ASTM E800 Guide for Measurement of Gases Present or Generated During Fires
9. Analytical Methods for Nitrogen Oxides, Sulfur Oxides, and Carbonyl Sulfide
9.1 Oxides of nitrogen include nitric oxide (NO) and nitrogen dioxide (NO2). Fire gases contain mostly NO; NO2 formation is usually a secondary oxidation process. Sulfur oxides include sulfur dioxide (SO2) and sulfur trioxide (SO3), the former being the more prevalent in fire gases generated from sulfur-containing materials.
9.2 Nitrogen Oxides:
9.2.1 Chemiluminescence:
9.2.1.1 General Description - Chemiluminescence is the principle of operation of several process analyzers for nitrogen oxides. Either NO or total nitrogen oxide content (NOx) can be measured. The amount of NO2 is calculated from the difference between NO and NOx.
9.2.1.2 Sampling is continuous in either the NO or NOx mode; however, sampling volumes are large. Samples of NO cannot be stored for later analysis. Both NO and NO2 measurements can be obtained simultaneously in an electrochemical-type analyzer equipped with two cells.
9.2.1.3 Interferences - Significant interference from other nitrogen-containing compounds such as HCN, can occur if high temperature (760°C) thermal converters are utilized for the NOx determination (77). This has been eliminated, in most of the instruments, by using a catalyst at lower temperatures. The sensitivity to non-NOx nitrogen-containing species potentially present in the samples of interest should be established before the unit is used for making measurements. It appears that water and CO2 may act as third-body quenchers (78) and H2 can be a source of interference in the stainless-steel converter (79). Interference from halogen containing compounds can also occur (80).
9.2.1.4 Advantages and Disadvantages - Many instruments for analysis of nitrogen oxides were designed for ambient-air monitoring. Therefore their analysis range can be too low for direct fire gas determinations. Instruments for high concentration NOx are available.
9.2.2 Ion Chromatography:
9.2.2.1 Ion chromatography has been used to determine nitrogen dioxide (40, 41). If NO is first converted to NO2 by reaction with oxygen, total nitrogen oxide (NOx = NO + NO2) is measured (81).
9.2.2.2 Sample gas is collected in a polyethylene bag or impingers for a specified time period. A determination of the concentration-time history requires a series of samples taken at intervals.
9.2.2.3 A conductivity detector is used to detect and measure NO2- and NO3 which are formed when the NO2 is dissolved in water. If a conventional suppressor column is used, the response of the conductivity detector will change greatly as the suppressor is expended. Greater sensitivity and constant response is obtained with fiber suppressor columns (82).
9.2.2.4 Advantages and Disadvantages - The holding time while NO reacts with oxygen is long. The concentration range over which the methods are useful has not been established. The efficiency of collection of NO2 has not been established.
9.3 Sulfur Oxides:
9.3.1 General Description:
9.3.1.1 Nondispersive infrared (NDIR) spectrometry is often used to monitor oxides of sulfur. These instruments permit continuous sampling and analysis of gases.
9.3.1.2 Water-vapor interference occurs in the SO2 spectral region. Water removal, without appreciable loss of sulfur oxides, can be accomplished with a permeation distillation dryer (see 5.9.3.3). Condensation of moisture (and adsorption of sulfur oxides) should be avoided by using heated sample transfer lines.
9.3.1.3 Sulfur oxides can also be determined with either of two sulfur-specific gas chromatography detectors. These are the flame photometric (83) and Hall electrolytic conductivity detectors. Parts per billion concentrations can be measured with these detectors.
9.3.2 Advantages and Disadvantages - The advantages and disadvantages of methods for sulfur oxides are not apparent at this time, since these techniques are relatively new.
9.4 Carbonyl Sulfide - Generation of carbonyl sulfide (COS), which is highly toxic, during the pyrolysis of sulfur-containing polymers was recently reported (84). A gas chromatograph-mass spectrometer was used for the determination.
