SILICA, AMORPHOUS: METHOD 7501, Issue 3, dated 15 March 2003 - Page 5 of 8 NOTE:
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14.
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16. 17.
For quantitative determination of amorphous silica in bulk samples, wet-sieve a portion of the bulk sam ple thro ugh a 10-µm sieve with 2-prop ano l in an ultrason ic bath . Evaporate the alcohol and dry in an oven 2 h. W eigh out, in triplicate, 2-mg portions of the sieved dust and transfer to beakers. Add 10 mL 2-propanol, deposit on a PVC filter (step 5) and continue with steps 13 through 17. Perform the following for each sam ple, working standard an d blank filter. a. Mo unt the referen ce s pec imen. D eterm ine the net intensity, I r, of the reference specimen before and after each filter is scanned. Use a diffraction peak of high intensity that can be rapidly but reproducibly (Sr <0.01) measured. b. Mount the sample, working standard, or blank filter. Measure the diffraction peak area for each silica polymorph. Scan times m ust be long, e.g., 15 min (longer scan times will lower the lim it of detection). c. Measure the background on each side of the peak for one-half the time used for peak scanning. The sum of these two counts is the average background. Determine the position of the background for each sample. d. Ca lculate the net inten sity, Ix, (the difference between the peak integrated count and the total back ground c ount). e. Ca lculate and reco rd the norm alized intensity, î x, for each peak:
NOTE: Select a c onvenient norm alization s cale facto r, N, which is app roximately equivalen t to the net count fo r the re ference spe cim en p eak, and use this value of N fo r all analyses. Normalizing to the reference specimen intensity compensates for long-term drift in Xray tube intensity. If intensity measurem ents are stable, the reference specimen m ay be run less frequently and the net intensities should be normalized to the most recently-m easured re ference inten sity. Rem ove the PVC filter from the XRD holder, fold the filter carefully and place it in a platinum crucible in the furnac e. Ra ise the tem pera ture of the fu rnac e slow ly (ca. 50 °C/m in) to 50 0 °C to ash the filter. W hen ashing is com plete (ca. 0.5 h), rais e the tem perature to 1500 °C and m aintain at 1500 °C for 2 h (for fumed am orphous silica) or to 1100 °C and maintain at 1100 °C for 6 h (for other amorphous silicas). Turn the furnace off and let the crucibles cool overnight in the furnace. Place ca. 10 mg NaC l in the crucible and mix with the ash in the crucible. Transfer contents of the crucible to an agate mortar and grind to a fine powder using an agate pestle. Add distilled water from a polyethylene squeeze bottle to the mortar. Use a rubber policeman to stir. W hen the NaCl is dissolved, transfer the solution to a 100-mL beaker (hold the rubber policeman at the edge of the mortar to guide the flow into the beaker). Rinse mortar, pestle, crucible, and rubber policeman, collecting rinsings in the beaker. Cover the beaker with a watchglass and place in an ultrasonic bath for 2 to 3 min. NOT E 1: Use extrem e care to avoid air c urrents when working with the dry s am ple, as it is easily lost in th e fo rm of a n aerosol. NOT E 2: Use uniform grinding techniques to produce similar particle size in standards and samples. W ash the underside of the watchglass and collect rinsings in the beaker. Place a silver m em brane filter in the filtration appa ratus and filter the contents of the beake r (step 5). Mount the silver mem brane filters in the XRD instrument and: a. Analyze for the three silica polymorphs (step 13); and b. De term ine the norm alized coun t, îAg, of an interference-free silver peak on the sample filter following the sam e procedure. Use a short scan tim e fo r the silver pe ak (e.g., 5 % of s can tim e for analyte peaks) throughout the method.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition
