Evaluation of an Open-Path Fourier Transform Infrared Spectrophotometer Using an Exposure Chamber

Abstract

This study investigated the use of a controlled outdoor exposure chamber to evaluate the accuracy of an open-path Fourier transform infrared (OP-FTIR) spectrometer, using path lengths that are found in the workplace. A range of stable homogenous pollutant concentrations of toluene and cyclohexane were generated in the chamber, multiple backgrounds were taken throughout each sampling day, and open-path measurements were compared with point samples taken in the chamber and analyzed by gas chromatography-flame ionization detection. For toluene, the OP-FTIR spectrometer either overestimated or underestimated concentrations by up to 36 percent, depending upon the spectral library (Environmental Protection Agency, Hanst, or ETG) used for quantification. The classical least-squares and subtraction analysis methods gave similar and lower results than the integration method. For cyclohexane, the OP-FTIR spectrometer overestimated concentrations by an average of 29 percent. The apparent overestimation, when compared with the point samples taken from the chamber, may have been due to chemical losses on the walls of the sampling lines and Tedlar bags, systematic calibration errors, or the spectral libraries. When backgrounds were used from different times during a sampling day, toluene results were significantly affected if the partial pressure of water vapor varied by more than 10 percent. Cyclohexane results were essentially unaffected by the use of backgrounds from any time during the sampling day or the sampling week, regardless of changes in the environmental conditions. Further research is necessary to evaluate the use of changes in water partial pressure as a tool for determining the frequency of background collection. This chamber provides a useful way to evaluate OP-FTIR spectrometers for use in the industrial hygiene application for detection of organic vapors.