Page:NIOSH Manual of Analytical Methods - 2507NIT.pdf/3

 CALIBRATION AND QUALITY CONTROL:

8. Calibrate daily with at least five working standards over the range 1 to 45 μg NG or EGDN per sample.


 * a. Add known amounts of calibration stock solution to ethanol in 10-mL volumetric flasks and dilute to the mark.


 * b. Analyze together with samples and blanks (steps 11 and 12).


 * c. Prepare calibration graph (peak area vs. μg NG or EGDN).

9. Determine desorption efficiency (DE) at least once for each lot of Tenax-GC 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 DE 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 together with working standards (steps 11 and 12).


 * e. Prepare a graph of DE vs. μg NG or EGDN recovered.

10. Analyze three quality control blind spikes and three analyst spikes to insure 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 2507-1. 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 with ethanol, reanalyze and apply the appropriate dilution factor in calculations.

12. Measure peak area.

CALCULATIONS:

11. Set gas chromatograph according to manufacturer's recommendations and to conditions given on page 2507-1. 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 with ethanol, reanalyze and apply the appropriate dilution factor in calculations.

12. Measure peak area.

CALCULATIONS: 13. Determine the mass, μg (corrected for DE) of NG or EGDN 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 Wb > Wf/10, report breakthrough and possible sample loss.

14. Calculate concentration, C, of NG or EGDN in the air volume sampled, V (L):

EVALUATION OF METHOD:

Method S216 [3] was issued on November 11, 1975, and validated over the range of 0.56 to 3.2 mg/m 3 for nitroglycerin and 0.51 to 1.8 mg/m 3 for ethylene glycol dinitrate [2]. Eighteen 15-L samples for each analyte were collected from dynamically-generated test atmospheres. Eighteen additional samples for each analyte were spiked (six each at one-half, one and two times the OSHA standard) directly. The average recovery for all three levels for nitroglycerin was 97.6% and had a pooled precision ( Sr) of 0.051. The average recovery for the three levels for ethylene glycol dinitrate was 92.0% with a pooled precision ( Sr) of 0.063. Since the collection efficiency of the Tenax-GC for both compounds was established as 100%, the corrected "found" value of the generated sample was taken as the measure of the generated air concentrations. Breakthrough for each compound was evaluated at a concentration twice the OSHA standard. Duplicate samples were collected at interim periods of 60, 120, and 240 min at 1.1 L/min. Breakthrough of EGDN occurred at some point between 130 and 257 L of air sampled.