204
Anal. Chem. 1085, 57,204-208
Analytical Standards for the Analysis of Chrysotile Asbestos in Ambient Environments J o h n A. Small,* Eric
B. Steel, a n d Patrick J. S h e r i d a n
Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
Results of a program for the development of standard materials for the analysls of chrysotlle asbestos In nonworkplace envlronments are presented. These standards conslst of carbon-coated polycarbonate fllter sectlons whlch contaln predictable loadlngs of chrysotlle asbestos flbers mixed wlth an urban alr particulate matrlx so that they resemble partic;ulate-loaded fllter samples. One standard contalns approxlmately elght asbestos flbers per 0.01 mm2 of filter. Because of the low flber counts and large standard devlatlon In the average flber loadlng, thls standard does not have a certlfled value for asbestos fiber loading. Instead the results of the analysis are presented In an analysls report format. The second standard contalns a slightly higher loadlng of asbesto!s, approxlmately 30 fibers per 0.01 mm2 of fllter. Thls standard Includes a certlfled flber loadlng with the uncertalnty In thle loadlng expressed as a 95 % tolerance Interval about a flvocount mean.
The measurement of asbestos in the ambient environment is currently accomplished by filtering the air sample through a membrane filter and then analyzing the collected particles by transmission electron microscopy (TEM) (1). In 1977, a multilaboratory study of ambient air samples was conducted by the Environmental Protection Agency (EPA). This study involved the use of the transmission electron microscope and required no specific sample preparation or asbestos counting procedures. The results from the different laboratories on the analysis of split samples varied by several orders of magnitude for the asbestos fiber loading in the air (2). Since that time, the EPA has been developing a standard methodology for the analysis of asbestos in ambient environments. As part of this program, the National Bureau of Standards (NBS) and the EPA have been cooperating on the development of a series of Research Materials (RMs) and Standard Reference Materials (SRMs) to aid in the analysis of the asbestos-fiber loading in ambient samples. The final goal of the NBS program is to provide reference materials with a certifiable loading of chrysotile asbestos fibers on a substrate suitable for sample preparation and analysis by either TElM or analytical electron microscopy (AEM). Such a set of sarnples can be used by field laboratories to test, refine, arid calibrate their sample preparation and analysis procedures. These materials consist of sections from polycarbonate filters which contain chrysotile asbestos mixed with an urban-air particulate matrix. NBS Standard Reference Material 1648 (Urban Air Particulate) was chosen as the matrix material so that the asbestos standards would mimic samples which were collected from the environment. SRM 1648 was developed as a multielement standard and has not previously been analyzed for asbestos. It was chosen as the matrix material because it contains many types of anthropogenic and natural particles such as fly ash, pollen, and automobile exhaust which are common to urban environments (3). EXPERIMENTAL SECTION Research Material. The Research Material (RM) filter contained a low level of chrysotile and was designed to mimic an
ambient sample with a fiber loading comparable to a general urban background. It represents one of the most difficult samples to analyze since the fibers are small, generally less than 0.05 pm in length, low in number, and difficult to see. This filter contained only the chrysotile which was present in the SRM 1648 urban air particulate matrix. The RM filter was prepared so that the fiber loading was approximately 500 fibers per 1.0 mm2of filter surface. To prepare the RM fiiter, a concentrated solution of SRM 1648 was made by adding approximately 0.025 g of 1648 material to 100 mL of filtered, distilled water containing 50 mg of surfactant to aid in the separation of fiber bundles. This was mixed with an ultrasonic probe at 100 W for 10 min to break up particle aggregation which may have resulted from particle collection and/or storage. Eight milliliters of this concentrated solution was then added to 400 mL of water, mixed in an ultrasonic bath for an additional 5 min, and filtered onto a 154 mm diameter polycarbonate filter with a 0.4 pm pore size. The polycarbonate filter was backed with a cellulose acetate filter to minimize areas of inhomogeneous fiber deposition resulting from contact between the filter and the support disk in the filtration apparatus. Standard Reference Material. The SRM was designed to mimic an ambient sample collected near an asbestos source. It has a medium-loading density of chrysotile asbestos of approximately 3000 fibers per 1.0 mm2of filter surface. It contains SRM 1648 and a short fiber chrysotile obtained from the National Institutes of Environmental Health Sciences (4). To prepare the SRM filter, a concentrated asbestos solution was made by adding approximately 160 pg of the chrysotile to 300 mL of filtered, distilled water and 20 mL of ethanol (added as a wetting agent). The solution was mixed with an ultrasonic probe at 100 W for 15 min. A dilute solution was prepared by adding 8 mL of the concentrated solution to 400 mL of the filtered, distilled water. This intermediate solution was mixed with ultrasound for an additional 10 min. A final working solution was prepared by adding 25 mL of the dilute solution and 0.7 mL of the concentrated urban air particulate solution (see above) to 1500 mL of filtered water. The resulting solution was then mixed in an ultrasonic bath for an additional 5 min and filtered onto a 154 mm diameter, 0.4 pm pore size polycarbonate filter. After filtration, the RM and SRM filters were coated with approximately 30 nm of carbon by vapor deposition. The carbon rod was vaporized in short bursts to minimize any alterations in the polymeric structure of the polycarbonate filter as a result of heating. The filters were divided into 3 mm X 3 mm squares for distribution with each unit of the RM or SRM consisting of four 3 mm x 3 mm sections. The relative location of each square was recorded so that the spatial distribution of the asbestos fibers over the filter surface can be reconstructed if necessary. Selected filter squares from the RM and SRM filters were prepared for analysis by condensation washing (5). After sample preparation, the Urban Air Particulate and the asbestos fibers were contained in a carbon film approximately 30 nm thick which was supported by a 200 mesh, indexed copper TEM grid. To test the EPA-recommended preparation procedure, select filter sections were also prepared by a modified Jaffe-washer method (6). No difference in asbestos loading was noticed between the sections prepared by condensation washing and the Jaffe-washer procedure. Counting of the asbestos fibers was done on the individual openings in the 200 mesh grid which measure approximately 100 pm x 100 pm in size. One usable grid opening was selected at random from each filter section prepared for counting. The openings were traversed at a magnification of 20000X using a back-and-forth pattern such that the entire grid opening was counted for asbestos.
This article not subject to US. Copyright. Published 1984 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
To accurately characterize the fiber loadings on the filters, the counting was replicated at two levels: First multiple operators counted each grid opening and second a large number of grid openings were counted from the SRM filter. The counting and identification of the asbestos fibers were done with a “verified counting” procedure developed at NBS for this project (7). For each fiber observed during the “verified count”, the operator recorded the following parameters which were used for identification and location of the fibers: (1)the x,y coordinates, which indicated the relative location of the fiber within the grid square; (2)whether a fiber image displayed the characteristic “hollow tube“ morphology of chrysotile; (3) whether the electron diffraction pattern of the fiber matched the electron diffraction pattern for chrysotile asbestos; (4) whether any fibers overlapped a grid bar or were obscured by other particles (Fibers which overlapped grid bars were counted as fibers.); ( 5 ) fiber length and width were recorded in millimeters at a magnification of 20000X;(6)fibers were identified as individual fibrils, fiber bundles or mats (Individual fibrils were single chrysotile crystals with a length-towidth ratio of 3 or more. Fiber bundles consisted of units of two or more individual fibrils. Mats were identified as either individual fibrils or bundles which had length to width ratios less than three. For closely associated fibrils, fibers, and fiber bundles, counting rules following those proposed by the EPA were used (8)J; ( 7 ) the fiber, bundle, or mat was identified as chrysotile, possible chrysotile, amphibole, possible amphibole, or other. Following the analysis of a grid opening by several operators, the “verified counting” procedure was used to minimize discrepancies among counters. This procedure uses the relative locations of the fibers recorded by each operator to construct a map which shows the position of each fiber for each operator. The fiber count on the grid opening is then verified by having one or more of the operators repeat the analysis on any fiber over which there was disagreement. This includes fibers that were found by only one operator and fibers for which identificationwas disputed because of ambiguous morphology or diffraction data. Because of this detailed fiber verification, the “verified count” represents the best estimate of the actual number of asbestos fibers on a given grid opening. Since the size and shape of the grid openings varied, it was necessary to standardize the ”verified” asbestos counts for each grid opening to verified counts per unit area of filter. This was done by photographing the analyzed grid openings at 200X magnification with a light microscope. The photographic negatives of the openings were then projected onto graph paper with 1 mm squares. The number of squares contained within the borders of each grid opening was determined. From the calibrated magnification of the photomicrographs and the image projection the actual areas of the grid openings were determined. The final areas were expressed as 0.01mm2and the values used for the SRlM certificate are given as counts per 0.01 mm2 area of filter. The fiber loading is expressed per 0.01 mm2of filter surface because this is approximately the area of the filter which was analyzed for each count during the certification process. To check the accuracy of the area measurements, two different analysts did separate determinations of the same set of 11 grid openings. The relative differences between the two seta of measurements for all 11 openings were considerably less than the variations observed for the fiber loadings from grid opening to grid opening, RESULTS AND DISCUSSION T o characterize the fiber loading across the filter, the “verified” fiber count on a given grid opening was taken as the true fiber count without error, although there is a small probability that all counters could have missed one or more fibers (9). In preliminary work with 47 mm diameter filters, i t was found that at medium loading densities (20-80 fibers/O.Ol mm2of filter surface area), the chrysotile fibers could be deposited homogeneously and that the distribution of counts was consistent with Poisson statistics. Such filters were impractical to use for the standards, however, because of the low yield of RM and SRM units per filter (less than 10). As a result, 154 mm diameter filters were used for the RM and SRM. This size filter yielded approximately 150 units per filter.
205
Table I. Counting Results for RM Filter
section
1
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23
5 7 5 1 6 2 4 2 8 8 6 6 7 8 16 3 7 5 11 5 8 6 2
operators 2 3
filter area analyzed in (0.01 mm2) units 4 verified counts
5 6 4
2 6 2 4 3 9 8 10 5 9 7 16 4 5 4 7
7 4 3
5 5 4 6 16 4 8 3 9
6 5 3 5
5 7 5 2 26 2 4 3 10 8 11 6 9 8 16 5 8 5 12 6 10 7 5
0.861 0.885 0.909 0.956 0.933 0.969 0.812 0.915 0.903 0.880 0.963 0.812 0.880 0.921 0.885 0.799 0.947 0.760 0.880 0.885 0.897 0.897 0.812
Research Material. Twenty-three grid squares distributed evenly over the surface of the RM filter were prepared and counted for asbestos fiber loading (Table I). The verified counts ranged from a low of 2.0 fibers to a high of 16.0 fibers. The average loading was 7.9 fibers per 0.01 mm2 of filter surface with a single measurement standard deviation of 3.7 fibers. Because of the low fiber loading on the filter, and the large standard deviation, it was impractical to specify the ambient-level filter as an SRM with a certified value for the fiber loading. Assuming the fiber distribution could have been described by Poisson statistics (a best-case example), the number of filter squares used in the certification of this material as an SRM would have consumed most of the filter, leaving only a few units available for distribution (IO). As a result, the cost per unit would have been prohibitive. As an alternative, this filter was released as a research material with a report of analysis (11). Standard Reference Material. A rapid survey method was developed for assaying the homogeneity of the fiber loading on the candidate SRM filters. This method was based on doping the final chrysotile solution with submicrometerdiameter iron oxide particles. After filtration, the intensity of the Fe K a X-radiation was determined by electron probe microanalysis for each of the 3 mm X 3 mm filter sections. The assumption in this method was that the iron oxide particles deposit on the filter in a manner statistically and spatially similar to the asbestos fibers. This assumption was tested by counting both iron and asbestos on filter sections taken from a test filter which was deliberately prepared to be inhomogeneous. The results of the asbestos counts indicated that the fiber loading across the test filter ranged from 12 fibers per grid opening on one side of the filter to 66 fibers per grid opening on the other side. By comparing the plot of iron counts vs. filter position to the plot of asbestos counts vs. filter position, we were able to determine that the iron counts could be used as a gross-screening method for filter homogeneity. However, the uncertainties in this screening method were large enough to mask any low level gradients in the asbestos fiber loading across a filter. As a result, this screening method was only useful for rejecting candidate filters which had severe inhomogeneities in fiber loading similar to
