LEAD SULFIDE: METHOD 7505, Issue 2, dated 15 August 1994 - Page 5 of 7
where:
Î x = normalized intensity for sample peak b = intercept of calibration curve (Î ox vs. W) m = initial slope of calibration curve, counts/µg
R = sin ( ΘAg)/sin ( Θx) T = Î Ag/(average Î Åg = transmittance of sample Î Ag = normalized silver peak intensity from sample average Î Ag = normalized silver peak intensity from media blanks (average of six values) NOTE: For a more detailed discussion of the absorption correction procedure, see references [4] and [5].
EVALUATION OF METHOD: The measurement precision, Sr, initially determined with 30 samples in the range 30 to 2000 µg PbS per filter was 0.0047 [6]. The same 30 samples plus five blank filters were analyzed by X-ray fluorescence for lead and sulfur and by ICP-AES for lead in order to verify the accuracy of the PbS filter depositions. A recovery study with 20 spiked samples in the range of 30 to 150 µg PbS per filter indicated a 102.6% average recovery and a 0.05 relative standard deviation. Another recent study [1] involving 54 spiked samples in the range 40 to 250 µg PbS per filter indicated an 0.0081 precision and a 98% recovery. The overall method was also evaluated in the study using eight sets of samples collected in galena mills. A set consisted of six filters collected side by side. Average PbS in air concentrations for these sets ranged from 12 to 35 µg/m 3 and the pooled SˆrT for seven of these averages was 0.103. The eighth set was an outlier [1]. Two stability studies [6] were performed: a short-term (47 days) and a long-term (6.5 months). In the short-term study, 15 PbS samples were exposed to various conditions from intense light to no light, from moist air to vacuum; all were analyzed at intervals. During analysis, they were all subjected to a total of over 2 h exposure to X-ray radiation at 1400 watts. Under continuous intense light, the PbS degradation was 14.4%. The smallest degradations were with no light (1.2 to 3.8%). In the long-term study, a PbS bulk sample exposed to ambient laboratory air and light conditions and a total of 36 h of X-ray radiation degraded 37.6%. A filter similarly exposed in the lab for 158 days and a total of 28 h of X-radiation degraded 12.2%. It is, therefore, recommended that the samples be protected from light.
REFERENCES: [1] Abell, M. T. Evaluation of a Sampling and Analytical Method for Lead Sulfide, NIOSH/MRB Internal Report (September 30, 1983). [2] NIOSH Manual of Analytical Methods, 2nd. ed., V. 7, P&CAM 350, U.S. Department of Health and Human Services, Publ. (NIOSH) 82-100 (1981). [3] Criteria for a Recommended Standard...Occupational Exposure to Inorganic Lead (Revised), U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 78-158 (1978). [4] Leroux, J. and C. Powers. Direct X-Ray Diffraction Quantitative Analysis of Quartz in Industrial Dust Films Deposited on Silver Membrane Filters, Occup. Health Rev., 21, 26 (1970). [5] Williams, D. D. Direct Quantitative Diffractometric Analysis, Anal. Chem., 31, 1841 (1959). [6] Palassis, J., D. D. Dollberg and M. S. Hawkins. Air Sampling and Analysis of Lead Sulfide in Galena Mining by X-Ray Powder Diffraction and X-ray Fluorescence Spectrometry, paper presented to the ACGIH, Cincinnati, OH (June 8, 1982). [7] Snell, F. D. and L. S. Ettre. Encyclopedia of Industrial Chemical Analysis , 15, 161-169, Interscience Publishers, NY (1972). NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
