Page:NIOSH Manual of Analytical Methods - 3700.pdf/2

 BENZENE by portable GC: METHOD 3700, Issue 2, dated 15 August 1994 - Page 2 of 4 REAGENTS:

EQUIPMENT:

1. Benzene* in air, working standards prepared in the field by filling Tedlar bags with commercially prepared and certified standards (preferred) or prepared in the field by injecting known amounts of pure benzene into Tedlar bags containing a metered volume of pure air or nitrogen. 2. Cylinder of air, nitrogen or helium for use as carrier gas and field blanks.*



See SPECIAL PRECAUTIONS.

1. Portable gas chromatograph (GC), with photoionization detector, preferably with gas sampling loop and (if appropriate) strip chart recorder. 2. Personal sampling pump, 0.02 to 5 L/min or other rate suitable for filling sample bag, with flexible connecting tubing. NOTE: Pumps lubricated with petroleum products should not be used. 3. Sample bags, Tedlar, 1- to 20-L or other appropriate sizes. 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: Benzene is a suspect carcinogen [3,4]. Shipment of compressed gases must comply with 49 CFR 171-177 regulations regarding shipment of hazardous materials. SAMPLING: 1.

2.

Start GC instrument and recorder and allow to warm up according to manufacturer's instructions. NOTE: A straight baseline should be attained at the highest sensitivity likely to be used. Select one of the following sampling modes: a. Spot sample. Draw air sample into the 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 large contributor to random error in the method is imprecision of replicate injections. To improve precision: (1) use a gas sampling loop for injections, if available; (2) make at least three replicate determinations per sample; (3) use an injection volume large enough to be precisely readable, and consistent with that used in calibration. b. Integrated air sample for TWA determination. (1) Evacuate a clean sample bag using the inlet port of a personal sampling pump. NOTE: To reduce memory effects and contamination, use only previously unused sample bags. Alternatively, purge used bags at least two times with clean air, then fill with clean air and analyze for benzene as in step (4) below. (2) Attach the sample bag to the outlet port of a personal sampling pump with a minimum length of flexible tubing. To the inlet port of the pump attach a length of Teflon tubing sufficient to reach the desired site. Alternatively, a rubber squeeze bulb can be used in place of a personal sampling pump when sample size is small. In either case, tubing and pump/bulb should be flushed with sample air prior to, and clean air subsequent to, sample collection.

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