Obtaining Qualitative Information on Trace Species in Continuous Open-Path Fourier Transform Spectroscopic Measurements Using Target Factor Analysis and Related Techniques

Abstract

We report the application of target factor analysis (TFA) to the identification of trace analytes in open-path Fourier transform infrared (OP/FT-IR) spectra. Results showed that for components such as methane and ammonia, for which the path-integrated concentration was greater than approximately 100 ppm.m, TFA yielded results that closely match corresponding reference spectra. Furthermore, it was shown that the rotation of certain eigenvectors allowed the presence of trace analytes to be recognized when the SNR of these molecules in individual spectra was below 1.0, even though no prior knowledge that a particular molecule was present in the atmosphere being monitored was available. The presence of the analyte was confirmed by conventional TFA. The presence of ethanol and ozone was observed in OP/FT-IR spectra measured at certain locations near dairy and hog farms in this way. No band that could be assigned to either analyte could be observed in any of the spectra in the data set. The application of TFA to OP/FT-IR spectra had the advantage that no preprocessing, e.g., compensation of water line absorption or baseline correction, is needed.