Page:NIOSH Manual of Analytical Methods - 1005.pdf/3

This page needs to be proofread.

METHYLENE CHLORIDE: METHOD 1005, Issue 3, dated 15 January 1998 - Page 3 of 4 b. Analyze together with samples and blanks (steps 11 and 12). c. Prepare calibration graph (peak area or height vs. µg methylene chloride). 9. Determine desorption efficiency (DE) at least once for each lot of charcoal tubes used for sampling in the calibration range (step 8). Prepare three samplers at each of 6 levels plus three media blanks. a. Remove and discard back sorbent sections of sampler and media blanks. b. Inject a known amount of methylenechloride directly onto the front sorbent bed of each charcoal tube. c. Allow the tubes to air equilibrate for several minutes, then cap the ends of the tubes and allow to stand overnight. d. Desorb the samples(steps 5 through 7) and analyze together with standards and blanks (steps 11 and 12). e. Prepare a graph of DE vs. µg methylene chloride recovered. 10. Analyze three quality control blind spikes and three analyst spikes to ensure that the calibration graph and DE graphs are in control.

MEASUREMENT: 11. Set gas chromatograph according to manufacturer’s recommendations and to conditions given on page 1005-1. Inject a 1-µL sample aliquot manually using solvent flush technique or with an autosampler. NOTE: If peak area is above the linear range of the working standards, dilute with appropriate solvent, reanalyze and apply the appropriate dilution factor in the calculations. 12. Measure peak areas. CALCULATIONS: 13. Determine the mass,µg (corrected for DE), for methylene chloride found in the sample front (W f) and back (Wb) sorbent sections, and in the average media blank front (B f) and back (Bb)sorbent sections. NOTE: If Wb > Wf/10, report breakthrough and possible sample loss. 14. Calculate concentration, C, of methylene chloride in the air volume sampled, V (L):

C

(Wf

Wb

Bf V

Bb)

, mg/m 3

NOTE: µg/mL mg/m3

EVALUATION OF METHOD: The method was originally evaluated for methylene chloride (Method S329) and validated over the range 1700 to 7100 mg/m3 at 25 C and 763 mm Hg using a 1-L sample [2, 7]. Overall precision (rT) using flame ionization detection was 0.073 with a recovery of 95.3% (non-significant bias). The concentration of methylene chloride was independently verified by calibrated syringe pump. Desorption efficiency was 0.97 over the range of 1.3 mg to 5.3 mg methylene chloride per sample. Breakthrough (5% on backup section) 3 occurred at 18.5 min when sampling an atmosphere containing 6726 mg/m methylene chloride at 0.187 L/min at 0% RH. The stability of methylene chloride on charcoal was not determined in the original method development. Method 1005 was evaluated for methylene chloride using electron capture detection (ECD)[3]. Toluene was determined to be the best desorption solvent for use with ECD. Desorption efficiency was determined to be 75% at a 230 ng spiking level. After 30 days storage at 5 C, the stability of 320 ng methylene chloride samples was determined to be 87.3% [3]. The NIOSH storage stability criterion of90% recovery was within the 95% confidence interval of 80.9 to 93.7%.

REFERENCES: NIOSH Manual of Analytical Methods (NMAM), Fourth Edition

This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.