ASTM E800 Guide for Measurement of Gases Present or Generated During Fires
10. Analytical Methods for Total Hydrocarbons and Aldehydes
10.1 General Comments:
10.1.1 Numerous organic compounds may be generated during pyrolysis and combustion of materials. Identification and quantification of each individual compound in such a mixture is difficult, and beyond the scope of "total hydrocarbons" analysis as described below. However, analytical studies have been reported (85, 86, 87) on the use of gas chromatography and mass spectrometry for the identification of complex mixtures obtained by thermal degradation of polymers.
10.1.2 Determination of "total hydrocarbons" and analysis of certain compounds of interest have become commonplace. "Total hydrocarbons" includes many compounds containing oxygen, nitrogen, sulfur, or halogen, in addition to those comprised solely of carbon and hydrogen. Methods for total hydrocarbon analysis, including gas chromatography and infrared, will be presented. Then, the specific techniques used to measure aldehydes will be described.
10.2 Measurement of Total Hydrocarbons:
10.2.1 General Description - The most widely used methods for measuring total hydrocarbons are those relying on a flame ionization response. Such methods use either a flame ionization detector (FID) alone or in combination with a catalytic converter or gas chromatograph (25). Another method for analyzing total hydrocarbons is based on nondispersive infrared (NDIR).
10.2.2 Apparatus and Procedure:
10.2.2.1 In the flame ionization approach, the organic sample is ionized in a hydrogen flame. The FID analyzers are usually calibrated in terms of a gas, such as methane or hexane, and the output read in parts per million of carbon. Sampling is continuous in some commercial analyzers. Sample pretreatment recommendations and cautions (5.9) should be followed, and proper maintenance to the system (5.11) is important.
10.2.2.2 In order to detect the non-methane portion of total hydrocarbons, two methods based on a selective catalytic combustor were reported by King (88). A dual FID analyzer has also been reported (89) to continuously monitor the methane and non-methane components in ambient air.
10.2.2.3 In the gas chromatographic approach to total hydrocarbon analysis, a predetermined volume of sample is introduced. As the various components emerge from the column, they are detected by an FID.
10.2.2.4 The nondispersive infrared (NDIR) technique is based on the absorption characteristics of the gases to be analyzed. If methane or ethane is used as the detector gas, unsaturated compounds will not be adequately measured. Mixtures of hydrocarbon gases for the detector have not been commonly employed.
10.2.3 Advantages and Limitations:
10.2.3.1 Most of the redaily available commerical instruments for measuring total hydrocarbons are based on an FID detector. Separation of methane and non-methane portions, while important for air pollution studies, is usually unnecessary for fire-safety studies. The response of the FID detector constitutes an approximate counting of carbon atoms. This relation, however, fails to hold for oxygenated molecules and certain aromatic and unsaturated hydrocarbons (104). The latter may be a limitation since it has been found (105) that even for fires of non-oxygenated fuels 1/4 to 1/2 of the unburned total hydrocarbons can comprise oxygenated species and a significant further fraction can comprise unsaturated hydrocarbons.
10.2.3.2 For NDIR, removal of water will avoid its interferences during analysis of combustion gases. The infrared spectral properties of hydrocarbons vary considerably so that an NDIR signal roughly proportional to hydrocarbon concentration can be obtained if the combustion-product sample stream contains hydrocarbons of only a single class. Quantitative results cannot be expected for a mixture of hydrocarbons of unspecified structure.
10.3 Measurement of Aldehydes:
10.3.1 General Description - Various materials produce aldehydes when involved in fires. Morikawa (90) studied evolution of formaldehyde and acrolein from wood, polyethylene, polypropylene, cellulose, glucose, and eight other materials. He used a colorimetric method for the determination of formaldehyde and gas chromatography for acrolein. These are the most common techniques.
10.3.2 Apparatus and Procedures:
10.3.2.1 The gas chromatographic approach utilizes packed columns and either a flame ionization detector (FID), a thermal conductivity detector, or mass spectrometer. There are a number of columns suitable for formaldehyde and acrolein. A number of these have been recently reported (90, 91). Sixteen liquid phases, used before 1967, are listed in an ASTM publication (92).
10.3.2.2 The FID is not sensitive to formaldehyde. When the concentration is high enough, a thermal conductivity detector can be used. However, water interference could be a problem in this case. Formaldehyde can be converted to another organic compound that is sensitive to FID (93, 94). For the purpose of increasing the concentration of aldehydes in sample gas, or to stabilize labile compounds, a short precolumn was used in cigarette smoke analysis (95, 96). Aldehydes were trapped and released by changing the temperature of the precolumn.
10.3.2.3 A colorimetric method specific for acrolein has been reported (97). It is based on a chemical reaction to produce a blue-colored product. Other aldehydes do not react to form products absorbing significantly in the same spectral region.
10.3.2.4 Instruments have also been developed for analysis of formaldehyde by gas filter correlation (see 7.4.2).
10.3.3 Advantages and Disadvantages - Analysis of aldehydes is, in general, difficult and must be performed with great care. Standardization is a major problem due to instability of the pure aldehydes.
