A longwave-infrared (LWIR) passive-spectrometer performance was evaluated with a short-pathlength gas cell. This
cell was accurately positioned between the sensor and a NIST-traceable blackbody radiance source. Cell contents were
varied over the Beer's Law absorbance range from the limit of detection to saturation for the gas analytes of sulfur
hexafluoride and hexafluoroethane. The spectral impact of saturation on infrared absorbance was demonstrated for the
passive sensor configuration. The gas-cell contents for all concentration-pathlength products was monitored with an
active traditional-laboratory Fourier Transform Infrared (FTIR) spectrometer and was verified by comparison with the
established PNNL/DOE vapor-phase infrared (IR) spectral database. For the passive FTIR measurements, the blackbody
source employed a range of background temperatures from 5<sup>o</sup>C to 50<sup>o</sup>C. The passive measurements without the presence of a gas cell permitted a determination of the noise equivalent spectral noise (NESR) for each set of passive
gas-cell measurements. In addition, the no-cell condition allowed the evaluation of the effect of gas cell window
materials of low density poly(ethylene), potassium chloride, potassium bromide, and zinc selenide. The components of
gas cell, different window materials, temperature differentials, and absorbances of target-analyte gases supplied the
means of evaluating the LWIR performance of a passive FTIR spectrometer. The various LWIR-passive measurements
were found to simulate those often encountered in open-air scenarios important to both industrial and environmental
Infrared airborne spectral measurements were collected over the Gulf Coast area during the aftermath of Hurricanes Katrina and Rita. These measurements allowed surveillance for potentially hazardous chemical vapor releases from industrial facilities caused by storm damage. Data was collected with a mid-longwave infrared multispectral imager and a hyperspectral Fourier transform infrared spectrometer operating in a low altitude aircraft. Signal processing allowed detection and identification of targeted spectral signatures in the presence of interferents, atmospheric contributions, and thermal clutter. Results confirmed the presence of a number of chemical vapors. All detection results were immediately passed along to emergency first responders on the ground. The chemical identification, location, and vapor species concentration information were used by the emergency response ground teams for identification of critical plume releases and subsequent mitigation.
The purged gas containment cell is composed of readily available materials. This cell is charged with analyte samples under the conditions of ambient temperature and pressure. The analyte samples are obtained from dilution of commercially available pure material in lecture bottles. This is achieved by injecting pure analyte material into a Tedlar® bags during filling with a known amount of nitrogen diluent. This study demonstrates the utility of the approach using a series of gas samples with concentration-pathlength products spanning the Beer's law range of infrared absorbances. These absorbance values and blackbody radiance levels are within the linearity range of both the active and passive Fourier transform infrared spectrometers that are used in this study. In addition, these conditions are representative of environments that are often encountered in open-air measurements.
Gravimetrically prepared aqueous binary solutions permit the generation of target vapors of methanol and ammonia in a portable vapor cell. A passive Fourier transform infrared (FT-IR) spectrometer monitors a short pathlength optical cell using a calibrated extended-blackbody background source. The temperature of the blackbody ranges from 5°C to 50°C in five degree increments. This temperature range simulates the radiance levels most often encountered for ambient temperature backgrounds in open-air field measurements. The solute liquid mole fractions determine the resultant vapor concentrations. The water component attenuates the target vapor concentration from that of the pure solute component depending on the solute liquid mole fraction. This study demonstrates the utility of a portable vapor cell using a series of binary aqueous solutions per target compound over the Beer’s Law range of infrared absorbances. These Beer’s Law infrared absorbances and blackbody radiance levels are within the linearity range of the passive FT-IR spectrometer and are representative of open-air field conditions.