Page:NIOSH Manual of Analytical Methods - 7502.pdf/4

 ZINC OXIDE: METHOD 7502, Issue 2, dated 15 August 1994 - Page 4 of 5 ZnO Peak (2-Theta Degrees) Primary Secondary Tertiary 36.26 16.

17.

31.75

34.44

Analyze the sample filters by step-scanning the most intense interference-free diffraction peak of zinc oxide and determine the integrated intensity. Measure the background on each side of the peak for one half the time used for peak scanning and add the counts from each side for a total (average) background. Determine the 2-theta position of the background for each sample. The net count or intensity, I x, is the difference between the peak integrated count and the total background count. The net intensity is normalized as in step 12 to obtain . Scan each field blank over the same 2-theta range used for the analyte. These analyses serve only to verify that contamination of the filters has not occurred. The analyte peak should be absent.

CALCULATIONS: 18.

The concentration of ZnO in the air sample is:

where:

= normalized intensity for sample peak b = intercept of calibration curve ( vs. W) m = initial slope of calibration graph (counts/µg) V = air volume sampled (L).

In heavily-loaded samples, particularly those rich in heavy elements, X-ray absorption may cause reduced intensities and underestimation of ZnO. If this is suspected, an absorption correction can be made [8]. The collection filters and blanks are mounted on a smooth metal plate (substrate) for XRD quantitation. The substrate should have a non-interfering diffraction peak which is measured at the same time as the ZnO peak. By measuring the substrate peak on both samples and blanks, an absorption correction like that in the silica method (Method 7500) can be made. The absorption correction factor must be calculated for each sample from the formula in Method 7500 which takes into account the diffraction angles for the particular substrate chosen. Altree-Williams used a silver filter under the collection filter [7].

EVALUATION OF METHOD: In a comparison of this method with atomic absorption spectrophotometry [3,4], zinc was determined on 15 Gelman DM-800 filters containing added zinc oxide in the range 250 to 1000 µg. The average percent difference for the 15 pairs was 2.7%. Method S316 was validated with generated samples in the range of 2.4 to 9.9 mg/m 3 [2,5,10]. A pooled SˆrT of 0.088 was found for 18 samples of 180 L collected from the aerosol.

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94