SILICA, CR YST ALLINE , by XRD: M ETH OD 7500, Issue 4, dated 1 5 Ma rch 2003 - Page 4 of 9
6.
7.
8.
5 m L 2-propanol to the filter funnel and allow to stand for 5 min. Apply vacuum and slowly asp irate the acid and alcohol in the funnel, washing with three successive 10-mL portions of distilled water. Release the vacu um . Carry both filters through the ashing step together. ii. Place the filter samples in porcelain crucibles, loosely cover and ash in muffle furnace for 2 h at 600 °C (8 00 °C if graphite is present). Ad d several m L 2-p ropanol to th e ash, sc rape the crucible with a glas s rod to loos en a ll particles and transfer the residue to a 50-mL beaker. W ash the crucible several more times and add wash to beaker. Add 2-propanol to the beaker to bring the volume to about 15 mL; or c. Filter Dissolution: Using forceps and a spatula, remove the filter from the cassette, fold the filter three times, and place in the bottom of a 40- or 50-mL centrifuge tube. Add 10 mL TH F and allow to stand for at leas t 5 m in. Cap the cen trifuge tube with alu m inum foil to prevent contamination. Gently agitate the centrifuge tube by hand or with a vortex mixer mak ing sure the THF does not go near the top of the tube. Place the tube in an ultrasonic bath (water level 2.5 cm from top) for at least 10 m in. (The filter should be totally dissolved.) Just prior to filtering, agitate the sample for 10 to 20 sec on a vortex mixer. Continue with step 6, substituting THF for 2-propanol and centrifuge tube for beak er. Cover the beaker with a watchglass and agitate in an ultrasonic ba th for a t least 3 m in. Observe the suspension to ma ke su re that the agglom erated particles are broken up. W ash the underside of the watchglass w ith 2-propanol, collecting the washings in the beak er. Place a silver filter in the filtration apparatus. Attach the funnel securely over the entire filter circumference. W ith no vacuum, pour 2 to 3 mL 2-propa nol on to the filter. Pour the sample suspension from the bea ker into the fu nne l. After the trans fer, rins e the bea ker several tim es a nd a dd rinsings to the funnel for a total volume of 20 mL. In order to minimize feathering o f the sample outside the deposition area, allow the suspension to settle for a few minutes prior to applying vacuum. Do not rinse the chimney after the material has been deposited on the silver filter. Rinsing the chimney can disturb the thin layer deposition. Leave the vacuum on after filtration to produce a dry filter. Place 2 drops of 1.5% parlodion solution on a glass slide. Remove the silver filter with forceps and fix the material to the filter by placing the bottom side of the filter in the parlodion so lution. Place the saturated filter on top of the Teflon sheet which has been heated on the hot plate at a low temperature setting. W hen thoroughly dry, mount the silver filter in the XRD sam ple holder.
CALIBRATION AND QUALITY CONTRO L: 9.
Prepare and analyze at least 6 levels of standard filters. NOT E 1: Calibration standards are limited to NIST and USGS certified standards of known purity, particle size, and sample-to-sample homogeneity. At least 12 materials, including 5-:m M inU-Sil, previously used by laboratories throughout the United States and Canada, have been evaluated, and non e ha ve been foun d to be acceptable alternatives to the certified standards cited within this method [10]. Standard reference materials should be corrected for phase purity. NOTE 2: Crystalline silica methods require calibration standards of known purity, specific particle size and distributio n, and sam ple-to-sam ple hom ogeneity. Establishing traceability of secondary calibration standards to the specified NIST and USGS primary standards requires the use of measurem ent methods with better precision and accuracy than the X RD, IR and visible absorption spectrophotometry methods comm only used in the industrial hygiene field can provide. In addition, particle size distribution measurem ents have considerable error. Therefore, the use of second ary calibration standards that are traceable to NIST and USGS certified standards is not appropriate. NOTE 3: NIST SRM 2950 calibra tion se t ("-quartz) and NIST SRM 2960 calibration set (cristobalite) may be useful for preparing working standards at known concentrations. a. Prepare two suspensions of each analyte in 2-propanol by weighing 10 and 50 mg of the standard material to the nearest 0.01 m g. Quan titatively transfer each to a 1-L glass-stoppered bottle using 1.0 0 L of 2 -propanol. b. Suspend the powd er in 2-propanol with an ultrasonic probe or bath for 20 m in. Im m ediately m ove the bottle to a m agn etic stirre r with therm ally insulated top and add a stirring bar. Allow the solution to return to room tem perature before withdrawing aliquots.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition