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
208
Table 11. Counting Data for SRM Filter
section
1
1
21 21 21
2
3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18
19 20 21
22 23 24 25 26 27 28
29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
24 30 28
33 35 27 23 19 38 36 21 28
36 42 25 22
27 27 29 21
23 27 31 31 21 31
2 21
23 23 28 27 31 34 33 26 24 26 38 39 17 29 37 39 29 25 27 26 34 23 23 29 34 32 21
25
32 25
22
21
17 17
16 16
21
22
23 30 56 38 41 43 41 40 22 33
27 29 53 38 42 44 40 40 22
35
counter 3 4 5 20 20 21 29 26 28 33 33 29 29 23 37 30 34
19 23 24 25 27 26 32 34 21
24 19 37 20 28 31 42 25
33 19
26
verified
count 21.0 26.0 22.5 29.0 29.5 31.5 34.5 33.5 27.0 23.0 25.0 37.5 38.5 21.0 30.0 36.5 41.0 30.0 26.0 29.0 28.0 32.0 24.0 24.5 31.5 34.0 33.0 22.0 33.0 27.0 21.0 17.0 16.0 21.5 28.0 30.0 56.0 38.5 42.0 44.5 41.5 40.5 22.0 35.0
filter area analyzed in (0.01 mm2) units 0.9031 0.8837 0.9250 0.9237 0.9181 0.8787 0.8512 0.9250 0.8394 0.9056 0.9375 0.9831 0.7844 0.9212 0.8962 0.8862. 0.8662 0.7544 0.8838 0.8969 0.8631 0.9106 0.9062 0.8962 0.9144 1.0475 0.9375 0.8272
22
27 25 38
29
those observed on the test filter. It was still necessary to use the time-consuming process of fiber counting to characterize the homogeneity of the final SRM filter. Once the final SRM filter was selected, the chrysotile asbestos fiber loading was determined with the “verified counting” procedure. Grid openings were prepared from 44 filter sections which were taken from suitably randomized locations on the SRM filter surface (Figure 1). Table I1 lists the individual operator and verified counts as well as the filter area analyzed, expressed as 0.01 mm2, for the 44 sections. As an added measure of quality assurance for the SRM, asbestos counters from two outside laboratories (U.S. Department of Interior Bureau of Mines, and U.S.Steel Gorp.) visited our laboratory and took part in the analysis of the SRM filter. The rationale for obtaining counts from individuals outside NBS was to determine if there was a systematic bias in the results from all NBS counters as a result of the NBS counting methodology. The fiber counts from the outside operators were within 15% of the NBS verified counts which is similar to the variations observed for NBS counters (9). As a check on the long-term stability and possible contamination of the prepared filter sections, duplicate counts were done on four filter sections during a time period covering a few months to 1.5 years after the preparation of the SRM
30
22 28
Figure 1. Relative locations for the 44 grid openings counted on the SRM filter. IO
8 2 6
0.8881
0.8912 0.8650 0.9483 0.8457 0.8873 0.8989 0.9167 1.0562 0.7487 0.9288 0.8993 0.8581 0.7687 0.9269 0.9894
28
c
Q)
2
= 14
LL
2
0 100 200 300 400 500 600 Chrysotile Asbestos Counts/O 01 mm2 Figure 2. Histogram of the asbestos counts determined for the 44 grid openings counted on the SRM filter.
Table 111. Data on Recounts of Selected Filter Preparations verified count operator original fiber count 1
2 5
36 (8182) 29 (8/82) 23 (6182) 33.5 (6/82)
recount
for grid opening
36 (12182) 28 (11/82) 22 (11/83) 35 (11/83)
39 30 25 34
filter. As shown in Table 111, the duplicate counts are well within the spread of counts originally recorded for NBS counters indicating that there was no long-term alteration or contamination of the prepared filter sections. Figure 2 shows a histogram of counts obtained on the 44 areas analyzed over the filter surface. Discounting the high count class centered at 240, the distribution appears to be fairly symmetrical. However, if one looks at a map of the coded deviations of the individual counts from the mean count for the sample, Figure 3, it is clear that there is in fact a gradient in the counts from the top of the filter to the bottom. The map indicates that the counts at the top of the filter tend to be low with respect to the mean while the counts at the bottom are high. The gradient is also evident from the nonzero slope of the least-squares straight-line fit to asbestos counts vs. location on the filter, Figure 4. In this case, the slope was
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
207
-
t
a
0 0
t
t
-
15
+ t
+
T
t
-
5~1,,1(,1,1,,,",,,,'1111111111 0 5 IO 15 20 25 0 0
15
10
5
FILTER
X
20
25
35
30
COORDINATE
Flgure 3. Map showing the deviations of the verlfied counts from the mean count for the SRM filter. Counts which are less than the mean are recorded as minus signs and those above the mean are recorded as plus signs.
