METHYLAL: METHOD 1611, Issue 2, dated 15 August 1994 - Page 3 of 3 a. b.
10.
Remove and discard back sorbent section of a media blank sampler. Inject a known amount (1 to 20 µL) of pure methylal or a standard solution of methylal in hexane directly onto front sorbent section with a microliter syringe. 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 methylal recovered. Analyze three quality control blind spikes and three analyst spikes to ensure that the calibration graph and DE graph are in control.
MEASUREMENT: 11.
12.
Set gas chromatograph according to manufacturer's recommendations and to conditions given on page 1611-1. 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 hexane, reanalyze, and apply the appropriate dilution factor in calculations. Measure peak area.
CALCULATIONS: 13.
14.
Determine the mass, mg (corrected for DE) of methylal found in the sample front (W f) and back (W b) sorbent sections, and in the average media blank front (B b) and back (B b) sorbent sections. NOTE: If W b > W f/10, report breakthrough and possible sample loss. Calculate concentration, C, of methylal in the air volume sampled, V (L):
EVALUATION OF METHOD: Method S71 [2] was issued on February 14, 1975, and validated over the range 1684 to 6670 mg/m 3 at 22 °C using 2-L air samples [1]. Samples of methylal in dry air were generated using a three-neck round-bottom generator flask containing the liquid methylal. An accurately measured flow of nitrogen gas was used to sweep the vapors out of the flask. Overall precision, s r, was 0.060 with average recovery of 104%. Desorption efficiency was 78, 81, and 88%, respectively, at 3.1, 6.2, and 12.3 mg methylal taken per sample. Breakthrough (5% on back section) occurred at 20 min when sampling an atmosphere containing 6250 mg/m 3 methylal at 0.183 L/min at 0% RH. Breakthrough at high humidity was not tested, nor was sample stability.
REFERENCES: [1]
[2]
Documentation of the NIOSH Validation Tests, U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 77-185 (1977), available as GPO Stock #017-033-00231-2 from Superintendent of Documents, Washington, DC 20402. NIOSH Manual of Analytical Methods, 2nd ed., Vol. 2, S71, U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 77-157-B (1977).
METHOD REVISED BY: Ardith A. Grote, NIOSH/DPSE; S71 originally validated under NIOSH Contract CDC-99-74-45. NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
