Silica, Crystalline by IR: Method 7602, Issue 3, dated 15 March 2003 - Page 3 of 6 currently used in the United States for the purpose of matching an earlier sampling convention [10]. In any case, a single sampler/flow rate should be used in any given application so as to eliminate bias introduced by differences between s am pler types and sam pler convention s [7]. SAMPLE PREPARATION: 3. Use one of the following methods to ash samples and blanks: a. Low temperature (RF Plasma) asher: Place the filters in lab elled alum inum dishes (previously rinsed with distilled water, followed by ethanol, and air dried). Place the dis he s in the low tem perature ash er so that sam ple ex pos ure to the plasm a is op timized. As h ac cording to m anufacturer's instructions. Carefully bring asher to atmospheric pressure and remove the dishes. b. Muffle furnace: For sa m ples containing a significant amou nt of calcite (> 20% of the tota l dust loading), wa sh filters with 9% w/w hydrochloric aicd. Othe r sam ples proceed to step 3.b.ii. i. Place a 0.5-µm , 47-mm PVC filter in the filtration apparatus. Remove sample filter from cas sette and center on top of first filter. Clam p fu nnel over the frit so that dust d eposit is com pletely exposed. Add 10 mL 9% w/w HCl and 5 mL 2 -propanol; allow to stand 5 min. Ap ply vacuum and slowly aspirate the acid and alcohol in the funnel. W ash with three successive 10-mL portions of distilled water. Release vacuum. ii. Place filter samples and blanks in porcelain crucibles, loosely cover, and ash in muffle furnace for 2 h at 600 °C (80 0 °C if graphite is present). 4. Add approximately 300 m g KB r, weighed to 0.1 mg and dried overnight at 110 °C, directly to each sample. Mix the sample ash and KBr thoroughly with a pestle. Transfer to mortar to complete mixing if necessary. Transfer the mixture to a 13-mm evacuable pellet die using glassine paper and cam el's hair brush. Press a pellet using standard technique. Weigh the finished pellet to 0.1 m g. Ca lculate the ratio (weight of finish ed p ellet/weight KBr initially added); it is us ually abo ut 0.98. C lean sam ple handling equipment with ethanol between samples. NOTE: A low relative humidity environm ent will facilitate sam ple handling when us ing KBr.
CALIBRATION AND QUALITY CONTRO L: 5. Prepare at least 5 working standard pellets: NOT E 1: Calibration standards are limited to NIST and USG S certified standards of known purity, particle size, and sample-to-sample homogeneity. At least 12 materials, including 5 µm Min-U-Sil, previously used by laboratories throughout the United States and C anada, have been evaluated, and none has been found to be an acceptable alternative to the certified sta ndards cited within this m eth od [7]. Sta ndard re ference m ate rials should be corrected for ph ase purity. NOTE 2: Establishing traceability of secondary calibration standards to the specified NIST and USGS primary standards req uires the use of m easurem ent m ethods with better precision and accuracy than the XRD, IR and visible absorption spectrophotometry methods com m only used in the industrial hygiene field can provide. In addition, particle size distribution m eas urem ents have considerable error. Therefore, the use of secondary calibration standards that are traceable to NIST and USG S certified standards is not appropriate. a. W eigh, to 0.001 mg, portions of NIST "-quartz SRM 1878a, NIST cristobalite SRM 1879a, USGS tridymite 210-75-00473 containing 10 to 200 µg standard. b. Add an accurately-weighed 300 mg KBr portion. Proceed as in step 4. c. Calculate the ratio (weight of finished pellet/weight of solids added); usually about 0.98. d. Determine the absorbance at 800 cm ! 1 for each stand ard pellet following the analytical procedure (step 8). Plot absorbances vs. µ g SiO 2. 6. If sam ples were low temperature ashed (step 3.a.) and kaolinite is present, prepare pellets containing 100 to 600 µg kaolinite. Determine the absorbance at 800 cm ! 1 and at 915 cm ! 1 as absc issa. Prepare at least five differe nt ka olinite concentrations. Use this curve to correct the absorbance value at 800 cm ! 1 for any sample containing kaolinite. 7. Carry m edia blanks an d filters spik ed w ith known am oun ts of q uartz through the sample preparation procedures (steps 3 and 4) to monitor for contamination and losses.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition
