Anal. Chem. 1992, 64, 320-325
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Quantitation of Asbestos in Synthetic Mixtures Using Instrumental Neutron Activation Analysis Pravin P. Parekh,*JRichard J. Janulis, James S. Webber, and Thomas M. Semkow' Wadsworth Center for Laboratories and Research. New York State Department of Health, P.O. Box 509, Albany, New York i2201-0509 A method has been developed for the quantltatlve determlnatlon of asbestos and other major components In bulk Samples. This method uses concentratbns of M a t u r e elements as lndlces of the contents of the respectlve components. I t Is based on analyzing one marker element for each component and subsequently solvlng a set of llnear equatlons. This approach was applied to Mnary and ternary component sys tems of asbestos-gypsum and asbestos-gypsum-flberglass prepared as proticlency test samples that represent reaCworM samples. Either chrysotlle or amoslte was used In these synthetk mMures; Co and Fe, respectlvely, were the optlmal markers for these two asbestos types. Markers for gypsum and fiberglass were Ca and Na, respectively. All these elements were measured wlth high preclslon udng Instrumental neutron actlvatlon analyds. Two addltlonal markers, Sc and Fe for chrysotlle and Sc and Co for amoslte were used to verlfy our results. Homogeneous dlstrlbutlon of asbestos flbers In the bulk proflclency test samples was ascertalned from the analyds of the three marker elements, Sc, Fe, and Co, In several repllcates randomly sampled from our bulk preparatkns.- R In the analysis of these elements In the repllcates was f2-4% (la) and sample homogeneity was found to be about 93% at the 95% confldence level. Concentratlonsof chrysotlle and amoslte as well as the other two components calculated from the marker element concentratlons In the pmfkhcy test samples agreed wen (wlthln f5-15%) wlth the formulated values at both 3% and 10% asbestos flber contents.
INTRODUCTION Asbestos is a well-documented health hazard. Exposure to airborne asbestos fibers can lead to diseases such as asbestosis, lung cancer, and mesothelioma (I). It is important therefore that samples of bulk materials suspected to contain asbestos be analyzed for these fibers. The most commonly employed technique is polarized-light microscopy (PLM), although X-ray diffraction, electron microscopy, and infrared spectroscopy are sometimes used (I). While PLM is well suited to asbestos identification, it typically yields poor accuracy and precision in quantitation because of the trivial percentage (usually less than 2%) of the collected bulk sample that can be examined, P L M s subjective nature, and the problems associated with disparate particle size distribution and resolution. Several studies have been carried out on the elemental analysis of pure asbestos (2-8). However, no work has been reported to quantitate the asbestos content of an environmental sample using elemental concentration(& The present study utilizes elemental analysis to determine the asbestos content of synthetic mixtures prepared in our laboratory as proficiency test (PT) samples. The PT samples were formulated with asbestos, gypsum, and fiberglass to represent Additional address: Department of Environmental Health and Toxicology, School of Public Health, State University of New York at Albany, Albany, NY 12201-0509.
