Measurement of the concentration ratio for 13С and 12С isotopes at atmospheric pressure by carbon dioxide absorption of diode laser radiation at ~2 μm

The ratio of ¹²СO₂ and ¹³CO₂ concentrations in the human exhaled air is measured by the method of diode laser spectroscopy using a three-channel optical scheme and multipass cell. Unlike the previous measurements in the spectral range of ~4.3 μm with a resolved rotational structure at low pressure of selected samples, the present measurements are performed in the range of ~2 μm, in which weaker absorption bands of CO₂ reside. In this case, it is possible to employ lasers and photodetectors operating at room temperature. The thorough simulation of the spectrum with collisional broadening of lines and employment of regression analysis allow one to take measurements at atmospheric pressure with the accuracy of ~0.04%, which satisfies the requirements to medical diagnostics of ulcers.