ZINC OXIDE: METHOD 7502, Issue 2, dated 15 August 1994 - Page 3 of 5 7.
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Prepare a series of standard filters using the 10 and 50 mg/L suspensions. Using appropriate pipets, prepare a sufficient number of standards in triplicate to cover the analytical range (or sample range if known). Standards at 20, 30, 50 100, 200 and 500 µg are usually sufficient. Mount a filter on the filtration apparatus. Place several mL 2-propanol on the filter surface. Turn off the stirrer and shake vigorously by hand. Within a few seconds of setting the bottle down, remove the lid and withdraw an aliquot from the center of the suspension. Do not adjust the volume in the pipet by expelling 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 If more than the desired aliquot is withdrawn, return all of the suspension to the bottle, rinse and dry the pipet. Transfer the aliquot from the pipet to the filter funnel, keeping the tip of the pipet near the surface of the liquid but not submerged. Rinse the pipet with several mL of 2-propanol, draining the rinse into the funnel. Repeat the rinse several more times. 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 to avoid disturbing it. Transfer the filter to the XRD sample mount. Perform step scans on the standards using the same diffraction peaks and instrumental conditions as for samples (step 16). The resulting intensities for standards, , are normalized in the following procedure. Determine the net count, I r, of the reference specimen before or after each unknown, standard or blank is scanned. Use a diffraction peak of high intensity that can be measured rapidly but reproducibly (less than 1% S r). Select a convenient normalization scale factor, N, which is approximately equivalent to the net count for the reference specimen peak. This factor never changes and should be used for all measurements made on a particular diffractometer. Calculate and record the normalized intensity, , for the analyte or silver peaks on each sample, field blank, media blank and standard:
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, I x, should be normalized to the most recently measured reference intensity. Prepare a calibration graph ( vs. µg analyte). NOTE: Poor reproducibility at any given level indicates problems in the sample preparation technique and new standards should be made. The data should lie along a straight line. A weighted least squares (1/ σ2 weighting) is preferable. Curvature can be eliminated with absorption corrections based on the mass absorption coefficient of the analyte [7,8,9]. Determine the initial slope, m, of the linear portion of the calibration graph in counts/µg. The intercept, b, of the line with the axis should be approximately zero. NOTE: A large negative intercept indicates an error in determining the background. This may arise from incorrectly measuring the baseline or from 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.
MEASUREMENT: 15.
Obtain a qualitative X-ray diffraction scan (broad 2-theta range) of the high-volume respirable sample to determine the presence of ZnO and any matrix interference. The expected diffraction peaks are as follows:
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
