Page:NIOSH Manual of Analytical Methods - 2011.pdf/3

FORMIC ACID: METHOD 2011, Issue 1, dated 15 August 1994 - Page 3 of 4 CALIBRATION AND QUALITY CONTROL: 9.

10.

11.

Calibrate daily with at least six working standards covering the range of 2 to 100 µg formate per sample. a. Spike appropriate amounts of calibration stock solution or a dilution thereof, onto sorbent tubes. b. Desorb in the same manner as field samples. c. Analyze working standards together with samples and blanks (steps 12 through 14). d. Prepare a calibration graph of peak height vs. amount (µg) of formic acid per 10 mL of sample. NOTE: Apply the conversion 1 µg formate = 1.022 µg formic acid. (Optional) Determine recovery for each lot of silica gel used for sampling in the concentration range of interest. Prepare four tubes at each of five levels plus three media blanks. a. Add a known amount of DE stock solution, or a dilution thereof, onto sorbent tube (see Appendix A). b. Analyze in the same manner as field samples. c. Prepare graph of recovery vs. µg formic acid. Analyze three quality control blind spikes and three analyst spikes to ensure that the calibration graph and recovery graph are in control.

MEASUREMENT: 12. 13.

14.

Set ion chromatograph to conditions given on page 2011-1. Inject 50-µL sample aliquot. For manual operation inject 2 to 3 mL of sample from the syringe (through inline filter) to ensure complete rinse of sample loop. NOTE: Flushing the column between injections with 0.1 M Na 2B4O 7·10 H 2O for one minute removes the later eluting ions quickly. Measure peak height. NOTE: If peak height exceeds the linear calibration range, dilute with eluent, reanalyze, and apply the appropriate dilution factor in calculations.

CALCULATIONS: 15.

16.

Read mass, µg of formic acid found on the front (W f) and back (W b) sections and the average media blank front (B f) and (B b) sections from the calibration graph. NOTE: If W b > W f/10, report breakthrough and possible sample loss. Calculate concentration C of formic acid (mg/m 3) in the actual air volume, V (L):

EVALUATION OF METHOD: This method was validated over the range 4.4 to 21.6 mg/m 3 at 23 °C and pressure of 759 mm Hg using 24-L samples [1]. Overall sampling and measurement precision, SˆrT, was 0.073, with average recovery of 96% in the range 4 to 22 mg/m 3 per sample [1]. Sample stability during storage was evaluated at 11 mg/m 3 per sample. Samples showed 91.5% recovery after one week of storage at ambient conditions compared to one-day old samples [1]. Breakthrough did not occur when a 25 mg/m test atmosphere (85% relative humidity) was sampled at 0.18 liter/minute for 300 minutes. This modification of NMAM 7903, which uses 2.5 m M Na 2B4O 7·10 H 2O eluent, resulted in a LOD of 2 µg/sample and a range 7 to 100 µg per sample. Samples were collected on silica gel, desorbed in 10 mL eluent, heated 10 minutes in boiling water, then analyzed.[3]

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94

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