Spectral matching quantitative open-path fourier-transform infrared spectroscopy

Abstract

Open-path Fourier-transform infrared spectroscopy (OP-FTIR) provides a rapid method of measuring concentrations of volatile organic compounds (VOCs) in environmental settings. The advantages of OPFTIR are the speed of data collection and the absence of sample handling. The quantitative difficulties with OPFTIR Include variation in source intensity caused by moving the source or instrument, rapid change in temperature, low intensity of passive sources, and changes in path conditions such as humidity and the presence of interfering compounds. These difficulties are the major obstacles in using OP-FTIR quantitatively. We are developing a method that has shown promise in combating source variability, the difficulty in obtaining reference spectra, and the problems with passive sources. Reducing the resolution and graphing the intensity of the analyte peaks against a convenient reference system produces quantitative curves without subtracting background spectra. These curves can be used to determine unknown concentrations to an accuracy of 20% as long as the intensity and the general shape of the unknown sample spectrum matches a representative spectrum of the quantitative curve. We have determined sample unknowns with the use of running car engines and hot car hoods as sources. The main problem still to be studied is that accurate concentration determination requires a good match between the temperature profiles of the quantitative curve spectra and the sample spectrum. Future studies will involve the relationship between source temperature and spectral shape, source temperature and absorption peak magnitude, and the effect of distance from the source to the instrument on spectral shape and peak magnitude.