Page:NIOSH Manual of Analytical Methods - 7504.pdf/4

 VANADIUM OXIDES: METHOD 7504, Issue 2, dated 15 August 1994 - Page 4 of 10 10.

11.

Perform step scans on the standards and reference specimen using the same conditions as for samples. Use steps 12 and 13 to determine normalized intensity, , for each peak measured. Use exactly the same normalization factor, N, as for samples (step 13). Prepare calibration graph ( vs. mg of each standard). Determine slope, m (counts/µg). NOTE 1: The intercept with the axis should be ca. zero. A large negative intercept indicates an error in determining background (e.g., incorrectly measuring the baseline, or interference by another phase at the angle of background measurement). A large positive intercept indicates an error in determining the baseline or that an impurity is included in the measured peak. NOTE 2: Poor repeatability at a given level indicates problems in the sample preparation technique and new standards should be made. Eliminate curvature with absorption corrections based on the mass absorption coefficient of the analyte (step 15, or from Tables 2 through 4).

MEASUREMENT: 12.

13.

Obtain a qualitative X-ray diffraction scan (broad 2 θ range) of high-volume sample to determine the presence of interferences. NOTE: If quantitative analysis is to be done on the bulk sample, wet-sieve it first through a 10-µm sieve. Mount the filter (sample, standard, or blank) in the XRD instrument and perform the following: a. Determine net intensity, I or, of the reference specimen before filter is scanned. Select a convenient scale factor, N, which is approximately equivalent to the net count for the reference specimen peak; use this for all analyses. b. Step-scan the most intense, interference-free diffraction peak of each compound to be determined, integrating the counts. NOTE: Useful analytical lines for the analytes are given in Table 1. Use strongest line of the analyte which does not have a matrix interference. Avoid lines in the proximity of Ag (JCPDS #4-0783 [3]) and AgCl (JCPDS #31-1238 [4]) (the latter often exists on the surface of silver filters). c. Measure the background on each side of the peak for one-half the time used for peak scanning. Add the counts from each side to obtain total (average) background. d. Calculate net intensity, I x, (different between peak integrated count and total background count). e. Calculate and record the normalized intensity for the analyte peak on each sample and standard:

f.

For each media blank, determine the net count for the analyte diffraction peak. Calculate the average normalized intensity, Î b, for the 6 media blanks. Determine net count, I Ag, of an interference-free silver peak on the filter following the same procedure. Scan times should be shorter for the silver peak (e.g., about 5% of scan times for analyte peaks) and should be consistent throughout the method. For each media blank, determine the net count for the silver peak. Calculate the average value, , for the 6 media blanks. NOTE: Normalizing to the reference specimen intensity compensates for long-term drift in X-ray tube intensity. If intensity measurements are stable, the reference specimen may be run less frequently. In this case, the net intensities of the analyte, blank, and silver peaks (I x, I b, and I Ag) should be normalized to the most recently measured reference intensity.

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