Estimation of Trace Amounts of Chrysotile Asbestos by X-Ray Diffraction Anthony L. Rickards Turner Brothers Asbestos Co. Ltd., P.O. Box 40, Rochdale, England X-RAY DIFFRACTION TECHNIQUES have been used for both qualitative identification and quantitative determination of different types of asbestos minerals, notably chrysotile, crocidolite, and amosite. Crable (I) has described a method based on the measurement of the area under a primary diffraction peak and comparison with an external calibration curve, to measure quantities of chrysotile in the 1-10 mg range. In the course of a detailed study of the chrysotile content of urban atmospheres, it became necessary to develop more sensitive techniques (2). It is the purpose of this paper to show that with suitable equipment and experimental precautions, the X-ray technique can be extended to detect much smaller quantities, of the order of 10 pg on membrane filters, when no gross interferences are present. Even when interference is present, the changes in intensity of a chrysotile diffraction peak as a result of known additions of chrysotile to the sample, relative to the intensity of an unknown line (3),can be used to detect amounts of 50-100 pg. EXPERIMENTAL
Apparatus. A Philips PW.1010 X-ray generator was used in combination with a PW.1050 vertical goniometer and a proportional counter. Copper K a radiation, nickel filtered, was used at 42 kV and 26 mA. The following slit system gave optimum resolution and peak-to-background ratios : divergence, 1"; scatter, 0.3 mm; and receiving slits, 1". A pulse height discriminator, provision of step scanning facilities with digital print-out, and a rotating sample holder contributed to the sensitivity achieved. The pulse height discriminator reduced the background, the step-scanning arrangement permitted the reliable measurement of small integrated areas, and the rotating holder enabled the total sample, collected on a 25-mm membrane filter, to be scanned. The background due to the membrane filter was minimized by using a Millipore Solvinert type filter. Preparation of Standards. A weighed amount (0.1 gram) of chrysotile was added to 250 ml of deionized water which had been first filtered through a membrane filter (0.05-pm mean pore size) to remove any suspended solids. An ultrasonic bath was then used to disperse and suspend the chrysotile. In order to stabilize the dispersed suspensions, sufficient anionic surface active agent was added to produce a 1 solution of surfactant. The ultrasonic treatment was continued for about one week, until a stable "smooth" suspension of chrysotile was produced. The suspension was then made up to 1 liter in a standard volumetric flask using deionized water and adding sufficient surfactant (normally 10 ml) to stabilize the suspension. One hundred ml of this suspension was then pipetted into a second volumetric flask and made up to 1 liter. Hence the original 0.1 gram of chrysotile contained in 1 liter was diluted by a factor of 10 to give 0.01 gram/liter. The amount of chrysotile present on the membrane filters was cross checked by analysis of the magnesium content using atomic absorption spectrometry.
x
(1) J. V. Crable, Amer. Znd. Hyg. Ass. J . , 27,293 (1966). (2) A. L. Rickards and D. V. Badami, N a m e , 234,93 (1971). ( 3 ) L. E. Copeland and R. H. Bragg, ANAL.CHEM., 30,196 (1958). 1872
10 1
10
100
1000
MASS OF CHRYXlTILE
(yg)
Figure 1. Calibration curve for known amounts of chrysotile
Electronic microscopic studies showed that both the calibration samples and the actual saomples consisted essentially of fibrils about 1pm long and 340A in diameter. External Standard Method. A series of chrysotile calibration samples was prepared by the diluted suspension technique described above. By filtration, amounts of 10 to 1000 pg were collected on membrane filters. Each sample was mounted on the diffractqneter and the chrysotile 002 diffraction peak (do,,* = 7.36 A) step-scanned from 11" to 13" 28, covering about 1" on either side of the maximum. The intensities on either side of the peak position were used to estimate an average background area which was subtracted from the integrated count for the whole scan, giving the integrated count for the peak. The final integrated area for the chrysotile 002 maximum was obtained by subtracting from the above, the area observed from a blank membrane filter over the same angular range. The final area was plotted (Figure 1) against the amount of chrysotile on the filters, the data quoted being the averages of two separate series of samples, in which four determinations were made on each sample. Internal Standard Method. The normal procedure employing internal calibration involves the preparation of a series of standards of known composition and the construction of a calibration curve. This implies a precise knowledge of the composition as well as the availability of sufficient material with the same history as the sample, to produce a series of standards. On occasions, neither of these requirements can be met. However, Copeland and Bragg (3) have described a method based on the addition of known amounts of the material to be estimated to a sample containing an unknown amount of it. The resultant increases in intensity of a diffraction peak are used to extrapolate back to give the amount of material that would give rise to the intensity originally observed in the sample. This technique has been used to determine the minor amount of chrysotile in samples with complex compositions. About 10 mg of the sample whose chrysotile content was to be determined was filtered onto the membrane filter and weighed. From an initial scan of the diffractometer pattern, the chrysotile 002 maximum was identified and a nonchrysotile maximum was chosen to be used as a reference peak. A