real-world samples found on the interior surfaces of building materials. Our technique is based on the principle that the amounts of asbestos as well as other major components in a sample can be estimated by determining concentrations of elements characteristic of these components. This elemental signature technique has found extensive application for quantitation in atmospheric (9-13)and cosmochemical (14-17)studies. A distinct advantage of this technique is the high precision and accuracy of results that can be obtained for individual bulk samples. Our department's Environmental Laboratory Approval Program synthesizes large (200-400 g) bulk PT samples with preweighed Components so that the overall percentage of asbestos is known (18). While it was assumed that components in the PT samples were spatially homogeneous, there were analytical impediments to verify this uniformity. With the exception of time-consuming and laborious transmission electron microscopy, PLM is the only analytical tool that can positively identify all six types of asbestos. Although polarized-light microscopy is not well suited for quantitative analysis, the EPA requires that the asbestos content in bulk materials be quantitated by that technique to determine whether concentrations are at or above the critical 1%composition (19). Quantitation is performed by either a visualestimation method or by point counting. While point counting is more accurate and precise than visual estimation (20,21), realistic evaluations of point counting usually ascribe detection limits of about 3% asbestos (221,the level at which precision is about k50% relative standard deviation (RSD). Thus it is difficult to use PLM for validation of homogeneity of PT samples when asbestos concentrationsare in the 1-10% range. The present study used the precisely measured concentrations of the marker elements to (a) determine the homogeneity of these PT samples and (b) quantitate the amount of each component within the sample. Elemental Analysis of Asbestos Fibers in the PT Samples. Wet HF-HC1 acid digestion has been used for atomic adsorption spectrophotometric (AAS) analysis of elements in asbestos (2-4).However, a drawback of this method was the observation of colloidal suspension in the aqueous solution (4,5).Such a suspension leads to poor precision in the measurements. For example, using the acid-digestion and the AAS method, the precisions obtained on the analysis of Cr, Mn, Fe, Co, and Ni in the Union International Centre le Centre (UICC) standard reference asbestos samples were highly variable, &3-31% (3). Another shortcoming of the acid-digestion/AAS method anticipated in the present study was the introduction of large amounts of insoluble CaFz and easily ionizable Na in the resultant solution (from gypsum and fiberglass used in our PT samples). These in turn would have produced marked matrix effects in subsequent analyses either by AAS or by inductively-coupled plasma emission spectroscopy and would degrade precision and accuracy. Nondestructive methods are better suited for the present study. Of the two nondestructivemethods, X-ray fluorescence analysis and instrumental neutron activation analysis (INAA), X-ray fluorescence has an overall uncertainty of >15% as
0003-2700/92/0364-0320$03.00/0 0 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992
Table I. Pertinent Nuclear Data for the INAA of Sc, Fe, and Co
nuclide asc
half-life 83.8 d
activation cross section, barn 23
69Fe
44.5 d
1.2
T o
5.3 y
37
24Na
15.0 h
0.53
activity produced,’ dPm
y-energy, keV (% intensity)
lo6 1.1 X lo6 0.4 X lo6
889 (100)
1.2 X
1121 (100)
1099 (56) 1292 (44) 1173 (100) 1333 (100) 1368 (100) 2754 (100) 1297 (82) 159 (73)
'ICs 4.5 d 0.25 (47S~b) 3.3 d Activity at the end of irradiation of 5.6 pg of Sc, 23 500 pg of Fe, and 39 pg of Co (mean concentrations present in 1 g of chrysotile; see Table 111) under the conditions used in this study (thermal neutron flux of 8 X 1OI2 n cm-2s-l and irradiation time of 4 h). b 4 7 Sin~transient eauilibrium with “Ca. ~
~
~~
