Page:NIOSH Manual of Analytical Methods - 7505.pdf/3
LEAD SULFIDE: METHOD 7505, Issue 2, dated 15 August 1994 - Page 3 of 7 3.
Take a high-volume bulk dust sample (e.g., 1 m 3 @ 3 L/min). NOTE: Protect the filters from exposure to intense light during storage.
SAMPLE PREPARATION: 4. 5. 6.
7.
Using forceps, place filters in 40-mL wide-mouth centrifuge tubes. Add 10 mL THF to each tube. Place the centrifuge tubes on the test tube rack in the ultrasonic bath for 10 min. Place a silver filter in the filtration apparatus and attach the funnel. Pour the suspensions from the centrifuge tube into the funnel. Wash the empty centrifuge tube with 5 mL THF and shake tube by hand or by vortex mixer for a few seconds. Transfer the THF to the filtration funnel. Repeat with two more 5-mL portions of THF. Control the filtration rate to keep the liquid level in the funnel near the top during rinsing. Do not wash the walls or add THF to the funnel when the liquid level is lower than 4 cm above the filter. Leave the vacuum on after filtration to produce a dry filter. Remove the filter with forceps and mount it in the XRD sampler holder.
CALIBRATION AND QUALITY CONTROL: 8.
9.
Select six silver membrane filters as media blanks randomly from the same box of filters to be used for depositing the samples. These will be used to correct for sample self-absorption. Mount each of the media blanks on the filtration apparatus and apply vacuum to draw 5 to 10 mL of 2-propanol through the filter. Remove, let dry and mount on XRD holders. Determine the net normalized count for the silver peak, Î Åg, for each media blank (step 11). Average the values for the six media blanks. Prepare and analyze working standard filters. a. Prepare two suspensions by weighing 10 and 100 mg of the dry, sieved lead sulfide to the nearest 0.01 mg. Quantitatively transfer each to a 1-L glass-stoppered bottle using 1.00 L 2-propanol. b. Suspend the powder in the 2-propanol using an ultrasonic probe or bath for 20 min. Immediately move the flask to a magnetic stirrer with thermally-insulated top and add a stirring bar. Cool to room temperature before use. c. Mount a silver filter on the filtration apparatus. Place 2 to 4 mL 2-propanol on the filter. Turn off the stirrer and shake the suspension vigorously by hand. Immediately remove the lid and withdraw an aliquot from the center of the suspension. Do not adjust the volume in the pipet by expelling part of the suspension. If more than the desired aliquot is withdrawn, return all of the suspension to the bottle, rinse, and dry the pipet and take a new aliquot.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. d. Rinse the pipet with several mL 2-propanol, draining the rinse into the funnel. Repeat the rinse several more times. e. Apply vacuum and rapidly filter the suspension. Leave vacuum on until filter is dry. Do not wash down the sides of the funnel after the deposit is in place since this will rearrange the material on the filter. Transfer the filter to the XRD sample mount. Prepare working standard filters in triplicate by this technique at, e.g., 5, 10, 30, 50, 100, 200, 500, 1000, and 2000 µg lead sulfide. f. Analyze by XRD (step 11). Use the same diffraction peaks and instrumental conditions as for samples. Designate the net and normalized XRD intensities for the working standard filters as I ox (step 11.d) and Î ox (step 11.e), respectively. Correct Î ox for matrix absorption for standards containing more than 200 µg PbS (steps 11.f and 12). g. Prepare a calibration graph by plotting Î ox vs. mass PbS (µg). A weighted least squares (I/ σ2) fit is preferable. h. Determine the slope, m (counts/µg), of the calibration graph. The intercept of the line should be zero ±5 µg. NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94