30
FILTER X COORDINATE
Flgure 6. Map showing the deviations of the verified counts from the mean for the bottom half of the SRM filter.
Table IV. Certified Values for the Fiber Loading on the SRM filter
batch
loading fibers/O.Ol mm2
95/95 tolerance interval
1876 1876a
30 37
f9 f13
std dev 10.lb
6.7a
"Determined from 21 expermental counts. *Determined from 23 experimental counts.
20-i l
o
l
5
,
I
IO
,
I 15
,
,
,
20
I
,
I
25
, 35
30
FILTER Y COORDINATE
Flgure 4. Stralght line flt to plot of asbestos counts vs. filter Y coordinate. 35
t
+
+
30-
+ + t
a 25W
c
+
-+
+ I 0
5
IO 15 2 0 25 30 FILTER X COORDINATE
35
Figure 5. Map showing the deviations of the verified counts from the mean for the top half of the SRM filter.
-0.497 with a 95% confidence interval of f0.288. The presence of this gradient in the fiber loading precludes the use of Poisson statistics to describe the fiber distribution on the entire filter. To minimize the effects of the gradient, the filter was divided into two p& along a left-right diameter with the filter orientation the same as that shown in Figure 1. This division is justified by the following: (1)It divides the filter in such a way that there are approximately 20 measurements on each half of the filter for characterizing the distribution of counts on each half. (2) The direction of the cut intersects the observed gradient thereby minimizing the range of the bias expected for each half. The maps of the coded deviations from the means are shown in Figures 5 and 6 for the top and bottom halves of the filter, respectively. Statistical analysis of the counting data for each
filter half suggests that the distributions of fibers on the two halves are not incompatible with an assumption of normality (12). As a result, it was decided to calculate normal-based tolerance intervals for the uncertainty statements on the SRM certificates instead of using other intervals such as broader nonparametric intervals. The division of the filter into more than one section requires that it be released as two separate SRMs each with its own statistical characterization. It is important to note that the gradient which has been characterized across the total SRM filter is a low-level gradient. The mean fiber loading per unit area for the high-count half is only 20% higher than the mean for the low-count half which compares favorably with the f50% variations observed in round robin studies (9, 13). A gradient of this magnitude would probably not be detectable by standard field counting methods. It was also decided that the users of the SRM would be required to determine the asbestos fiber loading on five separate areas of approximately 0.01 mm2 each and average the five values. This was considered to be necessary because there is sufficient variability in the "true" counts that a tolerance interval based on a single count would be too broad to be of any effective use. Since there is still the possibility of a nondetectable residual gradient, an additional constraint is placed on the users: The five areas selected for counting must come from areas which are distributed over the entire halffilter rather than one local area. As a result, the users of the SRMs will be required to prepare three of the four filter sections for analysis and count two areas on two of the filter sections and one area on the remaining filter section. For the convenience of the users, TEM grids are included in each SRM unit. If these grids are used, each area will equal one grid opening. If grids having different mesh sizes are used, then the appropriate number of grid openings equaling approximately 0.01 mm2 must be used. In either case, the actual size of the grid openings must be determined in order to calculate fiber loadings. Table IV lists the 95% tolerance intervals for the two filter halves expressed as fibers per 0.01 mm2 of filter surface. Tolerance intervals were chosen as a robust way of making credible statements about uncertainties in the mean value of the asbestos counts that the users are likely to obtain in their
208
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
their primary measurement method (17,18). The asbestos standards are available from the Office of Standard Reference Materials, National Bureau of Standards, Gaithersburg, MD. The reference number for the Research Material is 8410 and the numbers for the Standard Reference Materials are 1876 and 1876A.
100
so
>.
