Page:NIOSH Manual of Analytical Methods - 6602.pdf/3

 SULFUR HEXAFLUORIDE by portable GC: METHOD 5512, Issue 2, dated 15 August 1994 - Page 3 of 4 sampling period. While it is not necessary to know precisely the sample flow rate, it is important that the flow rate be constant over the duration of the sample. Within eight hours after completion of sampling, introduce an aliquot of the sample into the GC (as in step 2a above). NOTE 3: Bag samples may be stored for longer periods if integrity has been demonstrated in the bag being used. Obtain the peak height or area for the injected sample of sulfur hexafluoride. NOTE 2:

c.

CALIBRATION AND QUALITY CONTROL: 4.

5.

Perform the following in the laboratory before field work begins: a. Establish a laboratory calibration graph by at least three replicate determinations of at least five analyte volumes. Plot peak height (or area) vs. volume of sulfur hexafluoride. b. Determine detector drift, averaged over the time period expected to be used in the field. c. Determine the ability of the GC column to separate the sulfur hexafluoride from other substances known or predicted to be present in the field samples. Establish a daily field calibration graph of peak height (or area) vs. volume of sulfur hexafluoride by triplicate determinations of working standards under the same conditions as for samples (step 2a). Alternate analyses of samples and working standards, if possible.

CALCULATIONS: 6.

Calculate volume of analyte in the sample, V a (expressed in picoliters), by comparison of sample peak height or area with the daily calibration graph (step 5). Determine sulfur hexafluoride concentration, C (ppb) in the sample by dividing V a by the volume of the injection, Vi (mL):

NOTE 1: NOTE 2:

Some GCs will perform some or all calculations electronically.] Determination of sulfur hexafluoride concentration expressed in pg/mL (i.e., mg/m 3) can be accomplished by establishing lab and field calibration graphs (Steps 4 and 5) of peak height or area vs mass of analyte in mg. "V a" in Step 6 would then become "M a", mass of analyte.]

EVALUATION OF METHOD: This method was evaluated over the range 0.1 to 200 ppb (0.0006 to 1.2 pg/m 3) sulfur hexafluoride using an Ion Track Instruments (ITI) Model 505 portable SF 6 detector chromatograph modified by the addition of an injection port to allow syringe injections of samples. Mixtures of SF 6 in air were obtained from Scott Specialty Gases, Inc. at concentrations certified (±5%) of 1.02, 10.5, 36.6, 77.6, 105, 155, and 203 ppb. (The 10.5 ppb standard was diluted 1:100 to obtain a 0.105 ppb standard.) Calibration standards were prepared by dilution of sulfur hexafluoride (99.8% minimum purity, Union Carbide) with air from the laboratory compressed air supply. This dilution air was passed through activated charcoal to remove traces of hydrocarbons, but was tested prior to use since trace amounts of SF 6 may not be removed by the charcoal. The SF 6 was measured with the same type of gastight syringe used for sample injection, but the syringes used for the preparation of standards were not used for injections into the GC since they were shown to retain traces of SF 6 which would show up in such injections. The NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94