BENZENE by portable GC: METHOD 3700, Issue 2, dated 15 August 1994 - Page 3 of 4
3.
NOTE: Tygon tubing has been shown to adsorb some materials in complex hydrocarbon mixtures and to off-gas during later sample collection causing an increase in hydrocarbon concentration in those subsequent samples. (3) Pump the air sample into the bag at a rate calculated to fill ≤80% of the sample bag capacity over the sampling period. NOTE: The flow rate must be constant throughout the sampling period. (4) Within 4 h after completion of sampling, introduce an aliquot of the sample into the GC (as in step 2.a). Obtain the benzene peak height or area of the injected sample.
CALIBRATION AND QUALITY CONTROL: 4. a. b. c. 5.
Perform the following in the laboratory before field work begins: Establish a laboratory calibration graph with at least three replicate determinations of at least six working standards. Plot peak height or area vs. mass or concentration of benzene. Determine detector drift, averaged over the time period(s) expected to be used in the field. Determine the ability of the GC column to separate the benzene peak from other substances known or predicted to be present in the field samples. Establish a daily field calibration graph (peak height vs. mass or concentration of benzene) with triplicate determinations of working standards under the same conditions as for samples (step 2.a). Alternate analyses of samples and working standards, if possible.
CALCULATIONS: 6. Calculate mass, W (ng), of benzene in sample by comparison of sample peak height with daily calibration graph (step 5). Determine concentration, C, of benzene in the injected sample, V (mL):
NOTE: Some GCs will perform this calculation electronically.
EVALUATION OF METHOD: This method was evaluated over the range 0.03 to 100 ppm benzene using a Photovac 10S portable GC. Certified standard gas mixtures of benzene in air were obtained from Scott Specialty Gases Inc., and these were used to establish the calibration graph. Once this graph was established, bias was assessed by analyzing a bag sample containing an unknown concentration and comparing the concentration obtained with that obtained from analysis of replicate charcoal tube sample taken from the same bag. Additionally, a field evaluation was subsequently conducted in which results obtained using this method were compared with results obtained using conventional laboratory GC analysis and with results obtained using laboratory GC/MS analysis. Instruments used in the field evaluation include the Photovac 10S, the Sentex Scentoscreen and the MSI-301B by Microsensor Systems, Inc. Previous issues of this method discussed the use of portable GC with ambient temperature packed columns. That technique was considered to be limited in its ability to separate the analyte from any interferences present. Evaluation of this method, which uses a capillary column, indicates that benzene can be separated from butane, styrene, toluene and xylenes, and that a response corresponding to benzene can be identified in such complex mixtures as gasoline and diesel vapors [1].
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
