Optimization of Toxic Gas Sampling From Fire Effluents and Analysis by Fourier Transform Infrared Spectroscopy, 01-9145

Principal Investigator
A. Leigh Griffith

Inclusive Dates: 07/01/99 - Current

Background - A large number of fire-related fatalities result from smoke inhalation rather than exposure to the thermal effects of fire. For example, a person can be incapacitated by inhaling only 150 to 250 parts per million of hydrogen cyanide during a five-minute time frame. A system to evaluate the toxic potency of material in a fire requires three components. First, a fire test apparatus is needed to generate combustion products by exposing a specimen of the material to thermal and environmental conditions representative of a real fire. Secondly, a gas-sampling probe with a pathway from the combustion zone to the measuring device is necessary to sample the smoke. Thirdly, a calibrated device that can identify and quantify toxic gases is needed that can analyze the smoke rapidly enough to collect samples dynamically, thus creating a concentration versus time profile. Many forms of chemical analysis rely on ex situ grab bag approaches, or are limited to one or two types of gas species. The use of Fourier transform infrared (FTIR) spectroscopy may result in a single inexpensive instrument that can perform in-line sampling of fire effluents from both static and dynamic fire test environments and that can analyze concentrations of multiple gases quickly with high accuracy. However, while toxic gas analysis based on FTIR spectroscopy is conceptually straightforward, the practical implementation has several potential problems. For example, using a high-resolution setting increases noise, making accurate low-level determinations difficult. Additionally, smoke samples must be transferred from the test apparatus to the spectrometer at a known temperature and rate to obtain accurate concentration versus time values. Another problem is that gases are quantified using a classical least squares fitting of calibration spectra, which does not account for unknown components in the smoke.

Approach - This program is a systematic study to optimize the accuracy of toxic gas analysis to a known value from fire tests by improving gas-sampling techniques, identifying ideal FTIR spectrometer operating parameters, establishing reliable calibration methods, and developing sophisticated mathematical techniques for data analysis. The first step in achieving these objectives is to obtain mixtures of the major gases of interest to establish multipoint calibration curves of concentration versus absorbance. Because many compounds present a nonlinear profile, four to five data points will be collected at each range of interest, covering a range of three orders of magnitude. A gas-mixing system based on partial pressure will be built to dilute the mixtures to any concentration between zero and the maximum value with high, known accuracy. The mixer will also blend different gases known to have overlapping spectral regions that interfere with each other. The second step will involve using these gas blends in a systematic study of sampling techniques, spectrometer operating parameters, and mathematical techniques. A survey of different chemometric techniques, for example, partial least squares, to analyze FTIR spectra will be conducted to determine the best mathematical technique to approach for this analysis. The third step is to validate the optimum procedure selected in step two, including all sampling and FTIR parameters and the quantification technique. This validation will be accomplished by metering known quantities of a gas into the products of combustion from the fire test apparatus. This procedure will allow the research team to determine the robustness of the recommended methods, when used to quantify the composition of real fire gases at high temperature, with soot, unknown components, and gases in the mixture with overlapping spectra.

Accomplishments - SwRI has constructed a gas-blending system using a constant-pressure sample cylinder and an ultra-high precision (0.035 percent) digital pressure gauge. Calibration gases CO, CO₂, HBr, HCl, HCN, HF, NO, NO₂, SO₂, acrolein, acetaldehyde, and formaldehyde have been ordered for calibration of the spectrometer. The survey of chemometric techniques is ongoing. The experimental part of the program, which consists of calibration, parameter optimization, and validation, is in progress.

Chemistry and Chemical Engineering Program
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