SULFUR DIOXIDE: METHOD 6004, Issue 2, dated 15 August 1994 - Page 3 of 4 NOTE: The SO 2 collected on the treated (back) filter is present as sulfite, which oxidizes in air slowly (over several weeks) to sulfate. The contributions of sulfite and sulfate found on the back filter must be summed, with appropriate stoichiometric factors applied, to give the SO concentration (step 11). 6. Pour each sample into a syringe fitted with an in-line filter.
2
CALIBRATION AND QUALITY CONTROL: 7. Calibrate daily with at least six working standards. a. Add known aliquots of sulfate calibration stock solution to eluent in 50-mL volumetric flasks and dilute to the mark to produce solutions containing 1 to 20 µg/mL SO 24-. b. Prepare sulfite standards in the same manner over the same range. c. Store working standards in tightly-capped polyethylene bottles. Prepare fresh working standards daily. d. Analyze working standards with samples and blanks (steps 8 through 10). Prepare a calibration graph for each anion [peak height (mm or µS) vs. µg sulfite or sulfate].
MEASUREMENT: 8. Set ion chromatograph to conditions given on page 6004-1, according to manufacturer's instructions. 9. Inject sample aliquot. For manual operation, inject 2 mL of sample from syringe to ensure complete rinse of sample loop. NOTE: All samples, eluents, and water flowing through the ion chromatograph must be filtered to avoid plugging system valves or columns. 10. Measure peak heights of sulfite and sulfate peaks. NOTE: If peak height exceeds linear calibration range, dilute with eluent, reanalyze, and apply the appropriate dilution factor in calculations.
CALCULATIONS: 11. Determine the mass, µg, of sulfate equivalent found on the front (W f) and back (W b) filters and in the corresponding average media blanks (B f and B b). NOTE: The sulfate equivalent is the sum of the sulfate peak, µg, and 1.200 times the sulfite peak, µg, on the chromatogram (1.200 = MW SO 24-/MW SO 23-): µg sulfate equivalent = µg sulfate + 1.200 µg sulfite. 12. Calculate the concentration, C SO2 , of sulfur dioxide, applying the factor 0.667 (MW SO 2/MW SO 24-):
13. Calculate the concentration, C sampled, V (L):
SO4
, of particulate sulfate (including sulfuric acid) in the air volume
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
