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