V
60
z w
3 0
40. LL
20
0. 0
2
4
FIBER
S
6
LENGTH IN
IO
ACKMO WLEDGMENT The authors wish to thank the following people for their help in the analysis of the RM and SRM filters: Barbara Thorne, Patricia Johnson, Robert Myklebust, and Sara Mathews of NBS, A1 Zermay of the United States Steel Corp., and Kim B. Shedd of the Bureau of Mines. We also wish to thank Stefan D. Leigh and Keith Eberhardt of NBS for their help with the statistical analysis of the data and Michael E. Beard for his help as EPA project officer. Registry No. Chrysotile, 12001-29-5.
/IM
LITERATURE CITED
Figure 7. Histogram of fiber length for all fibers counted during certification of the SRM filter.
attempts to characterize pieces of the SRM filter (14,15).A 95% tolerance interval for a five measurement mean indicates that 95% of the population of such five-count means will fall within the tolerance interval with a 95% confidence. Actual computation of the tolerance intervals was based on the factors given in Table A-6 of NBS Handbook 91 (see ref 16), with factors suitably modified to reflect the fact that the desired tolerance intervals are for means of specified numbers of counts as opposed to tolerance intervals for groups of raw counts (16). In a simple simulated test of the reported fivemeasurement mean tolerance intervals run by computing all possible five-measurementmeans derivable from the available counts, all such computed means fell within the prescribed limits. In addition to the tolerance intervals, the certificates for the SRMs include the single-measurement standard deviation for the each of the halves since this value is based directly on experimental measurements. Finally, Figure 7 is a histogram of the fiber lengths for the fibers counted for the SRM certification. This plot shows that the majority of the fiber lengths are below 1pm. In addition, fiber widths are generally less than 0.1 pm, As a result of the small fiber dimensions, both the SRM and the RM are designed for use with methods based on analytical or transmission electron microscopy. They cannot be used to calibrate methods which use light or scanning electron microscopy as
(1) Chatfleld, E. J. Scanning Electron Microsc. 1979, 1 , 576-577. (2) Montgomery County Asbestos Study, EPA Internal Report; U.S. €PA: Research Triangle Park, NC, 1977. (3) NBS Certiflcate of Analysis SRM 1648; Office of Standard Reference Materials, NBS: Washlngton DC, 1978.
(4) Campbell, W. J.; Hugglns. C. W.; Wylie, A. G. Rep. Inwesf.-US., Bur. Mines 1979* RI-8452. (5) Chatfield,’E. J.; Glass, R. W.; Dlllon, M. J. €PA Rep. 1978, €PA-6001 4-78-011, 32-35. (6) Samudra, A. V.; Harwood, C. F.; Stockman, J. D. €PA Rep. 1977, €PA-60012-77-778, 9. 17) . . Steel, E. 6.; Small. J. A.: Sheridan, P. J. NBS S m c . Pubi. 1980, No. 6 1 9 , 162-168. ( 8 ) Samudra, A. V.: Harwood, C. F.; Stockman, J. D. €PA Rep. 1977. EPA -60012-77- 176 1 I- 15. Steel, E. 6.; Small, J. A. Anal. Chem., following paper In thls Issue. Leigh, S. D.; Steel, E. B.; Small, J. A,; Sheridan, P. J.; Filllben, J. J. NBS Spec. Publ. 1980, No. 6 1 9 , 169-182. NBS Spec. Pub/. ( U . S . ) 1981, No. 2 6 0 , 1-2. Fllllben, J. J. ACS Symp. Ser. 1977, No. 6 3 , 30-113. , Chopra. K. S.; Beaman, D.; Cook, P. NBS Spec. Pub/. 1980, No. 6 1 9 , 121-131. (14) Welssberg, A.: Beatty. G. H. Technomefrlcs 1960, 2 , 483-500. (15) Natrella, M. G. “NBS Handbook 91”; 1963; Chapter 2, Section 5. (16) Eberhardt, K. R. Natlonal Bureau of Standards, personal communication, Jan 1982. (17) Chatfield, E. J. Royal Commlsslon on Matters of Health and Safety Arlslng from the Use of Asbestos In Ontarlo, Study No. 10, 1983, pp I
.
13-19. (18) Small, J.; Newbury, D.; Myklebust, R. Microbeam Anal. 1983, 148-1 50.
Received for review April 19, 1984. Resubmitted June 13, 1984. Accepted September 12,1984. The work presented in this paper was supported in part by the Environmental Protection Agency Grant AD-13-F-2-535-0.
