Quantitative analysis of small volumes of gas mixtures by Fourier transform infrared spectroscopy

Abstract

To analyse small gas volumes efficiently, a cell with a large and variable ratio of optical path to volume (20–100 cm per 100 ml) was developed and adapted to a reasonably high resolution (0.5 cm^–1) Fourier transform infrared spectrometer (Bruker IFS 48). This cell can be used for the analysis of corrosive gases up to 200 °C. A specific software was designed for fast data processing on a microcomputer. The calculation is based on multipeak analysis, with an independant set of peaks for each component. Non-linearity is taken into account for some species (CO, CO₂), and critical analysis of the calibration coefficients for each gas at different frequencies allows validation of the choice of peaks. Additional software manages the standard spectra of pure gases obtained under the same conditions as those of the samples, and the selection process for characteristic frequencies: the complexity of the spectra obtained for mixtures of more than 12 gases (CO, CO₂, N₂O, NO, NO₂, SO₂, HCl, H₂O, hydrocarbons, etc.) necessitates a careful choice of specific peaks for each component in order to avoid interferences, especially when major components such as H₂O, are present. The method was successfully used to analyse gases produced by the radiolytic transformation of geological materials under γ-irradiation. Examples are given, the possibilities and limitations of the method are discussed, and further developments are outlined.