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
feature of this technique is that it is not necessary to identify the reference peak or any peak other than that of chrysotile. Having selected a reference peak, integrated intensities for the reference peak (ZT)and the 002 reflection of chrysotile (I,) were obtained by the step-scanning procedure described previously. Next, an addition of a known amount of chrysotile (typically 50 pg) was made by filtering a known volume of suspended chrysotile through the sample filter, as already described. The sample was dried and the intensities of the two peaks reevaluated. This process was repeated for 5 additions of chrysotile. Finally, the ratios of the two peak intensities were obtained and plotted (Figure 2) against the amount of chrysotile added to the sample. The graph was extrapolated down to the X axis and the intercept used to estimate the amount of chrysotile in the original sample. RESULTS AND DISCUSSION
The two methods described above can be applied to most situations where an estimate of trace amounts of chrysotile is sought. In the evaluation of environmental airborne chrysotile referred to earlier (2), these particular samples (from 1000 m a of air) had no detectable chrysotile peak and the chrysotile content was therefore below the limits of the X-ray detection methods, Le., less than 0.1 pg/ma. The samples did contain appreciable amounts of quartz and kaolinite but quantitative estimation of these components was not attempted. Electron microscopic methods have now proved sufficiently sensitive to detect amounts of chrysotile of the order of 0.1 to 1.0 ng per cubic meter and will be reported in detail elsewhere. However, the X-ray methods can be used in a number of other cases, such as mixed industrial atmospheric dusts and other instances where the final samples can be obtained on a membrane filter. The ultimate amount of sample on the membrane filter must be small, to ensure that the specimens are thin and thus reduce errors due to specimen thickness. This is of particular importance in the internal standard method, where chrysotile is added to the sample on the membrane. If the sample is too thick, then there are errors due to layering of the chrysotile additions. In practice, additions of 50 pg units of chrysotile to 10 mg of sample hardly affected the intensity of the reference peak, confirming that the errors due to layering are minimal. The detection limits of the two methods were 10 pg for the external standard method and, depending upon the nature of
Mass of chrysotlle pg) added to the s=ample
Mass d ChWSOtlle (!.IO) in the original sample.
Figure 2. Extrapolated graph of additions of chrysotile plotted against diffraction intensity
the sample, between 50 and 100 pg for the internal method. Standard deviations for the external method were 10% for 10 pg and 2 z for 100 pg. The most reliable region is clearly 50-100 pg and in practice this has been found to be the most useful range within which to operate. The methods have been developed especially for evaluation of chrysotile. In principle, however, there is no reason why the methods cannot be applied to any crystalline material, subject to the limitation of producing suitable calibration samples of sufficiently low concentration. Further improvements to the detection limits will require the development of more sensitive electronic equipment and the use of more powerful X-ray tubes. ACKNOWLEDGMENT
The author thanks G . F. Heron and D. V. Badami of the Research and Engineering Division, T.B.A., for their support and encouragement, and the Directors of T.B.A. Co. Ltd. for permission to publish this work. RECEIVED for review February 11, 1972. Accepted April 18, 1972.
Gravimetric Analysis of Uranyl-Orthophosphate Mixtures J. M. Schaekers Atomic Energy Board, Department of Physical Metallurgy, Private Bag X256, Pretoria, South Africa
MIXTURES OF URANIUM AND PHOSPHATE are frequently encountered, especially in connection with the extraction of uranium from low grade ores, and with the study of U(1V) compounds. The analysis of these mixtures, either as a solution or as a solid, is rather tedious, as it normally requires a separation step on a cation exchange resin column in hydrochloric acid medium (1-3). A gravimetric method, in which the UOz2+is ~~
(1) J. A. Goudie and W. Rieman, ANAL.CHEM.,24, 1067 (1952). (2) S. M. Khorkar and K. de Anil, Anal. Chim. Acta, 22, 153
(1960). (3) I. M. Kolthoff and P. J. Elving, “Treatise on Analytical Chemistry,” Part 11, Vol. 9, Interscience Publishers, New York, N.Y.,
1960-63.
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precipitated as U02NH4P04 XHzO, has been described (4-7) but it has been criticized (8)as not being very accurate. A variation of the same method has also been used for the gravimetric determination (5, 6, 9) of P043- (‘‘P043-”is used (4) A. A. Smales and H. N. Wilson, Rept. BR-150(1943). ( 5 ) L. G. Basset, D. J. Pflaum, et al., Rept. A-2912 (1949). (6) C. J. Rodden. “Analytical Chemistry of the Manhatten Project,” McGraw-Hill Book Co., New York, N.Y., 1950. (7) G. W. C. Milner, D. H. Rowe, and G. Phillips, Rept. AEREA4906 (1965). (8) W. B. Schaaf, L. S. Andrews, and J. W. Gates, Jr., Rept. CD-4002 (1945). (9) E. R. Caley and C. W. Foulk, J. Amer. Chem. Soc., 51, 1664 ( 1929).
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