Page:NIOSH Manual of Analytical Methods - 2000.pdf/3

 METHANOL: METHOD 2000, Issue 3, dated 15 January 1998 - Page 3 of 4 in the calibration range (step 8). Prepare three samplers at each of six levels plus three media blanks. a. Remove and discard back sorbent sections of samplers and media blanks. b. Inject a known amount of calibration stock solution directly onto the front sorbent bed of each silica gel 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 working standards and blanks (steps 11 and 12). e. Prepare a graph of DE vs. µg analyte 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 2000-1. 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 water/IPA (95/5), reanalyze and apply the appropriate dilution factor in the calculations. 12. Measure peak areas. CALCULATIONS: 13. Determine the mass,µg (corrected for DE), for methanol found in the sample front (W f) and back (Wb) sorbent sections, and in the average media blank front (B ) and back (B )sorbent sections. f b NOTE: If Wb > Wf/10, report breakthrough and possible sample loss. 14. Calculate concentration, C, of methanol in the air volume sampled, V (L)

C

NOTE: µg/L

(Wf

Wb

Bf V

Bb)

, mg/m 3

mg/m3

EVALUATION OF METHOD: The method was reevaluated and revised. Improvements were made to enhance the recovery of methanol from silica gel, to improve baseline resolution, and to improve the precision of the method. Desorption efficiency (DE) was determined for three levels using the improved analytical parameters stated on page 2000-1. The average DE for methanol wasdetermined at 20 µg (0.942), 40 µg (0.945), and 79 µg (0.942) [1]. The LOD was determined to be 0.7 µg/sample [1]. The precision, as determined from the pooled relative standard deviation ( r) was determined to be 0.030 [1]. The original method S59 [3] was issued on January 17, 1975 and was validated over the range of 140 to 540 mg/m3 at 25 C and 758 mm Hg with 5-L sample volumes [2]. The estimated LOD determined for a user check was 0.010 mg/sample [5]. Overall precision ( rT) was determined to be 0.063 with an average recovery for 18 samples of 94.2%, representing non-significant bias. The atmospheres were generated by calibrated syringe pump. Desorption efficiency (DE) was determined to be 0.92 at 1.3 mg methanol and 0.84 at 0.65 mg methanol per sample. The breakthrough volume was 10.4 L (5% on back section) when sampling 3 a methanol atmosphere containing 541 mg/m in dry air at 0.2 L/min for 52 min. Data gathered using larger silica gel tubes (700 mg front section) under conditions of high humidity were published in P&CAM 247 [4]. The data are consistent with a recommended maximum sample volume of 5 L. REFERENCES: NIOSH Manual of Analytical Methods (NMAM), Fourth Edition