ASTM E800 Guide for Measurement of Gases Present or Generated During Fires
6. Analytical Methods for Carbon Monoxide, Carbon Dioxide, Oxygen, and Nitrogen
6.1 The gases carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2), and nitrogen (N2) will be considered as a group, since several of the analytical methods to be discussed can be applied to more than one of them, sometimes simultaneously. The techniques to be described are gas chromatography, infrared spectrophotometry, and "other methods" including electrochemistry.
6.2 Gas Chromatography:
6.2.1 General Description - Gas chromatography is an ideal batch method for analyzing nonreactive gases in combustion products (25). These gases can be separated on columns with solid stationary phases operated isothermally and detected using thermal conductivity (TC) detectors. Some of the column configurations and alternative detectors are described below.
6.2.2 Apparatus and Procedures:
6.2.2.1 Apparatus requirements are modest. A basic gas chromatograph with standard temperature controls and thermal conductivity detector can be used. A gas sampling valve is a very useful accessory. Temperature programming, automated valve operation, electronic integration, etc., are convenient but not necessary.
6.2.2.2 Complete separation of all of these gases normally requires the use of two columns - a molecular sieve, which separates O2, N2, and CO but irreversibly absorbs CO2 at normal operating temperatures; and a porous polymer column which readily separates CO and CO2 from air but does not resolve O2 and N2. The two columns have been used together, in various configurations and with column-switching valves, to achieve complete separation of the gases (26).
6.2.2.3 An arrangement, using dual columns and a column-switching valve, has been successfully used to analyze O2, N2, CO, and CO2 gases (27). Total analysis time was approximately 15 min.
6.2.2.4 Concentric single columns, consisting of an inner and an outer column of different packing, are also available (28). These will separate O2, N2, CO, and CO2 in a single pass. The use of such columns eliminates the column-switching valve required in the dual-column arrangement; however, their use to date has been limited.
6.2.2.5 The sensitivity of the gas chromatographic method depends on sample size, the type of detector, and temperature and filament current for TC detectors. Thermal conductivity detector filaments will deteriorate if large air samples are repeatedly measured at high current. These gases can be measured at concentrations as low as 0.05 %.
6.2.2.6 Lower concentrations of CO can be detected by converting CO to methane (CH4) by catalytic hydrogenation (29). The CH4 is then detected, using a flame ionization detector (FID).
6.2.3 Advantages and Disadvantages:
6.2.3.1 The major limitation of gas chromatography for monitoring combustion products is its inherent restriction to batch sampling, since each analysis requires several minutes to complete. Therefore, only a limited number of points can be obtained during a test. However, samples can be collected, intermittently during a run, in suitable gas-tight containers (for example, syringes with close-off valves or gas sampling bags) and the contents analyzed at a later time. The relative nonreactivity of these gases allows them to be stored for extended periods of time before analysis.
6.2.3.2 The gradual build-up of organic pyrolysis and combustion products in the analytical columns may result in eventual degradation of performance. When this occurs, columns can be purged overnight at elevated temperatures or back-flushed; however, after a long period of use, it may be necessary to replace the column.
6.3 Infrared Analysis:
6.3.1 General Description:
6.3.1.1 Infrared (IR) methods are useful for continuously monitoring the concentration of CO or CO2 in fire gases. Symmetric diatomic molecules, such as oxygen and nitrogen, cannot be detected because they are infrared inactive.
6.3.1.2 Infrared analysis is based on absorption of radiation at specific wavelengths when the species of interest is present. By varying the length of the sample cell, gas concentrations from a few parts per million up to 100 % can be analyzed.
6.3.2 Apparatus and Procedures:
6.3.2.1 A standard (dispersive) infrared spectrophotometer can be used to measure CO or CO2 by operating with the monochromator fixed at a particular wavelength; or a conventional infrared spectrum of the gas mixture can be obtained.
6.3.2.2 A nondispersive infrared (NDIR) analyzer continuously monitors a single wavelength or wavelength band (30, 31). Such instruments are often less expensive than dispersive instruments; however, they are restricted to a particular wavelength or chemical species. (See Test Method D3162.)
6.3.3 Advantages and Disadvantages:
6.3.3.1 Interferences can occur in infrared analyses when absorption bands of other components in the sample overlap the absorption band of the compound being analyzed. The magnitude of the interference is highly dependent on the specific instrument and on the relative concentrations of the gases.
6.3.3.2 The major interferences found are of CO for CO2 and vice versa. For most applications, CO interference with CO2 analysis is minor. The interference of CO2 with a CO measurement can be reduced (if necessary) by incorporating a trap (for example, soda-lime or granular lithium hydroxide (LiOH)) to remove CO2 from the sample stream before reaching the analyzer.
6.3.3.3 Water vapor can interfere with CO2 analysis; however, this is not usually a problem. If necessary, a moisture trap in-line can reduce this interference (see 5.9.3). Smoke particulates must be filtered out (see 5.9.2).
6.3.3.4 The instrument readings will be affected by the total gas pressure in the measuring cell. This arrangement is usually adequate if the measuring cell is vented to ambient conditions.
6.4 Other Methods:
6.4.1 General Description - Electrochemical techniques are available for measuring CO and O2 (32), but not for CO2. Such devices are usually designed for air pollution or stack gas monitoring. A standard technique for CO involves oxidation in an electrolytic cell. Techniques for measuring oxygen include galvanic cells, polarographic analyzers, and paramagnetic analyzers.
6.4.2 Advantages and Disadvantages - All of these methods can be accurate and specific, but have slower response than the IR methods previously described. Accurate measurement of oxygen concentration with a paramagnetic analyzer requires compensation for the effects of measuring cell pressure.
