Page:NIOSH Manual of Analytical Methods - 5011.pdf/3

ETHYLENE THIOUREA: METHOD 5011, Issue 2, dated 15 August 1994 - Page 3 of 4 5. 6.

7.

Pipet 7.0 mL distilled water into the vial to completely immerse the filter. Cap the vial. Place vials in a 60 °C waterbath (thermostatically controlled) for 45 min. The waterbath level must be above the water level in the vial. Shake each vial every 5 min. NOTE: Do not use an ultrasonic bath, because it breaks up the filters. Lift the filter with tweezers above the water level in the vial. Wash the filter two times with 4-mL aliquots of water using a 5-mL adjustable pipette. Collect the rinsings in the vial. Discard the filter.

CALIBRATION AND QUALITY CONTROL: 8.

9.

Calibrate daily with at least six working standrds over the range of 0 to 150 µg/mL. a. Prepare a 15 µg/mL ETU standard solution by pipetting 3.0 mL of the 1000 µg/mL calibration stock solution into a 200-mL volumetric flask and diluting to the mark with distilled water. b. Prepare working standards by pipetting 0- to 10-mL aliquots of the 15 µg/mL standard solution into clean vials. Bring the total volume to 15 mL with distilled water. Follow steps 10 and 11 to analyze the working standards. c. Prepare a calibration graph (absorbance vs. µg per sample). Determine recovery at least once for each lot of filters used. a. Place eight filters on a plastic test tube rack. Using an adjustable pipette, add to the center of each filter 0, 7.5, 15, 30, 45, 60, 90, 120, and 150 µL of 1000-µg/mL calibration stock solution. Let filters air dry overnight at room temperature. Extract (steps 4 through 7) and analyze (steps 10 and 11). b. Convert the absorbance of each sample to µg from the calibration graph. Calculate recovery, R (µg found/µg taken).

MEASUREMENT: 10.

11.

Complexation. NOTE: Perform this step at the same time for both standards and samples to reduce errors due to color degradation. a. Pipet a 1.5-mL aliquot of dilute complexing reagent into each vial. b. Allow the vials to stand for at least 30 min before analysis to insure full color development. Shake the vials every 10 min. NOTE: The color of the complexed samples varies with increasing concentration from yellow to light green to turquoise. Very high concentrations have Prussian blue color; dilute these with distilled water and use the appropriate dilution factor in the calculations. Measurement. a. Transfer the solution to a clean 5-cm cuvette. Wipe cuvette with a lens paper to remove any droplets on the cuvette windows. b. Place the cell in the sample compartment and measure absorbance at 590 nm vs. a reference sample (15 mL distilled water and 1.5 mL dilute complexing reagent) in a 5-cm cuvette. Record the absorbance for each sample. NOTE: Scan the absorbance of the bulk sample (dissolve several mg of bulk sample in 15 mL water and treat as in steps 10 and 11) in the range 350 to 700 nm to detect possible interferences.

CALCULATIONS: 12.

Determine the mass of ETU, µg (corrected for recovery), for the sample (W) and average media blank (B).

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94