Page:NIOSH Manual of Analytical Methods - 7506.pdf/4

 BORON CARBIDE: METHOD 7506, Issue 2, dated 15 August 1994 - Page 4 of 7

d. e.

f.

g. h.

part of the suspension. If more than the desired aliquot is withdrawn, return all of the suspension to the bottle, rinse and dry the pipet, and take a new aliquot. Transfer the aliquot from the pipet to the filter, keeping the tip of the pipet just above the surface of the delivered suspension. Rinse the pipet with several portions of 2-propanol, draining the rinses into the funnel. Apply vacuum and rapidly filter the suspension. Leave vacuum on until filter is dry. Do not wash down the sides of the funnel after the deposit is in place since this will rearrange the material on the filter. Prepare working standard filters in triplicate by this technique at, e.g., 50, 100, 200, 500, 1000, and 2000 µg boron carbide. Analyze by XRD (step 11). Use the same diffraction peaks and instrumental conditions as for samples. Designate the net and normalized XRD intensities for the working standard filters as Ixo (step 11.d) and ˆI xo (step 11.e), respectively. Correct ˆI xo for matrix absorption for standards containing >200 µg B 4C (steps 11.f and 12). Prepare calibration graph ( ˆIxo vs. µg B 4C ) using 1/ σ 2 weighted least squares. Determine the slope, m (counts/µg), of the calibration graph. The intercept, b, of the line with the ˆIxo axis should be zero ±5 µg.

MEASUREMENT: 10.

Obtain a qualitative X-ray diffraction scan (e.g., 10 to 80° 2- θ) of the settled dust bulk or high-volume respirable sample to determine the presence of boron carbide and any matrix interference (see APPENDIX). The expected diffraction peaks are: Peak (Degrees 2-θ) Primary Secondary Boron carbide Silver

11.

37.80° 38.12°

34.91° 44.28°

Mount the filter (sample, standard or blank) in the X-ray diffractometer and: a. Determine the net intensity, I r, of the reference specimen before each filter is scanned. Select a convenient normalization scale factor, N (approximately equal to the net count for the reference specimen peak). Use this value of N for all analyses. 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 and the net intensities should be normalized to the most recently measured reference specimen intensity. b. Measure the area of the most intense, interference-free diffraction peak of B 4C. Scan times must be long, e.g., 15 min. c. Determine the position of the background for each sample. 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 background. d. Calculate net intensity, I x (peak count minus total background count). e. f.

g.

Calculate the normalized intensity, Îx = I x · N/I r. Determine the net normalized intensity, ˆI Ag, of an interference-free silver peak on the sample filter following the same procedure. Use a short scan time for the silver peak (e.g., 5% of scan time for analyte peaks) throughout the method. Scan each field blank over the same 2- θ range used for B 4C and silver peaks. These analyses serve only to verify that contamination of the filters has not occurred. The analyte peak should be absent. The normalized intensity of the silver peak should match that of the media blanks.

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