observed by Upreti et al. (6) for pure asbeatos samples. Hence it is not the method of choice in view of the high precision and accuracy required. On the other hand, INAA has yielded high-quality analytical data in the elemental analysis of asbestos (7). We therefore decided to employ INAA in our study. INAA of Marker Elements. A prerequisite for a marker element is that the ratios of ita concentration in the component under study (asbestosin this case) to its concentrations in the remaining components (gypsum and fiberglass)should be high. The higher the ratio, the more suitable is the element as a marker. The major elements of the six types of asbestos, chrysotile, actinolite, amosite, anthophyllite, crocidolite, and tremolite, are Si, Mg, Fe, Na, and relatively small amounts of Al; however, tremolite and actinolite are also enriched in Ca (1). Of these, Si, Na, and Ca are certainly not good marker elements for asbestos in the present study because they are also the major elements of fiberglass (Si, Na) and gypsum (Ca). The other three major and minor elements, Mg, Al, and Mn, have short-lived (n,y) product nuclides, thus requiring a pneumatic carrier facility for quick sample delivery and an on-line counting system at the reactor site. In the absence of these facilities, we had to resort to long-lived radionuclides. Thus Fe is the only suitable major element for our purposes. The two lithophilic elements, Sc and Co, are present in the silicate matrix of asbestos, albeit at ppm levels (7,8, 23). From the standpoint of INAA of Fe, Sc, and Co in any given analysis, Sc and Co are determinable with precision and accuracy at ppm levels comparable to precision and accuracy for Fe at percentage levels. This is because, in a typical thermal neutron irradiation of these three elements for, e.g. 1day, the reaction yields are approximately 7000 times higher for the %(n,y)% and 600times higher for the SgCo(n,y)GOCo reactions than for the 58Fe(n,y)59Fereaction. Added advantages in selecting these three elements are their convenient long half-lives and the fact that each of them has two prominent y-lines for independent analysis. Table I gives the pertinent nuclear data for the INAA of Sc, Fe, and Co. Notice that the induced activities of %c, 59Fe,and 6oCoin 1 g of chrysotile under the experimental conditions employed in this study are similar. For other constituents of PT samples, fiberglass and gypsum, Na and Ca, respectively, were used as markers. These were determined using the 23Na(n,y)24Naand 46Ca(n,y)47Ca reactions. Their nuclear data are also included in Table I. EXPERIMENTAL SECTION Preparation of PT Samples. The PT samples consisted of
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binary mixtures of asbestos and gypsum or ternary mixtures of asbestos, gypsum, and fiberglass; the latter two were used as diluents for reasons given earlier. Two types of asbeatos, chrysotile and amosite, were used. Chrysotile was selected because it accounts for more than 90% of asbestos applications in the United States (1).Additionally, its extreme flexibility and positive surface charge increased the potential for forming clumps and thereby generating inhomogeneous subsamples. Amosite was chosen because of its widespread use in steam-pipe and boiler insulation and frequent occurrencein bulk samples. Homogeneity problems for amosite were expected to be less because of its rigidity and negative surface charge. Chrysotile and amosite were obtained from samples used in the National Toxicology Program carcinogenesissutdies (24,25). Chrysotile was obtained from the Jeffrey Mine, Quebec, Canada, and amosite from a mine in Penge, Transvaal, Republic of South Africa. Preweighed components of asbestos, plaster of Paris (commercialgrade obtained from USG Industries, Chicago, E), and fiberglass (commercial grade “mineral wool“ obtained from Wrap-on Co., Inc., Chicago, IL),were mixed with fdtered deionized water in a commercial blender, poured into a pan, and allowed to dry. In this way, about 200-400 g of sample were synthesized. This large sample was then divided into small sections (approximately 1-2 g) for distribution to regulated laboratories as PT samples. It should be noted here that although one of the original diluents was plaster of Paris, C&04-0.5H20,wet mixing converted it to gypsum, CaS04-2H20;hence the use of the word “gypsum* for our PT samples. Two batches of PT samples were prepared at two different times from different lob of chrysotile, amosite, plaster of Paris, and fiberglass. Henceforth these will be indicated as 1 and 2 suffixed to the relevant component(s). To test the homogeneity of our PT sample preparations, three portions approximately 2 g each (representing about 1% of the total) were removed from randomly selected spots of the PT sample-containing pan. Henceforth these subsamples will be referred to as A, B, and C. Six aliquots of 50 mg each of A, B, and C were analyzed for the concentrationsof marker elements in them. While these aliquots served to test the homogeneity within the randomly selected portion of a PT