Page:NIOSH Manual of Analytical Methods - 7501.pdf/4

 SILICA, AMORPHOUS: METHOD 7501, Issue 3, dated 15 March 2003 - Page 4 of 8 CALIBRATION AND QUALITY CONTRO L: 6. 7.

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W eigh ca. 100 m g dried am orph ous silica to the ne ares t 0.01 m g. Quan titatively transfer to a 1-L bottle using 1.00 L 2-propanol. The resulting concentration is 100 µg/mL. Su spend the pow der in 2 -propanol using an ultrasonic probe or b ath for 20 m in. Im m ediate ly move the 1-L bottle to a magnetic stirrer with thermally-insulated top and add a stirring bar to the suspension. Allow the solution to return to room tem perature before withdrawing aliquots. Prepare a series of standard filters, in triplicate, over the range 0.005 to 2 mg per sam ple. a. Mount a 37-mm PVC filter on the filtration apparatus. Place several mL 2-propanol on the filter. Turn off the stirrer and shake the bottle vigorously by hand. Imm ediately remove the stopper and withdraw an aliquot (e.g., 2 to 25 mL) by pipet from the center at half-height of the suspe nsion. Do not adjust the volum e in the pipet by expelling part of the suspension. If m ore than the desired aliquot is withdraw n, disc ard th e aliquot in a beak er, rinse an d dry the pipet, and take a new aliquot. Transfer the aliquot from the pipet to the filter, keeping the tip of the pipet near the surface but not submerged in the delivered suspension. b. Rinse the pipet with several mL 2-propanol, draining the rinse into the funnel. Repeat the rinse several more times. c. Allow the suspension to settle for a few minutes prior to applying vacuum. Apply vacuum and rapidly filter the suspension. Do not w ash dow n the sides of the fu nnel after the deposition is in place sinc e this w ill rearrange the m aterial on the filter. Leave va cuu m on until the filter is dry. W hen thoroughly dry, mount the filter in the XRD sample holder. Prepare and analyze standard filters exactly like samples. a. Ash in LT A; redepos it on 25-mm PVC filters (steps 3 through 5). b. Analyze by XRD (s tep 13). c. Heat to conve rt amo rphous s ilica to cristobalite; redeposit on Ag filters (steps 14 throu gh 16). d. Analyze by XRD (s tep 17). Prepare c alibration graph (Î ox vs. µg of standard). NOTE: Poor repea tability (i.e., S r > 0.1) at any given level indicates that new stan dards should be m ade. The da ta should lie along a straight line. A weighted least s qua res (1/S 2 weighing) is preferable. Curvature can be eliminated with absorption corrections (step 20) [7 ]. Se lect six silver m em brane filters to be analyze d as m edia blanks. Mak e the selectio n ra ndom ly from the same box of filters used for redepositing samples. Mount each media blank on the filtration ap para tus and a pply vac uum to draw 5 to 10 m L of 2 -propan ol throu gh the filter. Rem ove, let dry and mount on XRD holders. Determine net normalized intensity for the silver pea k, î Ag, for ea ch m edia blank. Obtain an average value fro the six m edia blank s, Î Aog. NOTE: The analyst is a critical part of this analytical procedure [3]. A high level of analyst expertise is req uired in order to optim ize instrum ent param ete rs and correct for m atrix interferences either during the sample preparation phase or the data analysis and interpretation phas e. XRD analysts should have som e training (university or short course) in mineralogy or crystallography in order to have a background in crystal structure, diffraction patterns and m ineral transform ation. In addition, an intensive sho rt cou rse in the fu nda m enta ls of X -ray diffra ction c an b e us eful.

MEASUREMENT: 12.

Ob tain a qua litative X-ray diffraction sc an (e.g., 10 to 80 °22) of the bulk (high-volume respirable) sample to determine the presence of free silica polymorphs and interferences. The diffraction peaks are: Mineral Quartz Cristobalite Tridymite Silver

Prima ry 26.66 21.93 21.62 38.12

Peak (2-T heta Deg rees) Secon dary 20.85 36.11 20.50 44.28

Tertiary 50.16 31.46 23.28 77.47

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition