Page:NIOSH Manual of Analytical Methods - 6602.pdf/2

 SULFUR HEXAFLUORIDE by portable GC: METHOD 5512, Issue 2, dated 15 August 1994 - Page 2 of 4 REAGENTS: 1.

2.

EQUIPMENT:

Sulfur hexafluoride in air at known concentration(s) for use in field calibration. Prepared in the field by filling sample bag (see "Equipment") with verified commercial standard compressed gas*, or by injecting known amount of pure (or high concentration) SF 6* into sample bag containing a metered volume of clean air. Compressed argon or other gas* at approximately 30 psi for use as carrier gas.



See SPECIAL PRECAUTIONS.

1.

Portable gas chromatograph (GC), with electron capture detector, gas sampling loop if desired (use of loop will limit variability of injection volume and therefore restrict range of method), and strip chart recorder if appropriate. 2. Personal sampling pump, 0.02 to 1 L/mim or other rate suitable for filling sample bags, with flexible connecting tubing. NOTE: Pumps lubricated with petroleum products should not be used. 3. Sample bags, 1 to 20 L or other appropriate sizes. Sources include: Calibrated Instruments, Inc., 200 Saw Mill River Rd., Hawthorne, NY 10532; SKC, Inc., 334 Valley View Rd., Eighty-Four, PA 15330-9614. 4. Syringes, gas-tight, of various sizes appropriate to the GC. NOTE: To reduce the possibility of contamination, use separate, previously unused syringes for working standards and samples. Test syringes for contamination occasionally by filling them with clean air and analyzing the contents. 5. Label tape and marking pen for labeling bags.

SPECIAL PRECAUTIONS: Shipment of radioisotopes, calibration materials, and compressed gases must comply with 49CFR171-177 regulations regarding shipment of hazardous materials. Appropriate safety precautions should be followed when working with compressed gases. SAMPLING AND MEASUREMENT: 1.

2.

Start GC and recorder and allow to warm up according to manufacturer's instructions. NOTE: A stable standing current and stable baseline should be attained at the highest sensitivity likely to be used. Select either or both of the following sampling modes: a. Spot sample - Draw air sample into gas sampling loop of the GC with the on-board sampling pump, if supplied. Alternatively, inject an aliquot of air to be sampled into the GC with a gas-tight syringe. NOTE: A potential source of random error in the method is imprecision of replicate injections. To reduce this error: i) use a gas sampling loop for injections; ii) make at least three replicate determinations per sample; iii) use an injection volume precisely readable, and consistent with that used in calibration. b. Integrated air sample for time weighted average (TWA) determination- Attach a clean sample bag to the outlet port of a personal sampling pump with a minimum length of flexible tubing. NOTE 1: To reduce memory effects and contamination, sample bags should be purged with clean air between samples. Ideally the bags should be filled with clean air prior to use, and an aliquot of that air from the bag injected into the GC to demonstrate that the bag is indeed clean prior to use. Pump the air sample into the bag at a rate calculated to fill <80% of the bag capacity over the desired NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94