Quantitative Fourier Transform Infrared Diagnostics of the Gas-Phase Composition Using the HITRAN Database and the Equivalent Width of the Spectral Features

Abstract

This paper presents a strategy for quantification of medium resolution Fourier transform infrared (FT-IR) spectra. The approach is based on a comparison of the values of the equivalent width of spectral features determined from the measured FT-IR spectra with those calculated from corresponding molecular spectra simulated using spectroscopic parameters tabulated in the HITRAN database. Although the equivalent-width method is routinely applied in many high-resolution experiments, its potential is often ignored when the spectral resolution of the measurements is bigger than the width of the studied molecular transitions. Here with an eye on application of the method to the study of the biomass thermal decomposition products, we demonstrate the capability of the method for analysis of FT-IR spectra with moderate resolution. The method is validated for a number of molecules (NH₃, CO, CH₄, C₂H₂, C₂H₄, and NO) that are important products of biomass thermal decomposition. Namely, known amounts of gases were placed in a sample cell and their concentrations were determined using the method for different FT-IR settings. The agreement between the concentration values determined using the present method and those found in the sample cell was generally better than 10%. The paper also shows examples of application of the developed methodology for the analysis of FT-IR spectra from biomass pyrolysis.