explained below. The ideal condition of spinning would he that the x-ray beam coincides with the full radius or diameter of the spinning filter (Figure Za) so that all dust particles are exposed to the primary x-rays. Even under this ideal condition, the exposure of the central area of O.lr is 19 times longer than that of the annulus of the outermost 0.1r (Figure 2b) because: n(O.lr)*/{ar*- a(0.9r)21= %9
Under any other conditions (Figure ZC) the difference in exposure is worse. A multiple-exposure approach (Figure 2 4 has been adopted in our laboratory to cope with this situation. The calibration curve of external standards was prepared
The four stars in Figure 3 indicate the location of results obtained in our laboratory. T o illustrate the scatter of x-ray intensities from multiple exposure of differentareas of the Same filter, the experimental data for the two samples which exhibited the best agreement are presented in Table I. Table 1. Scatter of X-ray Intensities and Silica Concentration PAT round robln
sample 541-2
Figure 2. Spinning-samplevs. multiple-exposure
Y-rav intensib.
cls (muitiple expbsures of the strongest reflection)
270 280 185
av intensity, cls Si02 found, mglfilter PAT arithmetic mean, mg/filter PAT geometric mean, mglfilter
231
isn
0.093 0.097 0.077
"0.
541-4
190 260 305
isn
226
0.092 0.097 0.064
Volume 12, Number 10, October 1978 1209
There were no outliers in this round of silica analysis according to the statistics used for the PAT program. Therefore, the arithmetic and geometric means were calculated from the results of all 62 laboratories submitting data. The wide scatter of x-ray intensities from the same membrane filter confirms the conclusion from image observation that the distribution of dust particles over the filter is not uniform. The excellent agreement of the amount of silica found demonstrates that the multiple-exposure approach is effective and realistic.
of other laboratories participating in the PAT program. This in situ x-ray diffraction method achieves substantial timesaving, delivers reliable data, and may be used to substitute the more demanding NIOSH method for fast screening analysis of silica in airborne dust.
Conclusions For a maximum of 5 mg total dust collected on a membrane filter (overloaded if higher than 5 mg), the effect of particle distribution overrides the matrix effect in x-ray diffraction analysis for silica by an external standard method. Through an image development technique, it has been shown that the dust distribution on the filter is not uniform. To cope with this nonuniform distribution and to preserve the simplicity of analysis, a multiple-exposure x-ray diffraction method for silica analysis has been developed for routine and PAT samples. The results obtained are in good agreement with those
Literature Cited
Acknowledgment The author acknowledges the review and discussion of Richard W. Scott. (1) Anderson, P. L., A m . Ind. Hyg. Assoc. J., 36 (lo), 767-78
(1975). (2) U.S. Dept. of Health, Education and Welfare, “NIOSH Manual of Analytical Methods”, GPO, 1974. (3) Chung, F. H., J . Appl. Crystallogr., 8,17-19 (1975). (4) Chung, F. H., Lentz, A. J., Scott, R. W., X - R a y Spectrom., 3, 172-5 (1974). ( 5 ) Chung, F. H., Adu. X - R a y Anal., 19,181-90 (1976). A m . Ind. Hyg. (6) Allen, G. C., Samimi, B., Ziskind, M., Weill, H., ASSOC. J., 35 ( l l ) , 711-7 (1974). Received for review August 2,1977. Accepted April 24,1978.
Correction In the article, “Composite Hazard Index for Assessing Limiting Exposures to Environmental Pollutants: Application Through a Case Study” [Enuiron. Sci. Technol., 12, 802-7 (1978)],by Elizabeth M. Rupp, Dennis C. Parzyck, Phillip J. Walsh*, Ray S.Booth, Richard J. Raridon, and Bradford I,. Whitfield, on page 804, column 2, the second line of the caption to Figure 2 should read Helena smelter (ng/m3), not (mg/m3) as printed.
1210
Environmental Science & Technology
