Near-IR laser-based spectrophotometer for comparative analysis of isotope content of CO2 in exhale air samples

We present a laser spectrophotometer aimed at high-accuracy comparative analysis of content of ¹²CO₂ and ¹³CO₂ isotope modifications in the exhale air samples and based on a tunable near-IR diode laser (2.05 μm). The two-channel optical scheme of the spectrophotometer and the special digital system for its control are described. An algorithm of spectral data processing aimed at determining the difference in the isotope composition of gas mixtures is proposed. A few spectral regions (near 4880 cm^-1) are determined to be optimal for analysis of relative content of ¹²CO₂ and ¹³CO₂ in the exhale air. The use of the proposed spectrophotometer scheme and the developed algorithm makes the results of the analysis less susceptible to the influence of the interference in optical elements, to the absorption in the open atmosphere, to the slow drift of the laser pulse envelope, and to the offset of optical channels. The sensitivity of the comparative analysis of the isotope content of CO₂ in exhale air samples, achieved using the proposed scheme, is estimated to be nearly 0.1‰.