sample, the analysis of A, B, and C determined an overall variation of asbestos fibers in the pan. Preparation of Sample and Standard for Irradiation. Materials handled in this study were the six types of asbestos, gypsum, fiberglass, cellulose binder (Whatman ashless tablet pulverized to a fine powder),and a multielement standard basalt BCR-1 (suppliedby US. Geological Survey). Although chrysotile and amosite were the only two types of asbestos used in our PT samples, the remaining four types of asbestos, actinolite, anthophyllite, crocidolite, and tremolite, were also included to determine the feasibility of extending this study to these types. All materials, except BCR-1, were pulverized in a liquid nitrogen mill (SPEX 6700 freezer-mill) for approximately 4 min. The basalt BCR-1 was used as a multielement comparator with reference concentrations of 2.42% Na, 4.95% Ca, 33 ppm Sc, 17 ppm Cr, 9.38% Fe, 38 ppm Co, and 26 ppm La (26, 27). These values, independently confirmed by us using single-element standards, were used in the evaluation of elemental concentrations in the PT and other samples. About 0.5 g of chrysotile,amosite, gypsum, fiberglass, the PT samples, and BCR-1 were weighed to 0.1 mg and then mixed and homogenized in a Vortex Genie mixer with about 1 g of the cellulose binder, also weighed to 0.1 mg. Six replicates of chrysotile, PT samples, cellulose binder, and BCR-1, triplicates of gypsum, fiberglass, and amosite, and duplicates of actinolite,anthophyllite, crocidolite,and tremolite were prepared as follows. About 0.15 g of the homogenized mixture was weighed to within 0.1 mg and then pelletized at about 15000 psi using a SPEX 13-mm evacuable pellet die and a manual hydraulic press. The pellets were then sealed in high-purity polyethylene bags. In contrast to loose powder, a pelletized sample is more convenient to handle and provides a highly reproducible geometry for y-ray counting. Sample Irradiation. The sealed samples, standards, and blanks were stacked inside a clean polyethylene vial and then placed in a polyethylene irradiation rabbit. The standards and the blanks were placed at the top, middle, and bottom of the sample stack with equal numbers of samples sandwiched between
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992
Table 11. Homogeneity Test of the Proficiency Test Samples concentrationb
A. CHI
+ GY1
Fe Sc Fe Co
A. CHI
+ GY1+ FG1
2
0.60 0.24 3.93 0.66 0.26 4.04
0.62 0.26 4.07 0.68 0.27 4.33
0.55 0.23 3.51 0.54 0.23 3.37 0.53 0.21 3.25
0.56 0.23 3.51 0.60 0.25 3.80 0.57 0.24 3.67
3
4
5
6
mean (flu)
% RSD
0.64 0.25 4.36 0.67 0.28 4.19
0.67 0.27 3.93 0.67 0.27 4.40
0.63 f 0.02 0.26 f 0.01 4.08 f 0.16 0.67 f 0.01 0.27 f 0.01 4.23 f 0.12
3.2 3.8 3.9 1.5 3.7 2.8
0.58 0.23 3.56 0.57 0.24 3.70 0.56 0.24 3.58
0.56 0.23 3.49 0.55 0.23 3.48 0.57 0.24 3.58
0.56 f 0.01 0.23 0.004 3.51 f 0.06 0.56 f 0.02 0.24 f 0.01 3.54 f 0.19 0.55 i 0.02 0.23 f 0.01 3.47 i 0.18
1.8 1.7 1.7 3.6 4.2 5.4 3.6 4.3 5.2
Binary Mixture" Sc Co
B. C H l + G Y 1
1
0.64 0.26 4.09 0.65 0.27 4.22
0.62 0.25 4.11 0.66 0.26 4.20
Ternary Mixture" Sc Fe Co
B. CH1 + GY1 + FG1
Sc Fe
C. CH1 + GY1 + FG1
Sc Fe
Co Co
0.55 0.23 3.41 0.57 0.23 3.57 0.53 0.22 3.47
0.58 0.24 3.58 0.54 0.23 3.31 0.53 0.23 3.25
*
"A, B, and C are randomly selected samples from a given PT sample lot. Suffii 1 refers to a lot (see text). Constituent abbreviations: GY for gypsum, FG for fiberglass, and CH for chrysotile. Sc and Co in ppm while Fe in %. 1 through 6 are the six splits of samples A, B, and C.
two standards and blanks. This arrangement enabled us to correct for neutron flux gradient (if any) across the entire 7-cm stack length. Irradiation was carried out at the Research Reactor of the Massachussetts Institute of Technology, Cambridge, MA, at a thermal neutron flux of 8 X 1OI2 n cm-2 s-l for 4 h. y-Ray Spectrometric Measurements. The irradiated samples, standards, and blanks were mounted on cardboard sample holders such that the counting geometries were highly reproducible. The samples were counted by means of a 12% efficient Ge (Li) detedor (resolution: 1.9 keV fwhm for the 1333keV y-line of Boco) coupled to a 409E-channel pulseheight analyzer (Canberra Series 40). The y-ray spectza were stored on a computer interfaced to the analyzer and reduced by a computer program INTRAL (28). The counting schedule was arranged for optimal counting of the nuclides of interest. Cooling times were 2-4 days for 24Naand 47Ca-47Sc,2-6 weeks for and 59Fe, and 2 months for @To. Samples were counted on the face of the detector, and the counting dead time was
