Page:NIOSH Manual of Analytical Methods - 2527.pdf/3

 NITROMETHANE: METHOD 2527, Issue 2, dated 15 August 1994 - Page 3 of 4 CALIBRATION AND QUALITY CONTROL: 8.

9.

10.

Calibrate daily with at least six working standards. a. Add known amounts of calibration stock solution to ethyl acetate in 10-mL volumetric flasks and dilute to the mark. Use serial dilutions as needed to obtain nitromethane concentrations in the range 0.01 to 0.4 mg/mL. b. Analyze with samples and blanks (steps 11 and 12). c. Prepare calibration graph (peak area vs. mg nitromethane). Determine desorption efficiency (DE) at least once for each lot of sorbent used for sampling in the range of interest. Prepare three tubes at each of five levels plus three media blanks. a. Remove and discard back sorbent section of a media blank sampler. b. Inject a known amount (2 to 20 µL) of calibration stock solution 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 with working standards (steps 11 and 12). e. Prepare a graph of DE vs. mg nitromethane 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 2527-1. 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 ethyl acetate, reanalyze and apply the appropriate dilution factor in calculations. Measure peak area.

CALCULATIONS: 13.

14.

Determine the mass, mg (corrected for DE) of nitromethane found in the sample front (W f) and back (W b) sorbent sections, and in the average media blank front (B f) and back (B b) sorbent sections. NOTE: If W b > W f/10, report breakthrough and possible sample loss. Calculate concentration, C, of nitromethane in the air volume sampled, V (L):

EVALUATION OF METHOD: Method S220 was issued January 19, 1979 [2]. Synthetic atmospheres of the analyte in humidified air were generated dynamically at 20 to 24 °C and 767 to 774 torr over the range from 123 to 514 mg/m 3 by calibrated syringe pump injection of the analyte into a dilution airstream [1,4]. The concentrations were monitored by total hydrocarbon analyzer. A 600-mg bed of the sorbent retained 2.6 mg of the analyte before 5% breakthrough occurred at 5.0 L, when a challenge atmosphere of 518 mg/m 3 of analyte in humid air was sampled at 0.05 L/min. There were no statistically significant differences in the recoveries of 3-L samples collected from synthetic atmospheres containing 236 mg/m 3 nitromethane in humid air and stored for one or seven days at ambient temperatures prior to analysis. Desorption efficiency averaged 0.971 in the range 0.34 to 1.34 mg per sample. A previous study showed that samples of nitromethane collected on activated charcoal were unstable [5]. NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94