SILICA, CR YST ALLINE , by XRD: M ETH OD 7500, Issue 4, dated 1 5 Ma rch 2003 - Page 5 of 9 c.
10.
Mount a silver filter on the filtration apparatus. Place several m L of 2-propano l on the filter. Turn off the stirrer an d sh ake vigorous ly by hand . Imm ediately rem ove the stopper and withdraw an aliquot from the center at half-height of the 10 mg/L or 50 mg/L suspension. Do not adjust the volume in the pipet by expelling part of the suspension. If more than the desired aliquot is withdrawn, discard the aliquot in a beaker, rinse and dry the p ipet, an d tak e a new a liquot. Transfer the aliquot from the pipet to the silver filter, keeping the tip of the pipet near the surface but not submerged in the delivered suspension. d. Rinse the pipet with several mL 2-propanol, draining the rinse into the funnel. Repeat the rinse several times. e. Allow the su sp en sio n to settle for a few minutes prior to applying vacuum. Apply vacuum and rapidly filter the suspension. Do not wash down the sides of the funnel after the deposit is in place since this will rearrange the material on the silver filter. Leave vacuum on until filter is dry. Place 2 drops of 1.5% parlodion solution on a glass slide. R em ove the silver filter with forceps and fix the material to the filter by placing the bottom side o f the filter in the parlodion solution. Place the saturated filter on top of the heate d T eflon she et. W hen thoroughly dry, mount the silver filter in the XRD sam ple holder. Prepare working standard filters, in triplicate, at e.g., 10, 20, 50, 100, 250, and 500 :g. f. Analyze the working standards together with samples and blanks (step 12). The XRD intensities for the working standards (step 12.d) are designated Iox and are then n orm alized (s tep 12.e) to obtain Îox . Correct the intensities of working standards >200 :g for m atrix abso rption (step s 12 .f and 13). g. Prepare a calibration graph (Î ox , vs :g of each s tandard). NOTE: Poor repeatability (>10% above 0.04 mg silica) at any given level indicates that new standards should be made. The data should lie along a straight line. A weighted least squares (1/F 2 weighting) is preferable. h. Determine the slope, m, of the calibration graph in counts/:g. The intercept, b, on the abscissa should be within ± 5 :g of zero. NOTE: A large intercept indicates an error in determ ining the background, i.e., an incorrect baseline or interference by another phase. NOTE: The following proce dure for abso rption correction is not necessary in situations that have been previously documented as requiring no corrections. Select six silver mem brane filters as media blanks randomly from the same box of filters to be used for depositing the samples. These will be used to test for sample self-absorption. Mount each of the m edia blanks on the filtration a ppa ratus and app ly vacuu m to draw 5 to 10 m L 2-propan ol through the filter. Rem ove, let dry, and mount on XRD holders. Determine the net normalized count for the silver peak, îAg, for ea ch m edia blank (step 12.g). Ob tain an average value for the six m edia blank s, Î Aog. NOTE: The analyst is a critical part of this an alytical proced ure [1 2]. A high level of analyst expertise is required to optimize instrument parameters and correct for matrix interferences either during the sample preparation phase or the data ana lysis and interpretation pha se [15]. The analyst 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 mineral trans form ation. In addition, an inten sive short course in the fun dam entals of X -ray diffractio n can be useful.
MEASUREMENT: 11. Obta in a qualitative X-ray diffra ction scan (e.g., 10 to 80 °22) of the area air sample (or bulk settled dus t) to determine the presence of free silica polymorphs and interferences (see APPENDIX). The diffraction peaks are: Mineral Qu artz Cristoba lite Tridym ite Silver
Peak (2-Th eta Degre es) Prima ry Secon dary Tertiary 26.66 20.85 50.16 21.93 36.11 31.46 21.62 20.50 23.28 38.12 44.28 77.47
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition
