A single-beam titration method for the quantification of open-path Fourier transform infrared spectroscopy

This study introduced a quantitative method that can be used to measure the concentration of analytes directly from a single-beam spectrum of open-path Fourier Transform Infrared Spectroscopy (OP-FTIR). The peak shapes of the analytes in a single-beam spectrum were gradually canceled (i.e., "titrated") by dividing an aliquot of a standard transmittance spectrum with a known concentration, and the sum of the squared differential synthetic spectrum was calculated as an indicator for the end point of this titration. The quantity of a standard transmittance spectrum that is needed to reach the end point can be used to calculate the concentrations of the analytes. A NIST traceable gas standard containing six known compounds was used to compare the quantitative accuracy of both this titration method and that of a classic least square (CLS) using a closed-cell FTIR spectrum. The continuous FTIR analysis of industrial exhausting stack showed that concentration trends were consistent between the CLS and titration methods. The titration method allowed the quantification to be performed without the need of a clean single-beam background spectrum, which was beneficial for the field measurement of OP-FTIR. Persistent constituents of the atmosphere, such as NH₃, CH₄ and CO, were successfully quantified using the single-beam titration method with OP-FTIR data that is normally inaccurate when using the CLS method due to the lack of a suitable background spectrum. Also, the synthetic spectrum at the titration end point contained virtually no peaks of analytes, but it did contain the remaining information needed to provide an alternative means of obtaining an ideal single-beam background for OP-FTIR.