DIFLUORODIBROMOMETHANE: METHOD 1012, Issue 2, dated 15 August 1994 - Page 3 of 4
b. Allow standards to come to room temperature, and dilute to the mark. c. Analyze together with samples and blanks (steps 11 and 12). d. Prepare calibration graph (peak area vs. mg difluorodibromomethane). 9. Determine desorption efficiency (DE) at least once for each lot of charcoal used for sampling in the calibration range (step 8). Prepare three tubes at each of five concentrations plus three media blanks. a. Remove and discard back sorbent section of a media blank sampler. b. Inject a known amount of cold (<0 °C) difluorodibromomethane directly onto front sorbent section with a microliter syringe. NOTE 1: Perform this step in a cold room or cold box. NOTE 2: For small quantities (0.4 to 4 mg), use 2 to 20 µL of a 200 mg/mL standard solution of difluorodibromomethane in pentane. c. Cap the tube. Allow to stand overnight. d. Desorb (steps 5 through 7) and analyze together with working standards (steps 11 and 12). e. Prepare a graph of DE vs. mg dibromodifluoromethane recovered. 10. Analyze three quality control blind spikes and three analyst spikes to ensure that the calibration graph and DE graph are in control. MEASUREMENT: 11. Set gas chromatograph according to manufacturer’s recommendations and to conditions given on page 1012-1. Inject sample aliquot manually using solvent flush technique or with autosampler. NOTE: If peak area is above the linear range of the working standards, dilute an aliquot of the desorbed liquid with 2-propanol, reanalyze, and apply the appropriate dilution factor in calculations. 12. Measure peak area. CALCULATIONS: 13. Determine the mass, mg (corrected for DE) of difluorodibromomethane found in the sample front ( ) and back ( ) sorbent sections, and in the average media blank front ( ) and back ( ) sorbent sections. NOTE: If > /10, report breakthrough and possible sample loss. 14. Calculate concentration, , of difluorodibromomethane in the air volume sampled, (L): , mg/m³. EVALUATION OF METHOD: Method S107 was issued on April 11, 1975, and evaluated at 470, 932, and 1876 mg/m³ [1]. Spiking of samples and standards preparation were done in a cold room (0 °C). Samples of difluorodibromomethane in air were generated and collected on activated coconut charcoal (SKC Lot 105). The air concentration was independently determined by gas chromatographic analysis using a 5-mL sampling loop and comparison to gas bag samples. The mean desorption efficiency over the range 4.4 to 18.5 mg difluorodibromomethane per sampler was 99.5%. The breakthrough volume for 100-mg charcoal beds was 15.6 L at 1875 mg/m³, low relative humidity, and a flow rate of 0.2 L/min. No storage stability study was done.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition
